Computer Graphics Formats
Phil Wells 3/27/99
- GIF:
- Graphics Interchange Format: Uses LZW
compression, maximum 8 bits, 64k x 64k pixels. The most widely
used graphics interchange & storage format in the world.
Latest version is GIF89a.
- JPEG
- Joint Photgraphic Experts Group: Algotithm looks for changes
the eye cant easily detect and removes them. Works more on chroma
than greyscale data sincee the eye is less sensitive to color
changes. Possibly to achive > 20:1 compression. This is actually
a LOSSY compression format. Typically JPEG files are stored
as JFIF's (JPEG File Interchange Format). JPEG is also used with
other formats like TIFF.
- TIFF
- Tagged Image File Format: 24 bit, bitmap with more pixels
than a GIF file can have. Uses RLE, LZW,
JPEG, and other compression schemes.
- PICT
- This Macintosh format is limited to 8 bits and 72 dpi. Mostly
vector data. Basicially, it's a QuickDraw (Mac's native printing
protocol) Data Metafile.
- PTNG
- Old Mac paint file. 1bit 75 dpi bitmap
- PNG
- 24 bit bitmap. Called a "PING" file, it's becoming
a new universal format to replace GIFs using unpatented LZ77
compression.
Compression Schemes
- RLE
- Run Length Encoding: A "run" is a
group of repeated characters, e.g."AAAAA". RLE encodes
the character name and quantity and puts this in place of the
original string. Not a good system for encoding certain kinds
of graphics, like 24 bit graphics or like "noisy" scans
since the noise is random so unlikely to have many "runs"
of repeated bytes. A LOSSLESS compression scheme.
- LZW
- Lempel Ziv Welch: The original LZ77 is public
domain. LZ78 uses a dictionary to keep track of text strings
so the entry's code can be substituted for the string. Somehow,
using default dictionary of the ASCII codes at the start of the
file, the actual dictionary can be reconstructed at decompression
time without being included in the file. Welch modified LZ for
use with high speed disk systems. LZW is patented by Unisys.
Used in GIF graphics format for which Compuserve has a license
agreement. The license is only an issue when using or writing
an application with LZW encoding or decoding. Possestion
or tranferring of an LZW encoded file doesn't require a license.
LOSSLESS