Update cheatsheets

ppmntsc

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+---
+syntax: markdown
+tags: [tldr, common]
+source: https://github.com/tldr-pages/tldr.git
+---
+# ppmntsc
+
+> Make the RGB colors in a PPM image compatible with NTSC or PAL color systems.
+> More information: <https://netpbm.sourceforge.net/doc/ppmntsc.html>.
+
+- Make the RGB colors in a PPM image compatible with NTSC color systems:
+
+`ppmntsc {{path/to/input_file.ppm}} > {{path/to/output_file.ppm}}`
+
+- Make the RGB colors in a PPM image compatible with PAL color systems:
+
+`ppmntsc --pal {{path/to/input_file.ppm}} > {{path/to/output_file.ppm}}`
+
+- Print the number of illegal pixels in the input image to `stderr`:
+
+`ppmntsc --verbose {{path/to/input_file.ppm}} > {{path/to/output_file.ppm}}`
+
+- Output only legal/illegal/corrected pixels, set other pixels to black:
+
+`ppmntsc --{{legalonly|illegalonly|correctedonly}} {{path/to/input_file.ppm}} > {{path/to/output_file.ppm}}`