How to read pixels from framebuffer object

Question

I want to read the depth component of a scene rendered to a framebuffer object. I initially used glReadPixels() but found that it could only read pixels from the default framebuffer.

The answers to some relevant questions on this website suggest using PBO, but I haven't tried it yet. It seems that the PBO reading is asynchronous, therefore, using which command can synchronize the reading at the end?

Answer 1

A PBO won't help you here, because those are just a different kind of buffer to read into (instead of memory on the host into memory of the OpenGL implementation).

The usual way to go about making a depth component back-readable in OpenGL is to use a depth texture, attached to the depth attachment and after rendering using glGetTexImage to retrieve the date.

In the case of a normal color attachment you could use glReadPixels with a previous call to glReadBuffer to select the GL_COLOR_ATTACHMENT<i> of the bound FBO.