There are many different file formats for still graphics, but they fall
into roughly two camps.
There are vector based graphics. A good example of this would be an Autocad
file, which describe it's graphical elements in terms of end points and
lines.
The other type of graphic is the bitmap file. A bitmap can be thought of
as a grid, where individual cells are turned on and off to make an image.
The term "depth" is used to describe the amount of color information
that can be stored in a given bitmap format.
An 1bit bitmap is monochromatic. An 8bit bitmap holds 256 colors, and a
24 bit bitmap can contain up to 16.8 million colors.
The biggest problem with bitmapped images is that they can be large. Bitmapped
files are hard-coded for a particular resolution, where as Vector based
images are not. This means that high resoulution bitmapped images with many
colors can be very difficult to store and work with. By using schemes to
reduce the amount of information that it takes to represent an image, bitmapped
images can be made more manageable.
Lossless compression uses compresson schemes such as LZH to look for redundancy
in the image, and represent that redundancy with less information. Lossy
compression schemes throw away part of the image data to get a smaller size.
In many cases, the removed data is very hard to detect, and the savings
in file sizes can be prodigous. The drawback with lossy schemes is that
their effect is additive -- successive interations of saving the image will
begin to visably reduce image quality. For this reason, they are best used
as a format for final output.
BMP -- Windows Bitmap file,
as created by Windows paint and other software. While these are typically
8bit files, there is support for up to 24bit deep pallettes.
ILBM -- Interleaved Bit Map. Commonly used on the Amiga. There are
many subformats under this heading, notably HAM (12 bits per pixel, max
4096 colors), IFF24 (24 bit extension of the IFF graphic standard) and HAM8
(16 bits ,max 256,000 colors).
GIF -- Graphics Interchange Format. Very common file format, supported
on many platforms. Maximum color depth is 8 bit, with a maximum display
of 256 colors. There are two variants; GIF '87, and GIF '89, which added
support for color cycling and transparancy, among other things.
JPEG -- Named after the Joint Photographic Engineering Group. The
format that actually is used is called "jfif", and it supports
a maximum color pallette of 24bit, and variable "lossy" compression.
HPCD -- Also known as "PhotoCD". This format has 4 differently
scaled images inside -- for different display devices. Maximum color depth
is 24bit, but this is subject to change.
PCX -- PC Paintbrush file. Commonly seen on the IBM PC and compatable
platforms. Maximum color depth is 8bit, maximum colors are 256.
PS -- Postscript file. Can have an imbedded image, including support
for color images, or can be a pure vector format file.
PBM -- Pokanzer Bitmap, or Portable BitMap. A 24 bit uncompressed
RGB file produced by the NetPBM image processing suite. Normally used as
an intermediate format for conversion from one format to another.
TIFF -- The Interchange File Format. Used on various platforms for
high resolution, non-lossy image storage. There are extensions for file
compression that may be platform or application specific.
PICT -- Format that originated with MacDraw as a vector graphic format;
current implementations have support for a separate raster and vector layers,
in color depths up to 24 bit, as well as different compression formats,
such as jpeg.
XBM -- X11 bitmap. A non compressed image format that commonly is
only 1bit deep, for black and white images.
RAS -- Sun Raster file format. An uncompressed file with a 24bit
deep pallette.
TGA -- Targa graphic file format. Most often seen on the PC. Maximum
color depth 24bit, but there are 15bit versions available as well.