Generate a depth texture in OpenGL ES 2.0 or 3.0

Question

I have a scene I'm rendering using OpenGL ES in Android (using the Java libs). I want to apply some effects such as depth of field to the scene. I found a nice bokeh shader, and it needs a depth texture to do the job. What I need then, is to generate a depth texture to pass to the shader. I've been poking around the internet for a day and a half trying to figure this out, and can't seem to find a good example of how to generate a depth texture from an existing scene. It seems I need to do several passes, probably using an off screen render to a frame buffer, then generate a depth texture from that to pass to my bokeh shader to actually render the scene.

How do you generate a depth texture? A good working example would be much appreciated.

Answer 1

For any post processing effects, you will need to render your main scene to an FBO so colour and depth attachment data can be used in subsequent renders.

Rendering to a depth texture is very easy. You simply need to bind a depth texture to your FBO target, eg

glGenFramebuffers(1, &m_fbo);
glBindFramebuffer(GL_FRAMEBUFFER, m_fbo);

glFramebufferTexture2D(GL_FRAMEBUFFER, GL_DEPTH_ATTACHMENT, GL_TEXTURE_2D, m_depth_texture, 0);

The PowerVR SDK's RenderToTexture and ShadowMapping demos should have all the reference code you will need:

https://github.com/powervr-graphics/Native_SDK/blob/3.4/Examples/Intermediate/RenderToTexture/OGLES2/OGLES2RenderToTexture.cpp https://github.com/powervr-graphics/Native_SDK/blob/3.4/Examples/Intermediate/ShadowMapping/OGLES2/OGLES2ShadowMapping.cpp

Answer 2

You can access the per-pixel depth value from inside a fragment shader using gl_FragCoord . Write a fragment shader that writes the depth out to gl_FragColor, something like this:

gl_FragColor = vec4(gl_FragCoord.z, 0.0, 0.0, 1.0);

Render your scene to a texture using this fragment shader and you'll end up with a depth map.