[Height map] Add a tiny documentation

docs/gdevelop5/extensions/height-map3d/details.md

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+---
+title: Details
+---
+# Height map extension
+
+This extension allows to build a terrain with hills where objects with the 3D physics behavior can stand.
+
+
+## Create a height map from a 3d model
+
+3D modelers like [Blender](https://www.blender.org/) can be used to render a model height into a grayscale image. The model should only contain the terrain. Decorations like trees or buildings must be removed for this process.
+
+
+## Generate a realistic terrain
+
+[WGEN](https://github.com/jice-nospam/wgen) is a free and open-source height map generator. It can produce grayscale images that can be used with this extension.
+
+
+## Generate an endless terrain
+
+Basic terrains can also be generated directly inside GDevelop using the [noise extension](/gdevelop5/extensions/noise).
+
+[Open example in GDevelop](https://editor.gdevelop.io/?project=example://3d-endless-terrain){ .md-button .md-button--primary }
+
+[![](endless-terrain.png){ width="320" }](https://editor.gdevelop.io/?project=example://3d-endless-terrain)
+
+
+## Avoid stairs-like height maps
+
+The extension can only read images as 8 bits per channel. This can lead to terrain moving up like stairs. To avoid this issue, a 16 bits grayscale image can be converted into a 8 bits RGB image that uses red and green channels.
+
+```Python
+import numpy as np, cv2
+
+src = cv2.imread("greyscale-height-map.png",cv2.IMREAD_UNCHANGED)
+h,w = src.shape
+dst = np.zeros((h,w,3), np.uint8)
+
+dst[:,:,1] = (src & 0xff00) >> 8
+dst[:,:,2] = src & 0x00ff
+
+cv2.imwrite("red-and-green-height-map.png",dst)
+```
+
+This is a slightly modified version of the [original script](https://gist.github.com/companje/85e94ea96629ddaf1219f137b225fd69) written in [Python](https://www.python.org/).
+
+
+## Reference
+
+All actions, conditions and expressions are listed in the [Height map reference page](..).