Update cheatsheets

pbmmask

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+---
+syntax: markdown
+tags: [tldr, common]
+source: https://github.com/tldr-pages/tldr.git
+---
+# pbmmask
+
+> Create a mask bitmap from a regular bitmap.
+> See also: `pambackground`.
+> More information: <https://netpbm.sourceforge.net/doc/pbmmask.html>.
+
+- Create a mask bitmap separating background from foreground:
+
+`pbmmask {{path/to/image.pbm}} > {{path/to/output.pbm}}`
+
+- Expand the generated mask by one pixel:
+
+`pbmmask -expand {{path/to/image.pbm}} > {{path/to/output.pbm}}`