Credit: Kirk Born (St Sci/NASA)
Graphic File Types
The most popular Web graphic file types
are GIF and JPEG, for static images, and animated GIF,
Java Applets, MPEG, and Quick-Time,
for animations. Another graphic format,
PNG has been widely
discussed but is just now becoming significant. It is supported in
many Microsoft products, such as Image Composer, MicroSoft WORD, and
Internet Explorer.
Graphics File Sizes
Uncompressed pixel format files are always NxMxB bytes in
size, regardless of the complexity of the image, where
N and M are the number of rows and columns, and B is the
number of bytes per pixel. For a 1024x768 image we get
about 800 Kbytes for 1 byte/pixel and ~2.4 Mbytes for 3 bytes/pixel
(16.7 million colors). At a 33K baud transfer rate (which is
what most 56K modems actually give) we have
~ 3K bytes/sec, or 270 seconds to load 256 color image
and 800 seconds, more than 13 minutes, to load a single full
color
image -- much too long! For animation, at 30
frames/sec,
it would take 5 hours to load one seconds worth of
animation, or more than 2 years to load/display
a one
hour movie!
Image Compression
This is an absolute must for Web graphics. Types
include:
-
LZW (Lempil-Ziv-Welch): it builds its codes to represent
patterns based on the patterns it actually finds in the data,
so the data strings are replaced by the short codes, and
so the file is compressed. Excellent for logos and
comics but not so good for photographs: can get
compressions of 10-30 to 1. GIF files use LZW. It is
especially effective when the compression scheme is
adaptive; that is, it looks at the frequency of colors in
the original image and then picks the best 256 colors
to match the original.
-
Run-length-encoding (RLE): replaces runs of identical bytes
with a byte-count and the color, e.g.,
RRRRRRRGGGGGGBRGGGGGG is replaced by
7R6G1B1R6G
-
JPEG (Joint Photographic Experts group): main compression is done
by applying a Cosine Transform to each 8x8 block of pixels. This
generates a color frequency map. The 1st element in the map is the
average color, and the other elements represent less important
details. By throwing away those details in the frequency map
which are barely detectable by the human eye, the data is
compressed. Compression ratios of 15:1 are common. JPEG
is especially well suited to 24 bit photorealistic images.
However, there will soon be a new JPEG format, named JPEG
2000, that uses some version of the wavelet format discussed
below, and which offers excellent compression at values of
100-to-1 or more. See
JPEG 2000 and
Homesite of the JEG committee for more information.
- Wavelet compression per se is discussed and demonstrated
in these links:
Image Understanding Lab and
a comparison of JPEG and Wavelet compression.
-
MPEG (Moving Picture Experts Group): represents a set of
frames:
FFFFFFFFFFFFFFFFFFF by runs of the form:
IBBPBBPBBPBBPBBPBBP
where I = the first "F" and the B's and P's represent just
the "deltas" or differences between the images (the P is the
difference between the current frame and previous, whereas the
B is the bi-directional differences between previous-current-next
frame). Since each P or B represents just the differences between
very similar frames, rather than all the pixels in each frame,
they are much smaller than the corresponding F frames.
The frames are encoded in a different sequence than when
it is displayed. Then the entire subset of encoded frames are
subject to JPEG encoding. Compression ratios of 30:1 are
common.
Color Schemes
These include RGB (Red-Green-Blue) for most displays,
CYMK (Cyan-Yellow-Magenta-blacK) for hardcopy, and LAB
for the truest possible colors. In terms of color fidelity,
CYMK < RGB < LAB (approximately), but the differences
are not large in terms of quality. We will stick to RGB.
Color tables:
Index Red Green Blue
0 10 177 67
1 15 202 72
2 17 225 101
... ... ... ...
255 255 23 199
Graphic File Conversion
Obviously there are many types of graphic files and not all of them can be
used on the Web, so what do you do if you encounter a graphic file that
is not
in a compatible format? You convert it to the format that you desire.
Most graphic systems, such as PhotoShop, XV, PaintShop Pro, and many
others allow you to convert graphic files to other formats just by reading
in a file in one format and saving it in another. For example, PaintShop
Pro
lets you read in and save files in .bmp, .fpx, .gif, .jpg, .mix,
.png, .psd, .tga, and .tif and many other formats.
|
|
 |
|
|
