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doc/how_to_model_keys.md

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+# How to model a new key
+
+![Key in Inkscape](inkscape.png "SC4")
+![Key in OpenSCAD](key.png "152698")
+
+## How it Works
+1. Get some nice pictures of the side and tip of your key.
+I recommend a flatbed scanner.
+You may also consider finding a profile of your key in a [PDF
+from the manufacturer](https://www.imlss.com/images/pdf/KBD12.pdf).
+2. Using Inkscape, trace the key outline, warding, and engraving.
+If you got your image from a PDF, your job is much easier,
+and you only need to clean up the paths.
+3. Select a path and press `Control-Shift-O`. Give it a useful name, such as "outline" or "warding".
+4. Save the result as a SVG in the `resources` folder. Run `make` to generate OpenSCAD polygons from your paths.
+5. Look up online the various parameters of your key, such as plug diameter,
+cut depths and locations.
+5. Use the provided OpenSCAD functions `key_code_to_heights`,
+`key_blank` and `key_bitting` to generate a 3D model of your key.
+Use the provided OpenSCAD files as a template.
+6. To add your key to the database,
+edit the `Makefile` to include your `.scad` file in the `SCAD_SRC` variable.
+
+For this to work, your final file must look like this:
+
+```
+module key_name(...) {
+    name = "Human-readable name of your key";
+    /*
+        Description of your key, including
+        how to properly format a bitting,
+        possibly with examples
+    */
+
+    ...
+}
+
+// These defaults are overridden by
+// -D on the command line
+bitting = "default-bitting";
+outline = "default-outline";
+warding = "default-warding";
+key_name(...);
+```