Update cheatsheets

pnmnorm

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+---
+syntax: markdown
+tags: [tldr, common]
+source: https://github.com/tldr-pages/tldr.git
+---
+# pnmnorm
+
+> Normalize the contrast in a PNM image.
+> See also: `pnmhisteq`.
+> More information: <https://netpbm.sourceforge.net/doc/pnmnorm.html>.
+
+- Force the brightest pixels to be white, the darkest pixels to be black and spread out the ones in between linearly:
+
+`pnmnorm {{path/to/image.pnm}} > {{path/to/output.pnm}}`
+
+- Force the brightest pixels to be white, the darkest pixels to be black and spread out the ones in between quadratically such that pixels with a brightness of `n` become 50 % bright:
+
+`pnmnorm -midvalue {{n}} {{path/to/image.pnm}} > {{path/to/output.pnm}}`
+
+- Keep the pixels' hue, only modify the brightness:
+
+`pnmnorm -keephues {{path/to/image.pnm}} > {{path/to/output.pnm}}`
+
+- Specify a method to calculate a pixel's brightness:
+
+`pnmnorm -{{luminosity|colorvalue|saturation}} {{path/to/image.pnm}} > {{path/to/output.pnm}}`