Still images

A still image (or just image) is a 2D array of pixels, which each has a color.

The color is usually specified in RGB (red, green, blue) colorspace as 3 numbers from 0 to 255, i.e. there are = 16.8 million possible colors. Two important image classes exist:

24 bit images:

Images which allow for any number of the 16.8 million (24 bit) colors to be present simultaneously in a given image

8 bit colormapped images:

Images which allow only for a maximum of 256 (8 bit) colors to be present in a given image. These colors are drawn from the 24 bit palette, and the mapping between the up to 256 numbers and the 24 bit colors constitutes a colormap.

Some of the image file formats found on the Web are:

GIF (Graphics Interchange Format):

GIF images are 8 bit colormapped, and are compressed using a loss free algorithm (LZW). The compression scheme is well suited for images with large area of the same color, such as the images found in this Web exhibition.

JPEG (Joint Photo Experts Group):

JPEG images are 24 bit color, and are compressed using a lossy algorithm. The compression scheme is well suited for continuous tone natural images, such as scanned photographs. I have chosen not to use this image format at all in this project.
Actually, JPEG is a encoding scheme, not a file format. The file format used on the Web is called JFIF (JPEG File Interchange Format). For more details on JPEG, see the JPEG FAQ.

SGI / IRIS / RGB:

SGI is a native Silicon Graphics image file format. Images can be of different types, including 24 bit color. They can be uncompressed or run-length encoded (RLE). Files have extension .rgb, hence one of the names. This format has been used a lot in this project as an intermediate image file format; this will be described in more detail below. On a Silicon Graphics computer, man 4 rgb provides more information about this image file format.

PNG (Portable Network Graphics):

PNG is still in development, but is mentioned here due to it's promising specifications: it is lossless, well-compressed, and supports up to 48 bit color, thus being a likely replacer of GIF.

All of the above are described in further detail in the Graphics File Formats FAQ part 3 (see also part 1, 2, and 4.) Compression per se, including LZW and JPEG (but not RLE), is described in further detail in the Compression FAQ.