This program is part of Netpbm.
ppmhistreads a PPM image as input and generates a histogram of the colors in the image, i.e. a list of all the colors and how many pixels of each color are in the image.
The output is in one of two basic formats: a report for humans and a PPM image for use by programs. The PPM image is actually quite readable by humans too.
You get this format by specifying (or defaulting to) the -nomap option.
The format is one line for each color in the input image.
By default, there are two lines of column header at the top. Use the -noheader option to suppress those lines.
In each line, ppmhist identifies the color by red, green, and blue components. By default, it lists each of these in decimal, using the exact values that are in the PPM input. So if the image has a maxval of 255, the numbers in the listing range from 0 to 255. With the -hexcolor option, you can change these numbers to hexadecimal. With the -float option, the numbers are fractional, adjusted to a maxval of 1.
Each line lists the luminosity of the color. It is in decimal on the same scale as the rgb values (see above).
Each line lists the number of pixels in the image that have the color. This is in decimal.
You get this format with the -map option.
The output file is a genuine PPM image, but it is PPM Plain format and contains comments so that it is not a lot different from the human report described above.
As a PPM image, it can be useful as input to other programs that need some kind of palette. The image is a single row with one column for each distinct color in the image.
The default is frequency.
You may not specify this option along with -float or map.
You may not specify this option along with -hexcolor or map.
This option was added in Netpbm 10.19 (November 2003).
You may not specify this option along with -float or hexcolor.
This option was added in Netpbm 10.10 (October 2002).