Update cheatsheets

ppmfade

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+---
+syntax: markdown
+tags: [tldr, common]
+source: https://github.com/tldr-pages/tldr.git
+---
+# ppmfade
+
+> Generate a transition between two PPM images.
+> More information: <https://netpbm.sourceforge.net/doc/ppmfade.html>.
+
+- Generate a transition between two PPM images ([f]irst and [l]ast) using the specified effect:
+
+`ppmfade -f {{path/to/image1.ppm}} -l {{path/to/image2.ppm}} -{{mix|spread|shift|relief|oil|...}}`
+
+- Generate a transition starting with the specified image and ending in a solid black image:
+
+`ppmfade -f {{path/to/image.ppm}} -{{mix|spread|shift|relief|oil|...}}`
+
+- Generate a transition starting with a solid black image and ending with the specified image:
+
+`ppmfade -l {{path/to/image.ppm}} -{{mix|spread|shift|relief|oil|...}}`
+
+- Store the resulting images in files named `base.NNNN.ppm` where `NNNN` is a increasing number:
+
+`ppmfade -f {{path/to/image1.ppm}} -l {{path/to/image2.ppm}} -{{mix|spread|shift|relief|oil|...}} -base {{base}}`