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I assume the velodyne_points topic you are talking about is coming from the Velodyne driver. This publishes PointCloud2 data. If you use the rostopic tool, you can see the data format to understand it better. Essentially, it is an array of "x, y, z, intensity, laser number" points. So if you are trying to determine depth, you would need to play with algorithms on this 3D point cloud output.