I'm using pyopengl to get a depth map.
I am able to get a normalized depth map using glReadPixels(). How can I revert the normalized values to the actual depth in world coordinates?
I've tried playing with glDepthRange(), but it always performs some normalization. Can I disable the normalization at all?
When you draw your geometry, your vertex shader is supposed to transform everything into normalized device coordinates (where each component is between -1 and 1) via the view/projection matrix. There is no way to avoid it, everything outside of this range will get clipped (or clamped, if you enable depth clamping). Then, these device coordinates are transformed into window coordinates - X and Y coordinates are mapped into range specified with glViewport and Z into range set with glDepthRange.
You can't disable normalization, because the final values are required to be in 0..1 range. But you can apply the reverse transformation: first, map your depth values back to -1..1 range (if you didn't use glDepthRange, all you have to do is multiply them by 2 and subtract 1). Then, you need to apply the inverse of your projection matrix - you can either do that explicitly by calculating its inverse, or avoid matrix operations by looking into how your perspective matrix is calculated. For a typical matrix, the inverse transform will be
zNorm = 2 * zBuffer - 1
zView = 2 * near * far / ((far - near) * zNorm - near - far)
(Note that zView will be negative, between -near and -far, because in OpenGL your Z axis normally points towards the camera).
Although normally you don't want only depth - you want the full 3D points, so you might as well reconstruct the vector in normalized coordinates and then apply the inverse projection/view transform.