The Stereoscopic User Book

Transcription

THE STEREOSCOPIC USER BOOK
How to make good 3D stereo pictures with a
CineMonitorHD 3 DVi ew
Alain Derobe
All rights reserved © July 2009 Alain Derobe & Transvideo
No part of this publication may be reproduced, stored or transmitted, in any form or by any
means, in any countries, photocopying, including electronic imaging and any use on the net.
A limited license for personal use is given to the user(s) of Transivedo equipment under his (their)
responsibility.
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Equipment that basically need the help of Cinemonitor HD 3DView display.
B.Sp (BeamSplitter) setups : Two-camera rigs, set at right-angle with a beam-splitting mirror.
S.b.S (Side by Side) setups: All stereo rigs with two side-by-side cameras.
O.S (Other Systems) setups: All other two-camera setups, including setups with independent
cameras on different tripods, sometimes far from each other as well as dual lenses or specific
optical systems.
Note : Professional stereo acquisition involves anyway a pixel-to–pixel synchronization with both
cameras "genlocked" to the same timecode clock, and HDSDI monitoring outputs.
OVERVIEW
Stereoscopy is based on a dual point of view recording. To be able to see the deepness on a
display requires wearing specific eyeglasses.
(Only numerous point of view systems give the ability not to wear eyeglasses. The viewing
corresponding range of depth is very compressed).
The CineMonitorHD3DView have been created to clear the mind from any numeric datas or
calculations, and shows 3D adjustment by direct comparative visualization.
It gives a better way to resolve complex interaction in just a glimpse.
The chosen way, Including viewer’s eyes specificities, didn’t prevent from a pixel to pixel
accuracy, but better take in consideration the distinctive features of human sight, witch is never
forget it, “the main goal“.
As interactions between eye’s physiology and visual psychology are not yet well known enough
to be easily set in data or equation, visual adjustment on a screen is at the moment the better
way to set good 3D
Then, we better use “off set“ expressing, speaking about distance of homologous items in left
and right pictures, (measured on the screen in pixel unit), than “parallax” that involves boring
angles calculations.
In addition, we never know if “parallax“ term is used speaking about taking pictures or as the
result in screening, which makes confusion. Therefore, we will see through a glossary at the end
of this user’s book, that those two fields have to be completely dissociated in 3D stereo strategy.
Convention:
The terms used for shooting are underlined.
The terms concerning projection are in italics.
For instance, we will better use terms such as “angulation“ (in % of picture width), interaxial (in
cm or inches) and convergence plane (in meter or feet) as a result of the combination of the two
formers.
In this “stereoscopic user book“, a step to step process have been designed to drive 3D
professional consultant as well as 3D beginner, to set 3D cameras adjustment in a right logical
way.
“Natural depth method“ additional book drives the reader with virtual situations, in a justified and
a structured strategy on the path of the fundaments of the stereoscopic system. Both will give to
the reader, answers to most of 3D problems.
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MANUAL SUMMARY
CineMonitorHD3DView Allows Specific Stereoscopic Adjustments ............................... 7
A.
1.
PRELIMINARY CAMERA GEOMETRICAL ADJUSTMENTS. (Basic geometrical disparity). 9
1.1
TILT - IMAGE HEIGHT ADJUSTMENTS: ........................................................................ 10
1.2
ROLL - RELATIVE HORIZONTAL ADJUSTMENT........................................................... 11
1.3
ZERO ANGULATION CALIBRATION:.............................................................................. 13
1.3.1
FRONT END CHECKING. ......................................................................................... 15
1.3.2
Having the two lenses not exactly at the same height may have drastic
consequence on the image height adjustment...................................................................... 15
1.3.3
Mainly with macro stereoscopy, note that if one lens is in front of the other, the
background will be correctly superimposed but not the foreground. .................................. 15
2.
SHOT BY SHOT GEOMETRY ADJUSTMENTS AND SPECIFIC ADJUSTMENTS
ACCORDING TO FOCAL LENGTHS. ............................................................................................ 17
3.
CHECKING ANGULATION FOR EACH FOCAL LENGTH. ................................................... 19
4.
CONVERGENCE PLANE POSITIONING AND MASTERING. .............................................. 24
5.
USING THE GRID - DISTANT PLANES POSITIONING - COLORED BORDERS EQUAL TO
GRID PITCH - WIDER OR NARROWER COLORED BORDERS - HOW TO CALIBRATE
EFFECTS COMING OUT OF THE SCREEN .................................................................................. 26
6.
GRID USER CALIBRATION. GRID SPACING ADJUSTMENT ACCORDING TO FINAL
THEATER SCREEN WIDTH............................................................................................................ 27
7.
DETECTING SPECIFIC STEREO DISCREPANCIES. ........................................................... 28
B.
CineMonitorHD3DView helps to achieve two-camera photometric matching. ............ 29
1.
USING DUAL HISTOGRAMS. ............................................................................................... 30
2.
USING DUAL WAVEFORM MONITORING ........................................................................... 30
3.
USING DUAL VECTORSCOPE. ............................................................................................ 31
4.
DETECTING FLARE DISSYMMETRIES AND BRINGING FORWARD UNWANTED OR
UNMATCHED GLARE. ................................................................................................................... 31
5.
CHROMA FLICKER OR SWITCHER. .................................................................................... 31
C.
OVERVIEW OF A STRATEGY: “NATURAL DEPTH™“ METHOD. ......................................... 32
1.
Rules for shooting and displaying depth. ............................................................................. 32
2.
Working method: Step to step strategy................................................................................ 33
3.
For which screen width can adjustments be performed when the final projection size is
unknown?........................................................................................................................................ 34
4.
SCREEN WIDTH AND MATCHING BACKGROUND OFFSETS........................................... 35
5.
Two important bounce effects are of major importance to understand what is actually
different on large screens............................................................................................................... 36
6.
HOW TO BLOW OUT THE SCREEN BUT NOT THE VISION. ............................................. 38
D.
GLOSSARY ............................................................................................................................... 42
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Recommended setups for stereo acquisition.
The CineMonitorHD3DView does not involve any specific test chart nor particular equipment. It
is thus possible to adjust the equipment on location, during shooting, with the actual sets.
Nevertheless, apart from the usual camera assistant equipment, used for conventional
adjustments, the following additional items could be useful:
- A black flag with a small white cross will be set up on a stand in the frame, to simulate a
convergence point, when a character is not in his position or when no element stands out in this
specific area.
- On the other side, a double cross matching exactly the interaxial distance of a side-by-side rigs,
will help adjustment when pointing indoor short distance for parralelism control.
- A light-toned, over 5' (1.5 m) long rod will help to adjust relative horizontal level.
- When complex special effects are planed, an angle to level measuring tool and a laser beam
pointer instrument are very helpfull.
- Color test charts to help respective color rendering fine adjustments on the two cameras.
Preliminary warning:
-When looking to the monitor with “red / blue-green anaglyphs” without wearing the glasses,
keep in mind that a light-toned picture element with a red border on its left, means that this item
will be shown to the viewer behind the screen plane. On the contrary, if the element is dark, a
cyan border will show on the left for the backgrounds.
-It is very important to get used to this discrimination to obtain an immediate reading of the L/R
colored separation given out by the monitor.
-Of course, a light-toned item with a red border on its right will show a coming out of the screen
effect, just like a dark item must have a cyan border at its right if it is located in front of the
screen.
-With monochrome magenta/green function and appropriate glasses, ones can see as well
deepness or better, but without color rendering. When light bright primary tones are in the field,
this system will give less colored artifacts.
-Yellow to blue anaglyph system could be available on special order.
All those anaglyphic systems are very useful to inspect the picture, without wearing glasses, and
shows perfectly what belong to right or left picture.
Active glasses system, gives a nicer deep visual effect for fun, but didn’t give any identification
about left or right pictures adjustments.
This function is very useful to simulate a residual “ghost effect“ on the screen when lightly areas
are on a very dark background.
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A. CineMonitorHD3DView allows
Specific Stereoscopic Adjustments
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1. PRELIMINARY CAMERA GEOMETRICAL
ADJUSTMENTS. (Basic geometrical disparity).
To achieve these adjustments, it is recommended to begin with the focal length you plan to use
the most frequently. If not, choose the normal focal length closest to the horizontal width of the
sensor(s). It is the focal length that will be closer to an orthoscopic vision better for stereo.
Adjustments for other focal lengths will thus be distributed on both sides of the normal focal
length.
For instance with 2/3 inches chip ( 9.5mm horizontal HD size), a 9mm or 10 mm focal lens will be
considered as normal focal lens.
(Diagonal is better a reference to still picture work).
After setting up the equipment and connecting the cables according to the operating instructions
in the “Operator Manual“, set the monitor to “3D function” (for instance, red and cyan). Check
that the two lenses are positioned at the same level by inspecting from the front of the rig. If not,
the frame heights will never match for both distant and close-up shots.
Note that with general use from 3' (1m) to infinity, a 1 mm lens height difference or more can
remain unnoticed, when it gives some troubles with macro or very big close up.
With the beam-splitting mirror rigs (B.Sp), overlap the two lenses horizontally as well as vertically,
by looking through the mirror. Mark that starting position on the spacing scale as the null
interaxial distance. The cameras will be perfectly adjusted when the dual picture on the monitor
will not show any colored borders
On most rigs, the left camera will be the horizontal one. Choose it as reference camera. (Note that
on stereo HD Steadycam™ rigs, it will often be the reverse.)
With Side-by-Side rigs, (S.b.S ) adjustments are generally made by pointing at a very distant
scenery. If not, then use carefully the appropriate double cross target.
Levels of the lenses have to be controlled physically with a rod.
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1.1 TILT - IMAGE HEIGHT ADJUSTMENTS:
The camera rig has build in tilt adjustments. (If not, use spacers
at the back of one of the cameras to obtain a proper adjustment).
Point a target and check there are no colored borders above
small horizontal elements. Choose relevant items, preferably at
the same distance from left and right borders to avoid any
confusion with the Roll adjustment.
(If you think the two lenses are perhaps not strictly on the same
level, aim better at background items.)
When a light-toned item is upper lined with a red border, or if a dark item is upper lined by a cyan
border - like dark scenery on a background sky, lift up slightly the front of the left camera (red
picture). Use the display's 2x enlarging feature (F4) centered between right and left sides (using
blue thumbwheel) to fine adjust with a one-pixel accuracy. (Fig.1)
B.Sp. (Beam Splitter system). The adjustment is very easy for rigs with a beam-splitter mirror
system, if the two lenses have been previously perfectly overlapped by checking from the front.
With a null axial distance, the two pictures should then be strictly identical on their entire surface.
The smallest colored border shows there is a camera matching discrepancy.
(Possible defects: When the beam-splitting mirror is not strictly flat, or when the two lenses do
not produce perfectly overlapped pictures, tiny colored borders may appear only in a particular
picture area. For instance a strain on the mirror can be implied or other better matching lenses
may be found.
If the colored borders are over a few pixels, we suggest recording a grid or a constant-pitch cross
hatch to make systematic correction in post-production easier.)
S.b.S. (Side-by-Side system). For outside shots, better find a distant landmark so that the two
pictures can be overlapped perfectly. Otherwise, choose elements with horizontal lines, to
compare them with their own extension. No need to say that with side-by-side cameras, any item
that is not distant shows colored borders on the side.
When tilt adjustments are very small, you may prefer making fine adjustment during postproduction. In this case write down the number of pixels for each focal length, to make later
correction work easier.
A 1 to 3/1000th of the frame height precision is acceptable when shooting feature films (one pixel
accuracy is more professional work, but a 1% error for a very short time will not prevent the
majority of the audience to merge fleeting images.
(Note : Watch out for mechanical stress or play due to the weight of the cameras that can slightly
modify the vertical adjustment during extreme up and down tilt movements. We suggest testing
these positions during preparation and control an incidental baseplate loose).
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1.2 ROLL - RELATIVE HORIZONTAL ADJUSTMENT.
The camera rig has a build in horizontal roll adjustments. (If not,
use spacers under the side of the camera adaptors to obtain a
proper adjustment).
Watch now alternately and in the same manner the upper right
and left corners with the 2x enlargement. Upper or lower colored
borders with different colors from one corner to the other reveal a
picture roll between the two cameras, meaning a relative
horizontal discrepancy.
B.Sp. In the left corner of the screen, a red border above light-toned elements means that the left
camera leans to the left. Lift therefore its left side. Do the opposite if the upper colored border is
cyan.
In the right corner, it would be the opposite. (Fig 2).
Note : Remember, you can be confused when dark elements on a light-toned background show
reverse colored borders. Aim at horizontal lines as much as possible. Slanted lines are
misleading!
Viewers with delicate visual systems, even on large screens, will tolerate a vertical mismatch of
some 2 or 3 pixels in HD resulting in .02% to .03% of the picture height. However, trying to
obtain a one-pixel precision would be better for long durations and to achieve professional
quality.
Correcting roll mismatch in postproduction requires re-sampling and can destroy image quality.
Horizontal roll adjustment must then be done precisely as it is final and focal length independent.
After roll adjustment have been done, it can lead to a minor tilt adjustment (§.a). Check if needed.
S.b.S. For outside shots, finding a distant landmark (horizon, buildings) is the best. For interior
shots, look for a horizontal architectural item perpendicular to the optical axes (i.e. furniture,
railing) or set up a light-toned horizontal rod on a stand at a right angle with the axis of the shot
and covering the entire frame width. Check as above for reverse colored borders from one side to
the other.
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Figure 1 – Vertical tilt mismatch
Figure 2 – Clockwise left camera roll
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1.3 ZERO ANGULATION CALIBRATION:
The camera rig has build in angulation adjustment. (If not, turn the
camera horizontally).
First of all, any angulation adjustments on Beam Splitter Rigs have
to be set with the two lenses in absolute superimposition through
the mirror.
With Side by side rigs or other systems, lenses axes have to be
perfectly parallel.
B.Sp. With beam-splitter mirror rigs and a null optical interaxial distance, pictures should be
perfectly superimposed. If colored borders appear, correct the overlap with the angulation
adjustment and reset the zero mark. (This mark can be slightly different according to the lenses.)
This mark is mainly a reference one for user information, because the CineMonitorHD3DView
accounts for all accidental or repeated drifting.
Nevertheless, zero angulation will be useful during parallel shooting, either for an adjustment
aimed only at very large screens, or for making some special effects easier (i.e. tracking).
S.b.S. When a very distant background is available (more than a thousand times the interaxial
distance), adjust the angulation until the overlap has no colored side borders. (i.e. with an 6"
(15cm) interaxial distance, choose a background more than 500'(150m) away). For instance, at
1000ft (300m) a one pixel residual offset will remain.
If the item is not far enough, then try to appreciate the interaxial distance directly with the subject,
if it has known features. For instance, on a relatively close building, shift the two pictures by
comparison with the width of a brick. That can be accurate enough.
For inside shots, use a double cross with exactly the same offset distance as the interaxial
distance. A zero angulation marking can thus be corrected or defined.
Bear in mind that the angulation adjustment as well as the resulting convergence are generally
tweaked in post-production and then are not critical settings.
O.S. (Other systems). Two cameras positioned separately can also be calibrated and lined up. (Of
course, they must be re-calibrated for each setup).
-- For very distant landscapes: Mountains, etc. After having carefully checked the double tilt and
roll position of the cameras, proceed in the same way as for S.b.S. concerning the three main
adjustments by aiming distant items.
It is possible to adjust very distant cameras with a long BNC connexion. Just frame the first one
and then adjust visually the level of the other one with a fine tilt and roll adjustment knobs.
-- A specific case: Hyper Stereo on shallow subjects :
For instance, for tight close-up shots or to increase the depth of a “bas-relief” on a monument,
with a very large interaxial distance.
To obtain Hyper Stereo, a very strong angulation (several degrees) must be applied, bringing
inverted trapeze distortion (keystone cross effect) on the two pictures. Image height (like in "A")
and angulation (like in "C") adjustments must be performed as close as possible to the center of
the frame to overcome the trapeze effect. You must next evaluate the (as describe in "B") roll
error by choosing only elements half way up the frame or by judging by sight the same
symmetrical colored border values at the right and left of the frame. Especially for small depth
values, the unescapable trapeze distortion will be partly corrected in post-production.
Note that for an angulation usually set for a normal stereo effect on characters (generally under
1°), trapeze distortion remains below the audience's visual tolerance, even on large screens.
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Figure 3 - Zero angulation calibration
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1.3.1
FRONT END CHECKING.
With beam-splitter rigs, check from the front if the two lenses are correctly superimposed
vertically when the two lenses are at null horizontal spacing adjustment.
1.3.2 Having the two lenses not exactly at the same height may have
drastic consequence on the image height adjustment.
A height mismatch between foreground and background occurs
on the screen only if the height difference between lenses is a
significant part of the closeness of the foreground. By instance,
with a normal focal lens and a HD 1080 setting, a one
centimeter height difference (vertical parallax error) with an
adjustment to infinty, wll create 1 pixel mismatch for a local 17
meters foreground, and 10 pixels uneven level for what is at 1,77 meter camera distance.
See the diagram showing an inability to get a mountain, a tree and close-up character, leveled in
a same time when lenses are not leveled.
When you place horizontally two or three wires or strings in the deepness of the field,
CineMonitorHD3DView will help you, in showing colored underlines, to correct unnoticed
lenses height mismatch. (Fig. 4)
1.3.3 Mainly with macro stereoscopy, note that if one lens is in front of
the other, the background will be correctly superimposed but not the
foreground.
When you suspect that the two lenses are not exactly at same
distance of the subject, place a large graduate ruler at 1meter
distance from the normal focal lenses. If one picture is 1cm larger
than the other on the CinemonitorHD 3Dview, this means that one
lens is 1 cm behind the other one.
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Figure 4 - Height difference
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2. SHOT BY SHOT GEOMETRY ADJUSTMENTS AND SPECIFIC
ADJUSTMENTS ACCORDING TO FOCAL LENGTHS.
Note that if the rig is rigid enough, roll adjustments will remain unchanged whatever the focal
length.
You must of course keep track of the lenses, mark them with colored tape, red for left one and
blue or green for the right one, and never switch them around from one camera to the other. It is
indeed well known that axis offsets can occur depending on each specific lens.
This can come from either an offset between the lens axis itself and its mount or more often from
the offset between the sensor(s) and the camera mount. In this case the offset will regularly
progress together with the increase of FOCAL length.
Axis offsets coming from lens mounts appear when switching
different similar lenses on a same camera. Even with the most
professional equipment a slight axial mismatch does not
guarantee a sufficiently precise adjustment to shoot in stereo.
The adjustments made with the monitor offer the advantage of
taking them into account.
When the lenses have an available re-centering adjustment, such as the 17/112 Zeiss Digiprime
zoom, the CineMonitorHD3DView helps evaluating the adjustment to be done. With other
systems, a very delicate work in a specialized workshop can be planned.
a)- Then, a vertical correction may have to be done when the focal length changes. It can be
done on the spot and by sight as in paragraph § 2-1 a.
It will remain always identical to itself with the same lens, and can be marked on the tilt
adjustment knob (if so) to anticipate and spare time.
As long as tilt corrections remain small, the shot by shot adjustments by sight will be almost
immediate. When they are very small, some will choose to leave them to post-production, even if
a few pixels have to be cropped..
b)- When the CineMonitorHD3DView monitor is used in stereo mode, the horizontal
adjustment for each focal length is of course included in the adjustment by sight.
When a zoom is used like a series of fixed lenses, these adjustments can be made on the fly. A
dual remote control system will help. Fig 1-2B
For Daredevil people, some small focal re-framing can be done during the shot if a double zoom
motor is available. It must be extremely reliable because human vision does not tolerate unequal
image sizes. To remain on the safe side, focal lengths should be below twice or three times the
normal focal length.
Nevertheless, when shooting distant scenes with long focal lenses and/or large zoom amplitude,
such as sports or performances recording, sophisticated equipment with integrated "Motion
Control" must be used, with all the corrections pre-recorded on the computer.
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Figure 5 - Focal length mismatch
Figure 6 - Correct focal length adjustment
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3. CHECKING ANGULATION FOR EACH FOCAL LENGTH.
With each new shot, to keep distant landscape always similarly
behind the screen and to obtain the same depth view for the
audience, the angulation adjustment must physically be
performed, depending on focal lengths. Actually, to obtain the
same offset on the screen, (6,5 cm max for the background)
the applied angulation must decrease when the focal length
increases. (Fig. 7).
Figure 7 - Far Background adjustment
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Please refer to the "Natural Depth" tutorial, and remember that, whatever the size of the screen
is, items corresponding to background areas, have never to be offset over 6,5 cm (2,5 inches) on
it. This absolute rule ensues the physiological impossibility of having diverging sight.
With the CineMonitorHD 3DVIEW, the adjustments displayed on the monitor are the direct results
and do not depend on the focal length used. (Fig 1-3 bis).
With the user-defined grid pattern, a grid pitch is chosen generally during the whole continuity
according to the size of the final screen.
Then a constant “scenic box“ can be preserved (or breack out), depending on the option taken
by the stereo consultant.
Anyway, and whatever the lens used, offsets between left and right picture elements, that show
farest background items have to be adjusted with the choosen grid pitch.
To have a grid pattern instead of a simple crosshatch avoids moving the camera away to aim at a
particular element, and also give the opportunity to inspect all distant far planes together with one
look.
Note : For those who want to work with parallel cameras, and who wish to simulate convergence,
proceed in the following way: Use the "Shift" function of the CineMonitorHD3DView monitor,
and shift the left picture (red) of the equivalent of one step of the grid towards the left side of the
picture (this means the same number of pixels than the chosen grid pitch). This "virtual
angulation" will directly display on the screen the result of the convergence, which will be
reconstructed with an appropriate offset in post-production.
The convergence plane will thus be simulated (stereo window) and the same depth than with a
real angulation will be observed with 3D stereo display function.
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Figure 8 - Angulation Adjustment with various focal length
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Figure 9 - Angulation, Separation and Convergence Plane
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4. CONVERGENCE PLANE POSITIONING AND MASTERING.
An Angulation ( or a replacement "offset" simulation) having been previously positioned, as
mentioned above, the right way to change the convergence plane position. will be in modifying
only interaxial distance. (Fig. 9).
Perform a systematic adjustment of far backgrounds in setting correct angulation “α”, then adjust
convergence plane “C” by setting camera interaxial distance “ε”
Three type of elements will then be observed in the picture :
- All elements with light-toned parts, which show red borders on their left, will be the distant
planes.
- All elements whose light-toned parts show red borders at their right will be coming out of the
screen. (The elements coming out of the screen and that are cut off by the edge of the frame will
quickly be identified as disturbing).
- Then, all the areas with no colored borders are basically on the convergence plane. (Normally,
the convergence plane is supposed to be displayed on the screen plane, unless further horizontal
offset adjustment during post-production.)
Any change of interaxial distance will rearrange these areas with an obvious display of colored
borders, so the balance between them will be done at a glimpse of an eye. Warning: Viewing with
anaglyph (red and cyan) as well as active glasses (LCSs Liquid Crystal Shutters) will display a
pleasant 3D effect but with offsets and angulation adjustment not suited for a large screen. The
grid pattern is the only standard that allows calibration for large screens.
The convergence plane resulting from the adjusted interaxial distance will be positioned with the
following rules:
Strictly speaking, no element coming out of the screen should normally ever be cut off by the
edge of the frame. This means that the convergence plane should always be in front of the
nearest element. This rule can particularly be annoying, during dialogue scenes with a character
cut off by an over-the–shoulder shot! Usually you may cheat a little, but not up to the point where
the audience becomes aware that you do.
Recommendations on this “stereo window error“ are found in the paragraph about the "Natural
Depth" method and in detail in the corresponding book.
The front parts of the fence come out of the screen.
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5. USING THE GRID - DISTANT PLANES POSITIONING - COLORED
BORDERS EQUAL TO GRID PITCH - WIDER OR NARROWER COLORED
BORDERS - HOW TO CALIBRATE EFFECTS COMING OUT OF THE
SCREEN
All evaluations, all reasoning as well as precise calculations on the position of stereo picture
elements in the theater, become miraculously evident when an interocular distance (IOD – Inter
Ocular Distance) of 2.5" (6.5cm) is used as a standard unit.
For instance, the 2.5" (6.5 cm) IOD calibration will fit hundred times in a 25' (6.5m) theater screen
and corresponds to a x100 pitch displayed on the grid. (See figure in the central page)
So even if you don’t know the size of final screen, you have to choose one width to make a
consistent continuity.
Even if you intend to cheat, keep the grid pitch at exactly the ratio between IOD and theater
screen size, and prefer, if necessary, to calibrate visually beyond the grid pitch.
For instance, some far plane offsets beyond the distant elements calibration requiring little eye
divergence from the audience can momentarily be tolerated. 8,10 cm or (3, 4 inches) measured
on the screen. Regarding this, the audience beyond the “Depth bracket” will see confined
elements such as a far light in night background. Then the “Scenic box“ will locally overpass
“Depth bracket” limits.
The grid pitch representing exactly the IOD will also be very useful to appreciate the other depths
and specifically to quantify effects coming out of the screen. Then, It has to be used as a general
reference.
In this case, not only the distant elements will easily be calibrated, but (when a reversed offset)
the elements coming out of the screen showing the size of one, two or three column intervals, will
be respectively at half, one third or one fourth of the distance between the eye and the theater
screen.
It is the only way to define how much is the extension of the outcoming of the screen, while trying
to measure them in feet (or meters) would not make sense.
Can you imagine an easier system?
Thus, you can learn to "read" rapidly directly on the TransvideoHD 3D display the position of
each depth element.
Isn’t it every stereo consultant dream?
Consequently, every one scale division reversed offset (the red border of light element being on
the right side) will point out the elements coming out of the screen at halfway between eye and
screen. This means ½ OSD (ocular screen distance) : see diagram "E". The bottom of the table
on the graphic, simply shows that one, two, three or five crossed offset intervals show elements
coming out of the screen respectively at one third, one fourth or one sixth of the eye-to-screen
distance. ( see "F, G, H" diagrams for : 1/3 OSD, ¼ OSD or 1/6 OSD ).
Intermediate values will be properly appreciated at a glance. For instance, an element in the rear
with a colored border half the size of the grid's pitch value will be seen halfway between farest
plane and screen plane for the whole audience whatever is the position of the viewer. (see "C"
diagram). Is’nt it easy to use !
26
6. GRID USER CALIBRATION. GRID SPACING ADJUSTMENT
ACCORDING TO FINAL THEATER SCREEN WIDTH
That 3D stereo pictures must be "calibrated" according to a given screen size, comes as a
surprise to many ! What can then be done to account for different theaters ? Fortunately, depth
can be readjusted according to screen size with little trouble and minor losses by a full frame
sideways offset.
The “Natural Depth™“ method suggests two typical screen sizes: 21' (6.5 m), the most
convenient for any purpose use and as a shooting easier reference for beginners, and 30/33'
(9/10 m), better for feature or professional master delivery.
For instance, in HD and for a 30' (9 m) screen (140 times the interocular distance: IOD), a 140
lines grid (14 pixels in HD) will be used to calibrate immediately most of the far planes at their safe
value (See figure in the central page)
Note that for larger screens a grid with a narrower pitch would not be easily readable ! Therefore,
for screens 42' or 85' wide (13 m or 26 m), you can refer to 1/2 or 1/4 grid interval while keeping a
pitch of one hundredth.
Some will prefer using the picture offset, or the user crosshatch, both being equally adjustable
pixel by pixel. In this case the stereo 3D display on the monitor is degraded, there is no more help
for adjusting the interaxial distance and the information about the elements coming out of the
screen is lo
Nevertheless, one can think that with this giant size, the convergence plane obeys different laws
and the interaxial distance will be chosen according to other criteria. As window error decrease
with large screens we recommend to work as if for a 21’ (6,5 m) or 33 feet (10 m) width screen
and recalibrate if necessary for larger ones.
-For resolutions other than HD 1920, apply a rule of three, depending to the display and the pixel
numbers (for instance with 720p).
-In an other hand, for smaller screens than 6,50 m (21 ft), the far planes offset limit is no longer
proportional to the interocular distance. Indeed, there would be no sense in offsetting close to a
half of the whole picture on a 7" monitor!!!
The offset of the far planes on the screen, then have to be asymptotic to a fixed limit of around 2
to 2 ½ %. This represents a maximum limit of about 40/50 pixels in HD. See the graphs
corresponding to screen widths, from the "Natural depth" method.
(Numbers can be a little corrected, depending of the distance from the display to the viewer.
When watching still pictures this distance is usually bigger. Then the “one thirty’s rule“ that gives
3% of the distance to calibrate the offsets, cannot be applied to movies work.)
The cause lies in an inescapable "correlation" (called synchrony) between the viewer's crystalline
lens focusing on the screen (accommodation) and his two ocular axes attempting to converge on
a point at another distance. This physiological tyranny will particularly prevent displaying far
planes beyond a very small fraction of the width of small screens, because decorrellation
possibility is very restricted.
Consequently, with these small display widths, the elements coming out of the screen, far from
being proportional to their size, will be extremely reduced and flattened.
You can now understand better why even when you display stereo on the monitor for safety, or to
check for some discrepancies, YOU HAVE NOT TO CALIBRATE THE DEPTH AMPLITUDE WITH
THE STEREO EFFECT YOU CAN SEE ON A DISPLAY SMALLER THAN THE FINAL ONE.
ANYHOW YOU MUST COMPLY WITH THE GRID INDICATION TO GET A FITTED WORK.
27
7. DETECTING SPECIFIC STEREO DISCREPANCIES.
In stereo anaglyphic color mode, (with or without the eyeglasses), or in stereo magenta/green
monochrome mode, it will be easy to detect an isolated colored element or an element far away
from its counterpart in a complementary color. These solitary elements that strike the eye are
called "orphans".
It is often a leaf, a gaffer tape or a connecting wire hanging close to the camera and left
unnoticed. Superimposing on the monitor also points out a French flag in the field of only one of
the cameras.
Actually, using the main viewfinder can make the camera operator forget what is happening on
the other viewfinder, often neglected because of its upside-down picture on B. Sp. rigs or a lack
of room to place the viewfinder on S.b.S ones.
All set element partially in the field at the frame edge on one of the cameras and not on the other
will then be easily eliminated.
In the same way, "flares" and asymmetrical reflections, which are the greatest curse of stereo
picture pairs, will be easily eradicated by observing two superimposed pictures on the same
display.
A particularly sneaky defect with a regular pattern can appear: when for instance the offset
between the two images is the same as the pitch of a grating or any similar frequency like a fence
or an iron gate. In this case the superimposing attempt made by human vision becomes random
and hesitates between several neighboring patterns since they are similar. The perceived effect is
extremely unpleasant.
The grid will then be a valuable help to detect the dangerous areas.
28
B. CineMonitorHD3DView helps to
achieve two-camera photometric
matching.
29
1. USING DUAL HISTOGRAMS.
As well as a waveform allows to appreciate (in an analog way) the light amplitude displayed
vertically, the iris differential adjustment is made easier by the huge lateral offset displayed by a
double histogram.
Actually, if a specific area with the same brighness on each picture can be spotted, this area will
clearly undergo a digital category change over the histogram. Mostly its central significant part,
like its hight lights right part, will move rapidly sideways at the smallest exposure change.
Comparative lighting adjustment can therefore be executed with very high precision, way beyond
what just matching the two pictures requires.
Some mismatch on the two envelope between homologous areas will show up discrepancies in
specific high or low lights. That can reveal a sneaky one-eyed glare or reflexion.
2. USING DUAL WAVEFORM MONITORING
Double “waveform“ will remain useful to locate within the picture width if a specific reflexion is
clearly more intense on one of the cameras.
In general, it is advised to be concerned each time a waveform as well as a histogram envelope
difference is noticed.
For instance, if a different gamma or a white compression circuit does not match perfectly the
other camera, then you will see different "waveform" or “histogram” envelope.
30
3. USING DUAL VECTORSCOPE.
Masking in turn each camera can show up a color shift. Responses on color charts can also be
compared.
White and black balance can also be checked.
4. DETECTING FLARE DISSYMMETRIES AND BRINGING FORWARD
UNWANTED OR UNMATCHED GLARE.
A flare or an unwanted reflection is visible because it pulls up the black level. It can be detected
either if the left side of the double histogram is not overlapping, or if the bottom of the double
waveform is lifted up.
5. CHROMA FLICKER OR SWITCHER.
All general or local synchronization defects generate a particular flicker on the monitor. This
unpleasant picture instability is useful to warn the user that a temporal offset is present between
the two pictures and that the audience can be disturbed, for instance by car wheels on a pan
shot.
31
C. OVERVIEW OF A STRATEGY: “NATURAL DEPTH™“ METHOD.
1. Rules for shooting and displaying depth.
The "Natural Depth™" method aims at converting the real depth of the subject to obtain a limited
depth called "Scenic Box" through the theater’s screen.
It is well known that the Scenic Box shown in projection is different for each viewer seat. The
paradox is that after a few moments of accustomation, it will nevertheless appear as a fair
likeness of reality to every viewer at any seat at whatever position ! This is a visual adaptation
phenomenon and that is fortunate, otherwise who would buy tickets for one-seat theaters!
- In addition, the Scenic Box is practically never identical to natural layout. It obeys its own rules,
witch are very different ones from natural seeing the surrond.
Therefore, the comparison between the goal - a satisfying “Scenic Box“, and the real depth of the
set will lead us to the correct camera adjustments of the appropriate rig, whatever type it is.
Also, very different extensions of “Scenic Boxes“ cannot be successively displayed on the screen
whatever reality is represented, but a visual presentation continuity must rather be preserved.
In all circumstances, try to fill as better as possible the volume of the Scenic Box, without
minding anymore the real subject amplitude. The maximum “Bracket of Depth“ cannot be used
all along the continuity because of eyestrain and also because that did not give anymore
adjusting possibilities to post production.
So effective “Scenic Box“ will be generally less extended than maximum “Bracket of depth“.
Compressing or expanding reality seems then of little importance by comparison with managing
on the screen a flow of several Scenic Boxes that must follow their own laws. These laws aim at
satisfying the audience's vision and are an absolute priority, whatever the subject or how it is set
up.
32
2. Working method: Step to step strategy.
Look out for elements the farther away behind the subject, containing details that can be
analyzed. They will be fare backgrounds, and their left and right images must be shifted on the
display to be seen as much behind the screen as possible, preferably at the rear end of the
Scenic Box. (That means a 6.5 cm offset on large screen whatever its size. That offset have not to
be much overpassed).
By applying an angulation between the two cameras axis, the resulting offset between these far
planes will be correctly adjusted on the screen. II should match the grid pitch on the display.
(Remember this pich has been chosen according to the final projection width size.)
This offset can be slightly readjusted in post-production by a global offsetting of the entire image
to make shots even. This small correction does not change much the relation between
background and foreground planes.
Applying different optical interaxial distances to the same securely fixed angulation will
consequently push back more or less the optical axis crossing. This is how the convergence
plane will be adjusted forward or backwards, following the well known triangulation laws.
Changing convergence plane position during a scene will then have to be a modification of
interaxial distance, not of angulation.
Whether axial distance modification is made during the shot, or if it concerns successive shots,
the audience will not notice any variation. With moderate interaxial movements, various depth
elements will be neither flattened nor distorted and will quietly stay in position for the audience.
For the same screen width and with far planes at infinity, there is only one correct angulation per
field width, hence per lens. For each screen width, the number of grid intervals must fit to the
number of times the interocular distance holds inside the screen width.
Whenever a close obstacle, such as the wall of an indoor shooting, limits the distant background
of the picture, the angulation must be increased until the offset indicated by the grid is obtained
again. Of course, this interior wall is now a “restricted background“, but it haves to be considered
as well as the new far plane in the scenic box.
On the other hand, those who will choose against common sense to modify the angulation while
a character is getting closer, to keep the convergence plane on him, will necessarily greatly
increase the far plane offset on the screen. Beyond twice the intraocular distance on a medium
size screen, a large part of the audience will not be able to merge anymore left and right pictures,
and will see them double (diplopia) while, with the rest of the audience, eyes will hurt or constant
headaches will appear.
This unlookable background is much worse than a possible miniaturing effect or gigantism effect.
How to be protected from these abnormal size effects will be explained in detail with “Natural
Depth™“ method.
Benefits from using a stereo 3D monitor.
Of course, when screens of any size, lenses with different fields and various distances are
concerned, adjustments could eventually be meticulously calculated every time by geometry
drawing or with tables, software and formulae, but this is not the right way for a rapid setup on
the stage.
With this way of calculation, how appreciation or cheat can be applied depending on the nature
of the image, when human comparative feeling about significant areas in the picture is a
basement of 3D stereoscopic language ?
Formerly, when adjustments were executed blindfolded, mainly with film stock shootings, there
was no other possibility than these sluggish calculations.
Now the revolutionary CineMonitorHD3DView, allows precise real time adjustments, without
any calculation or tables. It also gives the possibility to modulate what the stereo operator can
(and must) appreciate regarding picture composition.
33
3. For which screen width can adjustments be performed when the
final projection size is unknown?
The use of a standard based on 21'(6.5m) screen for shooting, in addition from using a very
practical 1/100th (20 pixel) grid, corresponds directly with an average width of projection screens
used for screening or post production control.
Professionnal feature film makers will like better to work straight to very large screens with a
standard like a 10 m (33’) one, to get less trapezoid distorsion. Then they will better use a 14
pixels grid (14 pixels are 140 times in a 1920 pixels picture width).
When using the 21’ (6,5m) width screen you have to strictly respect the stereo window rules. It is
an additional mean to use it as a reference.
(We remind you that “stereo window error“ is the visual contradiction caused by an image
element seen in front of the screen being caught by the edge of the screen perceived far behind).
But we will see that it is much unlike with the very large screens.
By means, with a screen far away, the “scenic box“ have to be almost entirely coming out into
the theatre.
Anyway, If shooting have been done for a 21’ (6,5m) screen width, it will be necessary to
recalibrate a "Master" – for instance for a 30/33'(9/10m) screen standard, with a 5/6 pixel shift –
to comply with the far planes position in the theater.
If a Master rather calibrated for a 42'(13m) width would absolutely be wanted, then a little ten
pixels will have to be “dropped“ (what generally goes unnoticed).
34
4. SCREEN WIDTH AND MATCHING BACKGROUND
OFFSETS.
With large screens, background screen offsets are proportional to screen width:
Screen width
(meters)
%
Ratio
26
19.5
13
9.95
6.5
¼%
1/3%
½%
2/3%
1%
1/400
1/300
1/200
1/150
1/100
Screen offset
backgrounds measured
on screen plane
(centimeters)
From 6.5 to 10
Fron 6.5 to 9
From 6.5 to 8
From 6.5 to 7
6.5
Pixel offset value to get
comfortable backgrounds
5 to 7 pixels 2K or HD
20 pixels 2K or HD
With smaller screens, background screen offsets are not anymore proportional to screen width:
Screen width
(meters)
5
4
3
2
1.5
1
0.65
0.50
0.30
0.26
0.13
Mobile phone
%
Ratio
1.10
1.30
1.60
2.00
2.27
2.50
2.27
2.10
2.00
2.00
2.00
2.8 to
3.3
1/90
1/73
1/63
1/50
1/44
1/40
1/44
1/48
1/50
1/50
1/50
1/30
Screen offset
backgrounds measured
on screen plane
(centimeters)
5.5 to 6
5 to 5.5
4.8 to 5.3
4 to 4.5
3.4 to 4
2.5 to 3
1.5 to 2
1.0 to 1.3
6 mm
5 mm
3 mm
Depending of inspecting
distance
Pixel offset value to get
comfortable backgrounds
22 pixels
27 pixels
32 pixels
40 pixels
45 pixels
50 pixels
45 pixels
42 pixels
40 pixels
40 pixels
50 pixels
2K or
2K or
2K or
2K or
2K or
2K or
2K or
2K or
2K or
2K or
2K or
HD
HD
HD
HD
HD
HD
HD
HD
HD
HD
HD
© Alain Derobe Feb. 2009
35
5. Two important bounce effects are of major importance to
understand what is actually different on large screens.
Effect 1: As soon as the lights go out in the theatre, the viewer loses easily the exact sense of
distance to the screen and recreates, when he sees a gigantic picture, a better virtual resized
screen, with a more probably size, such as the 21' one (6.5m)
Effect 2: With a giant screen, the choice of interaxial distance during shooting will not establish
any more the window position, but will better define the more or less "expanded" or
“compressed“ progression of the subject's depth scale.
When the shooting have been adjusted to a 21’ (6,5 m) one, the depth scale will be seen two
times deeper on a 42' (13m) screen size and four times deeper on a 85' (26m) screen. This depth
expansion is perfectly admitted by the audience wishing a rather exaggerated depth/width ratio
on large screens.
Since the illusion of depth varies with the seat position in the theater, it means the
orthostereoscopic position would be then in the first rows, close to the screen.
When screen size increases, lack of respect of stereo window rule goes more and more out of
sight and is more and more unnoticed by the audience.
Those two effects explain why breaches to stereo window rules are well accepted for large
screens and also why it makes sense to work as if the screen width was a littler one.
Concerning public city theaters, where average screen sizes are between 33' (10m) and 66'
(20m), it is convenient to calibrate the shooting according to stereo window requirements,
corresponding for instance to a minimum 21' (6.50m) screen or a maximum 33’ (10m) one.
With larger ones, stereo window error is not much noticed by the audience.
On the contrary, if you would choose to work directly for a 42' (13m) screen, by using the grid at
half pitch (10 pixels), or less, remember that a too distant stereo window would make shooting
terribly strenuous. A so far stereo window will generate a weak depth in the incitement during
shooting, to wipe out close elements.
Nowadays, working systematically for a virtual 21' (6.5m) or a 32’ (10m) screen seems to be a
consensual way of getting good 3D.
36
37
6. HOW TO BLOW OUT THE SCREEN BUT NOT THE
VISION.
This is an extract of Alain Derobe’s book about 3D, Copyright apply
The inescapable adjustment of showing stereo 3D with screen size in theatre, sometimes caused
the odd incredulous reaction is quite understandable and calls for more explanation. Many
experienced stereographs – who often, have been soaked into the magic 3D mixture, many years
ago – have now grappled their way to understand several “paradox” of human vision.
These “specific visual rear path” now seem obvious to them but that is not the case for most
cinematographers and mostly to a large audience.
Common sense has a habit of believing what it sees and is frustrated when it comes to admitting
that the best 3D image on a screen will require a huge jump away natural seeing. Through a poor
knowledge of the complex mechanisms of vision, we sometimes don’t realise that some
essential, visual functions, have to be performed, even to the detriment of real size and
disposition.
The main priority is that the stereoscopic double image requires no major or lengthy ocular
divergence from the audience. The most popular feature films and the most restful ones for
delicate eyes – and especially the least traumatic for our vision health – do not require any
divergence of ocular axis.
This means that our visual perception of depth – really very well behaved – is able of finding,
without the slightest problem, a resemblance to stereoscopic pictures, that are far-removed from
what it shows. This surprising faculty leads to the non observance of any kind of proportionality, a
non-observance of proportionality in regard to reality, but also between different projections.
This is fortunate because proportionality respect is impossible to attain and this is why a
spectator who watches the stereo images of a given volume on a small screen, then the same
images on a giant screen, feels they are seeing similar depth and similar set of items.
However, it can be scientifically demonstrated that it is absolutely not the case, and that the
extent of the depth represented is not only completely different, but also considerably deformed.
More incredible, it has to be!
If – in spite of this – the audience feels a impression of realistic depth, it is because his perceptive
visual system included two, extremely powerful corrective functions: the ability to rebuilt space
depending on the “most probable depth possible” and the “consistency of the size of objects and
individuals”.
We therefore lose any hope of finding a straightforward relation of proportionality between two
different-sized projections, which can be illustrated with the following.
During the editing of a 3D film, a very close-up shot of a beautiful woman smiles at us from a 30
cm wide stereo monitor. The gracious oval of her face – nearly life size- - covers the full height of
the image.
Then, when it will be projected in an “Imax” theater with a 30 metre screen base, we shall be
delighted to see this friendly face again and not that of a giant whose 16 metre high head could
lead us to believe she is nearly 100 metres tall, and whose smile would terrify both King Kong and
Godzilla, clutching each other in fear. Obviously, it is our visual analysis system that performs the
correction to the scale and this banality seems obvious with a flat image.
38
It is far less obvious with stereo 3D, because depth information may hinders the re-dimensioning
of the perspective.
Fortunately, the perception of depth is completely separate from the analysis of width.
On a large screen, the perceptive function of the “consistent size” encourages us to mentally recalibrate the subject to its normal dimensions and at the same time, to see the whole image as if
it was a lot closer to our own face, with the help of darkness. The notion of actually placing the
screen further away is completely irrelevant.
Beside this, on a small monitor, the correct adjustment of a 3D image with the least possible
depth error, would show – for example – the eyes level on the very screen’s surface while ears
and hair would be showed inside monitor’s housing.
Similarly, the chin and nose would stand out towards the spectator to complete the shared
volume, which is nevertheless a very flat depth in relation to reality.
By the by, this kind of adjustment, which adds “presence” to such a close-up shot, is better
appreciated than if the whole face would have been behind the screen plane, which is visually
uncomfortable or if it would have stood completely off in front the screen, which would have
caused some eyestrain.
For example, we can notice an offset of 2 % between left and right pictures of a same item (6 mm
measured on the screen) for a lock of hair that sticks away from the back of the head; whereas
for the tip of the nose standing out forwards, the difference between the corresponding images
will be crossed over about 1 % (3 mm).
In fact, the rear 2/3 of the depth of the head will really seem to be beyond the screen ( regular
watching distance would give about an additional 5 % of eye screen distance) whereas the other,
prominent third will be showed, standing out by 2.5 % on the way from screen to eye.
This is a really flat 3D depth because seen from a distance of 30 cm, it will only occupy a
thickness of 2.5 cm for a face which is roughly 15 cm wide! And yet if the subject turns her head,
this completely squeezed “bas relief” will look at first sight, realistic enough if not completely
identical to live.
Pierre Allio ironically calls this kind of depth: a “Pizza box 3D”.
Obviously, the depth bears absolutely no relation to the width of the face, but the visual function
that “rebuild a familiar area” flies to the rescue of a genuine cervical volume to prevent it from
looking like a cushion that has been flattened by a Sumo wrestler!
And so the good looking face appears to be delightfully represented rather than deformed, which
is very fortunate because it would be fundamentally impossible to show the true volume of a
correctly proportioned skull inside a monitor of this size.
Now let’s project the same pictures in the same stereo adjustment in an “Imax” theatre,
respecting their inherent off sets. In view of the new dimensions of the 30 metre screen, the lock
of hair in the background would now have left and right local pictures, 60 cm apart and therefore
impossible to make visual fusion for anyone in the theatre. These unacceptable images would be
seen double and would stays incomprehensible.
In fact, our eyes cannot diverge and it is very painful to try and visually superimpose a couple of
too far-removed separated images. The eyes of a spectator sitting 30 metres from the screen
would have to diverge by about 2 degrees whereas a child in the front row, 15 metres from the
screen, would have probably to diverge by more than 3 degrees:
And yet only a few people are able of visual divergence up to 1 degree while other ones didn’t!
Defects such as this one have earned such a bad reputation for 3D in the past. Then an Imax
technician would probably come to readjust the global off set of the two images and “recalibrate” them. This readjustment involve now that the whole face will stays completely in the
theatre area, in front of the screen and no longer behind it. To do this, the technician has to do a
39
general offset of left and right pictures (in the laboratory) until the lock of hair at the back is
perfectly superimposed to that it appears in screen plane, which itself is far enough away for it to
be pointless to go any further. This adjustment will bring the depth position of the nose (with a
difference of (2 % + 1 %) = 3 % of the width of the image, i.e. nearly one metre measured directly
on the screen ( with about 60 pixels offset).
This huge difference is perfectly acceptable to audience’s vision because a very large ocular
convergence is easy. The nose will now appear at 90 % of the screen to spectator distance, i.e.
for a spectator sitting 30 m away, something like 27 meters in front of the screen that looks like 3
meters from the eye observing it.
The charming face will be represented now much deeper than wide, made oval like a rugby ball
and yet we shall still perceive it as a beautifully proportioned head.
This little trickery – granted we are using an extreme example – is designed to help you
understand the phenomena and the rules that apply to watching stereoscopic images.
The actual living depth and the picture depth represented on the screen are two, very
different things and it is pointless trying to apply the same size on the screen as in the subject.
The depth represented is known as the “scenic box”. It has its own rules for screening
in a darkened room, especially with regard to the width of the image shown.
Moreover, the scenic box has very different measurements depending on the distance
and the angle between each spectator and the screen and yet each spectator mentally corrects
the image to achieve a credible depth and thanks to this miracle, they all have the illusion of
seeing the same kind of depth.
Trying to maintain a particular measurement in the scenic box is a utopist concept.
Separating the foreground area (coming out the screen) and the background area by
means of accurately positioning the “stereo-window“ is only valid for a specific screen width.
By means, this convergence level (screen level) becomes a non-sense when the whole
volume has to appear in the theatre itself.
In fact 6.5 cm on a 30 metre screen is so insignificant that sometimes we no longer
take it into account. Moreover, far-off distances behind the screen no longer make any difference
visually.
Also, with the increase in the size of the screen, the spectator can no longer evaluate
the distance and systematically feel it as it was closer. The extent and the exact position of the
convergence level therefore reduce regularly in relation to the size of the screen.
On the other hand, small screens are perfectly situated by spectator’s eyes,
consequently they tolerate a large section of the image being behind their surface. This entails the
stereo window being further forward within the volume of the scenic box, just as the convergence
would have if it was closer to the camera when the shot was recorded. Therefore, simply moving
a whole image sideways in relation to the other one is sufficient to re-calibrate it and this
automatically positions the window (the screen level).
The re-calibration of images depending on the width of the screen fortunately
corresponds to the logical distance between the spectator and the screen. It is perfectly justified,
for small screens, to have a greater proportion of the volume presented further back and with
large screens, the scenic volume is presented much closer to the spectator so that the audience
feels more involved.
It is unacceptable to leave elements out of place in a direct line (in the distance), with a
difference of more than the space between the eyes. If this is not readjusted, the same difference
combined with the master results in varying differences on the screen, depending on the screen’s
size.
Re-calibrating objects in the distance by means of slightly moving the images will make
very little change to the ideal position of the stereo window and will not have a major influence on
any coming out of the screen.
40
The difference is generally small, 7 pixels (in 2K) when moving from a 10 to a 20 metre
screen and this protects the audience’s eyesight with little trouble.
Seven pixels opposite way, will allow one to make the adjustment for a 5 metre screen
and to add the necessary depth to scenic box otherwise too far forward when no re-calibrated.
We can see that with satisfactory re-calibrations, of a few pixels, we can consider that the
position of coming outs the screen – and stereo-window position – are not much altered.
However, diplopia and damage caused by uncontrolled divergence may have consequences on
the whole show, or even cause optical medical damage.
Alain Derobe, Stereograph.
© Alain Derobe 2008
41
D. GLOSSARY
The terms used for shooting are underlined. The terms concerning projection are in italics. It is important to
identify them fully. Many misunderstandings and wrong meanings ensue with approximate terms.
For instance, il is very important to keep the "screen offset " term for the space between corresponding left and
right points on the screen and to use "axial distance" to name the space between the two optical axes.
Interaxial will also be used.
The average distance between human eyes (2.5" = 6.5 cm) is called interocular distance (IOD) or also
interpupillary distance (IPD).
When terms are common to making pictures and to their watching, like the too frequently used “parallax“
notion, we recommend you say "scenic" for the first and " pictured“ or “screened“ for the second.
For instance, we can say that an actual scene scale during shooting can, depending on camera adjustments,
give a compressed or expanded screen scale for projection.
For projection:
Scenic Box:
A space represented by a volume, as it is presented to the audience's view and often much different from real
space from which it comes.
“Stereo real state“ means a comfort zone or the “ Maximum depth bracket” or “3D screen
Space“. It is the maximum amount of deep that can be showed through the screen. It can
be larger than the “scenic box”. Then, this scenic box can be a little moved more in front
or back the screen plane by a general offset in post production. (depth repositioning ).
The other terms can be ambiguous and may involve available screen space as well as
the portion of natural space recorded.
Orthoscopy:
Defines the viewer's position where the screen is seen at the same field angulation than the lens used for
shooting. Orthoscopy is used with flat images and does not garantee a scale corresponding to real depth in 3D.
Ortho-stereoscopy:
Defines the position of the one privileged viewer who sees the picture through the screen frame exactly as
through a room window in natural surroundings. For this only viewer, it is the combination of an orthoscopic
position and of a natural depth scale.
Depth compression:
When the scale is flattened. A cube seems flattened through loss of thickness. Viewers diversely perceive this
distortion.
Depth expansion:
When the scale is expanded. A sphere is distorted like an olive or a zeppelin facing to you. Variously perceived
by viewers.
Offset:
Space on the screen separating left and right corresponding (or homologous) points of the same subject
element. It is expressed in percentage of the picture width, hence of the screen base. When the image
resolution is known, it can be expressed in pixels for better post-production adjustments.
Rear depth:
Picture elements perceived beyond the screen.
Far planes:
Picture elements represented in the theater the farther away to the rear. Even with a L/R offset equal or larger
than interocular distance (2.5" = 6.5 mm) or more, and even if the viewer can sense deep far planes, he will
never really experience infinity. To get close to this infinity, he should be himself at a tremendous distance from
the screen plane.
Stereo window:
42
Where right and left images have no offset. It is also the borderline between rear depth and elements coming
out of the screen.
Screen plane:
Normally, the stereo window is perceived at the screen plane (as for a flat image when left and right views are
superimposed), except if , by a visible offset screen plane and stereowindow have been a little disconnected, or
if a “floating window“ has been artificially planned.
Floating window:
By moving the frame side edges, in and giving them an offset just as any picture element, vision can be tricked
and led to believe the screen plane is itself in stereo, a bit closer or farther than it physically is. This is practiced
in post-production with a small amplitude, mostly to dampen the shot to shot jumps.
Coming out of the screen (sometimes called theater effects or front depth):
Elements that seem to appear in front of the screen. They cannot be seen outside a pyramid joining the eyes of
the viewer and the four corners of the screen. The amplitude of the elements coming out of the screen, vary a
lot with the nature of the image, and criteria are mostly psychological.
Broken coming out of the screen effect or stereo window errors:
When an element supposed to be perceived in the theater is cut off by the edge of the frame.
- With medium-sized or small screens, the viewer's visual perception can then be to push back the entire image
as well as a large part of the Scenic Box and lead to ocular fatigue. Abuse of window errors allowed by some is
rejected by an important fraction of the public.
- The larger the screen, more the viewer loses the notion of distance to the screen. He has a tendancy to return
the picture dimensions back to their probable size and a virtual window is then created much closer to his eyes.
In fact, with giant screens, the great majority of the picture will be displayed with broken coming out of the
screen effects. With immersive setups like Omnimax TM, this can concern the entire image.
Roll:
Residual relative horizontal level defect of an image with respect to the other.
On most correction software this can be expressed in degrees or fraction of degrees.
Post-production correction might mean destroying quality.
Parallax:
Ambiguous! Used both in taking picture or when theatre space speaking. Unlikely it is one of the main roots of
misunderstanding about 3D trade.
Better to leave this term to scientific or calculation use. It can means the cause as well as the effect. Strictly, it
is generate by the distance between two separate point of view such as A and B in pointing a C target. When C
is the stereo window (or the screen plane) one says that positive parallax is behind the stereo window and
negative parallax is in the foreground.
43
Zone Industrielle
27135 Verneuil sur Avre
France
Tel +33 2 3232 2761
[email protected]
http://www.transvideointl.com
Rev 0.0 – Sept 3rd 2009
44

The Stereoscopic User Book

Transcription

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