pnmhisteq, pnmhistmap, pnmindex, pnminvert: add pages (#12189)

* pnmhisteq, pnmhistmap, pnmindex, pnminvert: add pages

* Update pages/common/pnmindex.md

Co-authored-by: K.B.Dharun Krishna <kbdharunkrishna@gmail.com>

---------

Co-authored-by: K.B.Dharun Krishna <kbdharunkrishna@gmail.com>

pages/common/pnmhisteq.md

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+# pnmhisteq
+
+> Histogram-equalize a PNM image.
+> More information: <https://netpbm.sourceforge.net/doc/pnmhisteq.html>.
+
+- Increase the contrast of a PNM image using histogram equalization:
+
+`pnmhisteq {{path/to/input.pnm}} > {{path/to/output.pnm}}`
+
+- Only modify grey pixels:
+
+`pnmhisteq -grey {{path/to/input.pnm}} > {{path/to/output.pnm}}`
+
+- Do not include black or white pixels in the histogram equalization:
+
+`pnmhisteq -no{{black|white}} {{path/to/input.pnm}} > {{path/to/output.pnm}}`

pages/common/pnmhistmap.md

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+# pnmhistmap
+
+> Draw a histogram of a PNM image.
+> More information: <https://netpbm.sourceforge.net/doc/pnmhistmap.html>.
+
+- Draw a histogram of a PNM image:
+
+`pnmhistmap {{path/to/input.pnm}} > {{path/to/output.pnm}}`
+
+- Draw the histogram as dots instead of bars:
+
+`pnmhistmap -dots {{path/to/input.pnm}} > {{path/to/output.pnm}}`
+
+- Specify the range of intensity values to include:
+
+`pnmhistmap -lval {{minval}} -rval {{maxval}} {{path/to/input.pnm}} > {{path/to/output.pnm}}`

pages/common/pnmindex.md

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+# pnmindex
+
+> Build a visual index of multiple PNM images.
+> See also: `pamundice`.
+> More information: <https://netpbm.sourceforge.net/doc/pnmindex.html>.
+
+- Produce an image containing thumbnails of the specified images in a grid:
+
+`pnmindex {{path/to/input1.pnm path/to/input2.pnm ...}} > {{path/to/output.pnm}}`
+
+- Specify the size of the (quadratic) thumbnails:
+
+`pnmindex -size {{50}} {{path/to/input1.pnm path/to/input2.pnm ...}} > {{path/to/output.pnm}}`
+
+- Specify the number of thumbnails per row:
+
+`pnmindex -across {{10}} {{path/to/input1.pnm path/to/input2.pnm ...}} > {{path/to/output.pnm}}`
+
+- Specify the maximum number of colors in the output:
+
+`pnmindex -colors {{512}} {{path/to/input1.pnm path/to/input2.pnm ...}} > {{path/to/output.pnm}}`

pages/common/pnminvert.md

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+# pnminvert
+
+> Invert a PNM image.
+> More information: <https://netpbm.sourceforge.net/doc/pnminvert.html>.
+
+- Invert the colors or greyscale values in a PNM image:
+
+`pnminvert {{path/to/input.pnm}} > {{path/to/output.pnm}}`