How Gazebo generate heigthmaps?

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Hello;

I'm trying to create a plugin , that , given a x,y coordinates, set the position of a model in the ground level of a heigthmap. I generate it through a greyscale image (following this tutorial: https://github.com/AS4SR/general_info...). Does anybody know how gazebo generate the 3D surface from this grayscale image.

I use Gazebo 7 in ubuntu 16.04.