Update cheatsheets

ppmshadow

@@ -0,0 +1,21 @@
+---
+syntax: markdown
+tags: [tldr, common]
+source: https://github.com/tldr-pages/tldr.git
+---
+# ppmshadow
+
+> Add simulated shadows to a PPM image.
+> More information: <https://netpbm.sourceforge.net/doc/ppmshadow.html>.
+
+- Add simulated shadows to a PPM image:
+
+`ppmshadow {{path/to/input_file.ppm}} > {{path/to/output_file.ppm}}`
+
+- [b]lur the image by the specified number of pixels:
+
+`ppmshadow -b {{n}} {{path/to/input_file.ppm}} > {{path/to/output_file.ppm}}`
+
+- Specify the displacement of the simulated light source to the left and the top of the image:
+
+`ppmshadow -x {{left_offset}} -y {{top_offset}} {{path/to/input_file.ppm}} > {{path/to/output_file.ppm}}`