INP
INP specifies the input file name(s). Normally, INP would be three
BYTE images, representing (red, green, blue) respectively. It could
also be a single 3-band file.
OUT
OUT specifies the names of the two output files. The first output file
is the name of an image file which will be a BYTE format VICAR image.
The DN of each pixel represents a color given by the pseudo-color table
in the second output file.
The second output file is the name of a lookup table file which contains
the 256 colors to be used to display the image. It is an IBIS "Tabular"
file, containing the three (red, green, blue) lookup tables. This file
is compatible with VIDS (using JLOAD-PSEUDO or JLOAD-STRETCH) and
Pixel Pusher (on the Macintosh; use "Open New Transfer" to view it).
NBITS
NBITS is the magic parameter. It is used as follows.
The color of each pixel in the input is determined by taking the NBITS
most significant bits of the red, green, and blue pixel values, and
shifting them together to fit into one integer. A histogram is collected
giving the frequency of each color in the input, and the 254 most common
colors (plus black and white) are selected. Each of the remaining colors
is then mapped to its nearest match (euclidean distance) in the 256 chosen.
NBITS should be 3, 4, or 5. Choosing 3 bits will force the 256 colors
to be further apart from one another, and often gives the best results
on a picture with a lot of disparate colors. Five bits will tend to allow
the chosen colors to be more similar, and is thus the best choice when
an image contains gradual shading. For a quick look, 4 bits almost always
gives reasonable results (and is thus the default).
I have found, however, that if you want the best, it pays to run all three
and compare the results.