Computer Graphics
Lesson 2
July 12, 2005
Image Formats
What are some formats you are familiar with?
There are 4 basic image format types:
• Uncompressed
• “Lossy” Compression
• Palletized Images
• Vector Images
Uncompressed Images
An image is considered to be uncompressed if each pixel is stored
individually with its full color value.
The Bitmap (BMP) file format is a good example of an
uncompressed file format.
BMP files may still be compressed using “lossless” compression
algorithms such as ZIP.
Uncompressed Images
Each Pixel is stored using 3 numbers representing the individual color
channels.
R = 243
G = 198
B = 114
R = 239
G = 190
B = 106
R = 243
G = 199
B = 115
…
R = 245
G = 197
B = 114
R = 245
G = 197
B = 112
R = 241
G = 192
B = 108
…
R = 244
G = 195
B = 109
R = 243
G = 190
B = 102
R = 239
G = 184
B = 99
…
…
…
…
…
Uncompressed Images
Calculating Size:
An uncompressed image requires 3 bytes (numbers) per pixel; one
for each color channel.
So… a 256x256 image would require 256*256*3 = 196608 bytes!
OMG!
… or 192 kilobytes… meh.
Compressed Images
The Joint Photographic Experts Group (JPEG) file format utilizes
image compression by approximation to reduce file size.
As you can see, there is some quality loss although the effect is
magnified here.
Without Compression
With JPEG Compression
Compressed Images
Compressed Images take advantage of visual approximation.
Most images do not need to be displayed exactly as they were
created. The resulting image need only resemble the original.
The compression comes at the expense of some image quality.
So, as is often the case in CS we have a memory  quality tradeoff.
Color Pallets
Another way of making images smaller is by restricting the number
of colors that can be displayed in one image.
Previously we discussed 1 byte (8bits) per color channel. This is
known as 24-bit color or True Color.
28 * 28 * 28 = 16,777,216 colors
While this provides good color depth, in many images less than a
handful of those colors are actually used.
Color Pallets
To the right is a 256 color table
containing every color used in this
Mona Lisa image.
Each color is 24 bits, but every
pixel contains only 8 bits which
index a color in the table.
Color Pallets
Vector Images
In a vector image, the computer stores lines,
shapes, and colors instead of pixels.
When it is time to show the image a
traditional “raster” image is generated for
display on a monitor.
The most popular file format for vectors is
called Scalable Vector Graphics (SVG).
Vector Images
Vector images look more like drawings, but they have
the advantage that that can be rastered at any level of
detail, meaning they are infinitely scalable.
Image Format Comparison
BMP
JPEG
GIF
SVG
File Size
Largest
Variable
Small
Small
Image Quality
Perfect at a given
resolution.
Variable
Some color depth
loss.
Perfect
Scalability
Limited
Limited and results in
more loss.
Limited
Perfect
Photorealism
Best
Very good
Poor
Bad