pnmhistmap Man page

pnmhistmap General Commands Manual pnmhistmap


pnmhistmap – draw a histogram for a PGM or PPM file


pnmhistmap [-black] [-white] [-max N] [-verbose] [pnmfile]


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 pro‐
duces 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.


-black Ignores the count of black pixels when scaling the histogram.

-white Ignores the count of white pixels when scaling the histogram.

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.

-max N Force the scaling of the histogram to use N as the largest-count
value. This is useful for inputs with a large percentage of
single-color pixels which are not black or white.

Report the progress of making the histogram, including the
largest-count value used to scale the output.

All flags can be abbreviated to their shortest unique prefix.


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 ele‐
gant solution.

Should allow the output size to be specified.


pgmhist, ppmhist, pgm(5), ppm(5)


Wilson H. Bent. Jr. (

25 October 1993 pnmhistmap

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