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 function of PPM output is essentially the same as the output of pnmcolormap all. ppmhist is much older than pnmcolormap.
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).
With this option, ppmhist works on images that contain invalid sample values. Normally, like most Netpbm programs, ppmhist fails if it encounters a sample value greater than the maxval that the image declares. The presence of such a value means the image is invalid, so the pixels have no meaning. But with -forensic, ppmhist produces a histogram of the actual sample values without regard to maxval. It issues messages summarizing the invalid pixels if there are any.
One use for this is to diagnose the problem that caused the invalid Netpbm image to exist.
There is a small exception to the ability of ppmhist to process invalid pixels even with -forensic: it can never process a sample value greater than 65535. Note that in the rarely used Plain PPM format, it is possible for a number greater than that to appear where a sample value belongs.
This option was new in Netpbm 10.66 (March 2014). But Netpbm older than 10.66 does not properly reject invalid sample values, so the effect is very similar to -forensic.