Image Compression and Storage Image Manipulation

Transcription

Overview of Computer Science
CSC 101 — Summer 2011
Digital Images:
C
Compression
i and
dM
Manipulation
i l ti
Lecture 15 — July 26, 2011
Announcements
•
•
•
•
•
Lab 5 today.
Midterm grades *should*
should be up soon
Writing Assignment #5 due Today
Writing Assignment #6 due Thursday
Quiz #3 on Friday
2
Objectives
• Digital images
– Storage
– Compression
– Manipulation
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1
Storage of Digital Images
• Digital images are numbers, stored in files
• Each image file is of some file type
– The file type
yp dictates how those numbers are stored in
the file
– Choice of file type typically dictated by intended use
• Most image file types compress the image data
– Compression allows a series of numbers to be stored
in a more compact way
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Storage of Digital Images
• If there is no compression
– The numbers for each pixel are stored in the file
– This kind of file is called a bitmap
• Filename will have a .bmp extension
– Bitmap images are no longer common because they
can take up so much storage space
• Good compression techniques have been developed
• A compressed image may appear similar or identical to the
bitmap, but be only a fraction of the size
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Image Compression
• Compression is the act of making a data file smaller
–
–
–
–
Takes up less space on disk
Transfers more quickly over a network
A compressed file must be uncompressed to be used
There are two categories of image compression commonly used ––
lossless and lossy compression
• Lossless image compression
– After compression and decompressed, the original image is reproduced
exactly
– No information (image detail) is lost
• Lossy image compression
– Exact reproduction of the original is not guaranteed
– Information (detail) that is lost cannot be recovered
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2
Digital Image File Formats
• JPEG (Joint Photographic Experts Group)
– .jpg, .jpeg, or.jpe filename extensions
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–
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Lossy compression with variable controls
Ideal for photographs
Not so good for solid-color graphics
Frequently used in Web pages
• GIF (Graphic Interchange Format)
– .gif filename extension
–
–
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–
–
Indexed color image (up to 256 colors)
Lossless compression
Ideal for simple graphics with solid colors
Not so good for photographs
Also frequently used in Web pages
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Digital Image File Formats
• PNG (“PNG’s not GIF”) (www.libpng.org)
– .png filename extension
– Uses both lossy and lossless compression
• Can be used both for graphics and photos, though JPEG usually best for photos
– Lossless
l compression
i similar
i il to GIF:
• Indexed color (but not limited to 256 colors)
• Run-length encoding (RLE)
– The GIF format was subject to royalties due to a controversial patent
(now expired) on a compression method
• The PNG format was created to avoid that patent
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Storing Digital Images
Graphic as BMP (307 KB)
Graphic as GIF (20 KB)
Graphic as PNG (17 KB)
Graphic as JPEG (12 KB)
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3
Storing Digital Images
Photo as JPEG
(99 KB)
Photo as PNG
(17 KB)
Photo as GIF
(20 KB)
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Image Compression
• Lossless techniques
– Used in GIF and PNG images for solid-color graphics
• Indexed color
• Run-length
R l th encoding
di
• Lossy techniques
– Used in JPEG and PNG images for photographs
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Indexed Color
• Indexed color is a lossless compression technique that avoids
storing unnecessary information (used by the GIF format)
• Only those colors actually
used in the image are stored
• The set of up to 256 chosen
colors is called a palette
– If more than 256 colors are
needed, then this may not be
an appropriate method
• Indexed color image files
are very compact
– Uses 1 byte/pixel (or fewer)
• instead of 3 (or more)
– GIF files are further compressed
using run-length encoding
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4
Indexed Color
• An indexed color image has up to 256 different colors
– Can choose any colors of the rainbow for those 256
– Different images will have different color palettes
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Lossless Compression
• Run-length encoding (RLE) is another method of
lossless compression
– Solid-color graphics often have many consecutive
pixels that are the same color
– Instead of storing each individual (identical) pixel in
a consecutive series, a pixel can be stored just once,
with a repetition count
– Consider Spongebob
• Let’s just look at his eyebrow
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Lossless Compression
60
20
• In this part of the image, the top row of 60 pixels is all one color
– Instead of storing 60 yellow pixels, RLE allows us to store just one
yellow pixel, and the instruction to repeat that pixel 60 times
– For solid color graphics, this can result in huge file-size reduction
• The full Spongebob image has 500×500 resolution (250,000 pixels), but the
GIF file is only 21,900 bytes because of run-length encoding
• 97% reduction in file size in this image
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5
Lossy Compression
• Indexed color and run-length encoding work well in solid-color
images, but what about images with complex coloring and
shading, such as photographs?
– Indexed color and RLE are not good choices, because they work best with
a small number of solid colors
– Other methods can be used—but to get a reasonable file size reduction,
lossy compression techniques are required
• The JPEG file format exploits the nature of human vision and
perception
– For example, we’re more likely to notice subtle variations in brightness
than subtle variations in hue
– Compression that reduces the detail in the variations of brightness may be
undetectable by eye
– JPEG makes use of a number of techniques that maximize compression
while minimizing noticeable reduction in quality
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Lossy Compression
• The JPEG file format uses
lossy compression
– User has a choice of degree of
compression used,
used and therefore
amount of information lost
• An example of information
loss on compression:
• Here is a JPEG image at basic
compression (‘Quality’ = 100)
– 111 KB file size
• (the original, uncompressed photo
bitmap is 496×770, or 1.15 MB)
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Lossy Compression
• The same image at
‘Quality’ = 85
– Still 496×770
– 61 KB file size
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Lossy Compression
‘Quality’ = 50
– Still 496×770
– 15 KB file size
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Lossy Compression
‘Quality’ = 25
– Still 496×770
– 8 KB file size
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Lossy Compression
‘Quality’ = 1
–
–
–
–
Still 496×770
4KB file size
Original JPEG file was 111 KB
While not good for a large
blowup, perhaps a high level of
compression provides sufficient
quality for a small version of the
image?
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7
Image Editing
• Digital images – like any digital information –
can easily be changed
– It’s just a bunch of numbers
• Reasons why someone may want to change an image:
–
–
–
–
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Enhance aesthetics (artistic reasons)
Correct ‘errors’ (judgment call about what constitutes an ‘error’)
Clarify information (e.g. “false color” technical images)
Influence opinion (editorializing; marketing)
Alter history
• Images form an important part of the historical record
– Alter current ‘reality’
• Our universe is only what we experience directly –
and much of that has been presented to us by someone else
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Image Manipulation
• Because images may be manipulated so easily,
we must critically evaluate the images that we see
• The ethics of image modification…
– When an image is changed, is it a deception,
or a clarification?
– What is the intent? Is it purposeful?
– What are the potential consequences?
– Context and expectation
• Almost anything is OK for marketing or entertainment
• “News” reporting must meet a higher standard
• PLEASE! Check out the website.
– http://www.cs.dartmouth.edu/~farid/Hany_Farid/PhotoTampering0
.html
• Many of the examples from today can be found here.
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False Coloring
• Commonly used with
technical images to make
information clearer
Gustav
Hearing
Hannah
Ike
Vision
Hurricanes Gustav, Hannah, and infant Ike (9/1/2008)
Richmond, VA
Brain scan
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Correcting ‘Errors’
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Image Manipulation
• Is seeing believing?
– A couple of fish stories… Are either of these true photos?
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Image Manipulation
www.snopes.com/photos/shark.asp
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Image Manipulation
As for the giant catfish:
www.snopes.com/photos/noodling.asp
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Another Shark Tale
www.snopes.com/photos/animals/scubashark.asp
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Another Shark Tale
www.snopes.com/photos/animals/scubashark.asp
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Some Image Manipulation Examples
• Almost any marketing image
• Even ‘respectable’ magazines
like National
Geographic
• Many
images in
supermarket
tabloids
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• Two magazines
had different
ideas of how
Cameron Diaz
should look
http:/www.thefab.net/topics/culture_general/cg02_doublediaz.htm
http://web.archive.org/web/20040619234038/http://www.thefab.net/topics/culture_general/cg02_doublediaz.htm
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• How about a
news anchor?
• Katie Couric
loses weight…
weight
www.popphoto.com/photographynewswire/2916/katie-couric-photo-airbrushed.html
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What is “Beauty”
An interesting Web site concerning the erosion
of self-image among young women based on
distorted ideals of human beauty as presented
by the media:
– campaignforrealbeauty.com
– The latter half of this 45-second video focuses
on digital image manipulation techniques
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• Brad Pitt and Angelina Jolie
– Rumored to have a ‘relationship’ (April 2005)
• It was just a rumor at the time…
• …since he was still married to Jennifer Aniston
– The p
picture of Pitt was taken on a Caribbean
island, in January 2005
– The picture of Jolie was taken in
Virginia some time in 2004
• On page 8 is a disclaimer noting the image
is a “composite of two photographs”
– This composite was purchased from a
London photography agency, for $500,000
• So, were they really “Caught Together!”
on vacation?
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• The previous examples
were modifications of
photographs
• Here’s a completely
artificial (computer
generated) image by
Mihai Anghelescu
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• Modifying images to be ‘politically correct’
– The Beatles’ Abbey Road album cover shows Paul holding a
cigarette
– Some US poster
companies have
air-brushed the
cigarette out
of the image
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www.thememoryhole.org/arts/abbey-road.htm
• Image editing for
marketing purposes is
usually not a problem…
• But, images can also be
manipulated for less
benign purposes…
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• Falsification of history in Stalin’s Soviet Union
– Before the age of digital images, but image manipulation was
still a major part of soviet politics
– The Commissar Vanishes by D
D. King (1999) has many examples
(http://www.newseum.org/berlinwall/commissar_vanishes/)
Nikolai Yezhov, commissar of
water transport, is shown with Stalin
in front of the Moscow-Volga canal.
By 1938 he was out of favor with Stalin.
In 1940 he was executed and
removed from the historical record.
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• Falsification of history in Stalin’s Soviet Union
– Before the age of digital images, but image manipulation was
still a major part of soviet politics
– The Commissar Vanishes by D
D. King (1999) has many examples
(http://www.newseum.org/berlinwall/commissar_vanishes/)
Leon Trotsky is shown here with Lenin
and others.
He came to disagree with the
Soviet Central Committee.
He was exiled in 1928.
Murdered in 1940.
And removed from the historical record.
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• 1960’s Soviet Union
– Refused to admit failures in the space race
– Eliminated all traces of cosmonauts who perished
• http://www.lostcosmonauts.com/default.htm
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• Heroic portrait of Lincoln widely posted in schools and
government buildings after his assassination
– Lincoln’s head superimposed on the body of SC senator John Calhoun
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www.museumofhoaxes.com/photos/02calhoun.html; memory.loc.gov/ammem/today/mar18.html
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• Iranian missile launch
demonstration (7/9/2008)
http://thelede.blogs.nytimes.com/2008/07/10/in-an-iranian-image-a-missile-too-many/
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Image Manipulation Examples
• Does the
cropping of
this photo
change its
meaning?
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More BP Controversy
Meta info for picture says it was created (taken) in 2001, not July 16 2010
But can you spot other mistakes?
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More BP Controversy
Meta info for picture says it was created (taken) in 2001, not July 16 2010
But can you spot other mistakes?
46
Image Manipulation Examples
• Egyptian president Hosni Mubarak moved to the
front of the procession in Egyptian newspapers
–
http://www.guardian.co.uk/world/2010/sep/16/mubarak-doctored-red-carpet-picture
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• The Ballplayer Vanishes
– After Ricky Clemons was
jjailed and expelled
p
from the
University of Missouri's
basketball team, his image
was removed from their
2003-04 basketball media
guide
http://www.columbiatribune.com/2003/Dec/20031204Feat001.asp
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16
• After spending time in
prison, Newsweek wanted
to show Martha Stewart as
Ready for Prime Time
Time”
“Ready
• But, she was not available
for a photo shoot
– Newsweek created the
photo by placing an older
head shot on a model’s body
www.nppa.org/news_and_events/news/2005/03/newsweek.html
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• In 1989, TV Guide wanted to attract
attention to an article about Oprah
– Placed Oprah’s head on the body from
a 1979 promo photo of
Ann-Margret
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www.museumofhoaxes.com/photos/oprah.html
Some (in)famous wartime photographs
– Gettysburg: “A Harvest of Death”
by Timothy O’Sullivan (1863)
• Rearranged corpses on the
battlefield for emotional impact
memory.loc.gov/ammem/cwphtml/cwpcam/cwcam2.html
– Spanish civil war photo
“Death of a Spanish Loyalist” (1936)
by Robert Capa
• Some have suggested
gg
it was staged
g
www.photographers.it/articoli/cd_capa/img/falling%20soldier.pdf
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• LA Times photographer Brian Walski
makes an image more dramatic (2003)
– He was fired two days later
More info:
www.sree.net/teaching/lateditors.html;
www.washingtonpost.com/wp-srv/photo/essays/vanRiper/030409.htm
Walski responds: www.pdnonline.com/pdn/search/article_display.jsp?vnu_content_id=1000456607
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• Modification of images in the news media
– National Guard fires on student protesters at
Kent State University in May, 1970, killing 4 students
• Original
O i i l Pulitzer-prize
P lit
i winning
i i news photograph
h t
h
• vs. version used later in Life and other publications
en.wikipedia.org/wiki/Kent_State_shootings; journalism_jobs.tripod.com/a.filo.html
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• June 1994: O. J. Simpson arrested for murder
– Time and Newsweek both used the police mug shot on their covers
– Appeared side-by-side
on newsstands
– Newsweek took it
straight; but Time
altered their version
– Time later defended
this by claiming it
wasn’t a “news photo”
but rather a
“photo illustration”
54
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• But,
Newsweek
can be
guilty too
guilty,
• The
McCaughey
septuplets
(1997)
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• Politics: John Kerry and Jane Fonda
www.snopes.com/photos/politics/kerry2.asp
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• Politics: George W. Bush
www.snopes.com/photos/bushbook.asp
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19
Image Manipulation
• Because images may be manipulated so easily, we
must critically evaluate the images that we see
– When an image is changed, is it a deception, or a
clarification?
– Our universe is only what we experience directly –
and much of that has been presented to us by someone
else
58
Ethics of Image Modification
• Suggested ethical guidelines for image use by news outlets
–
–
–
www.digitalcustom.com/howto/mediaguidelines.asp
www.nppa.org/professional_development/business_practices/ethics.html
www.nppa.org/professional_development/business_practices/ethicsdefault.html
• Credibility - some questions to ask
– Is the photograph a Fair and Accurate Representation of the info being presented?
– In what Context is the photo being used?
– Does this photograph Deceive the reader?
• A few other resources
snopes.com —probably the best place to check email and Web scams, hoaxes, etc.
– “Digital Tempering in the Media, Politics and Law.” Hany Farid. Dartmouth
College. www.cs.dartmouth.edu/farid/research/digitaltampering
• Farid, Hany. “Digital Doctoring: How to Tell the Real From the Fake.”
– Leach, Susan L. “Seeing is No Longer Believing.” The Christian Science
Monitor, 2 Feb. 2005, p. 15.
–
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Image Compression and Storage Image Manipulation

Transcription

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