Image representation is performed by dividing a source image into foreground,
background and selector planes. The foreground plane is selected to contain mainly
line type art or textual type information, the background plane mainly contains
image data, and the selector plane identifies whether the image data is maintained
in either a specific plane or a combination of planes. A color is selected, by
averaging or selecting an appropriate value based on overflow or other criteria,
to replace each color in the foreground plane. Error in portions of the foreground
plane resulting from replacing foreground colors is fed into corresponding portions
of the background plane. Each plane is then compressed using compression schemes
appropriate for the type of data maintained in each plane (LZW for the foreground,
and JPEG for the background and lossless fax LLITT, for example). Image reconstruction
is performed by decompressing each of the foreground and background planes, and
selecting pixels from each of the foreground plane and an additive image produced
by combining image data from both the foreground, background, and selector planes.
The selection is made based on the selector plane (selection mask), which identifies
where image data is maintained for the reconstructed image (i.e., the upper plane
or a combination of planes).
