Update cheatsheets

ppmdither

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+---
+syntax: markdown
+tags: [tldr, common]
+source: https://github.com/tldr-pages/tldr.git
+---
+# ppmdither
+
+> Reduce the number of colors in an image by applying dithering.
+> More information: <https://netpbm.sourceforge.net/doc/ppmdither.html>.
+
+- Read a PPM image, apply dithering and save it to a file:
+
+`ppmdither {{path/to/image.ppm}} > {{path/to/file.ppm}}`
+
+- Specify the desired number of shades for each primary color:
+
+`ppmdither -red {{2}} -green {{3}} -blue {{2}} {{path/to/image.ppm}} > {{path/to/file.ppm}}`
+
+- Specify the dimensions of the dithering matrix:
+
+`ppmdither -dim {{2}} {{path/to/image.ppm}} > {{path/to/file.ppm}}`