Section: User Commands (1)
Updated: 25 October 1993
Reads a portable anymap as input, although bitmap (PBM) input produces an error message and no image. Produces an image showing a histogram of the color (or gray) values in the input. A graymap (PGM) input produces a bitmap output. A pixmap (PPM) input produces pixmap output with three overlaid histograms: a red one for the red input, a green one for the green input, and a blue one for the blue input. The output is fixed in size: 256 pixels wide by 200 pixels high.
The -black and -white options, which can be used seperately or together, are useful for images with a large percentage of pixels whose value is zero or 255, which can cause the remaining histogram data to become unreadbaly small. Note that, for pixmap inputs, these options apply to all colors; if, for example, the input has a large number of bright-red areas, you will probably want to use the -white option.
Assumes maxval is always 255. Images with a smaller maxval will only use the lower-value side of the histogram. This can be overcome either by piping the input through "pnmdepth 255" or by cutting and scaling the lower-value side of the histogram. Neither is a particularly elegant solution.
Wilson H. Bent. Jr. (firstname.lastname@example.org).
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