Rendering to depth texture - unclarities about usage of GL_OES_depth_texture

2
votes

I'm trying to replace OpenGL's gl_FragDepth feature which is missing in OpenGL ES 2.0.

I need a way to set the depth in the fragment shader, because setting it in the vertex shader is not accurate enough for my purpose. AFAIK the only way to do that is by having a render-to-texture framebuffer on which a first rendering pass is done. This depth texture stores the depth values for each pixel on the screen. Then, the depth texture is attached in the final rendering pass, so the final renderer knows the depth at each pixel.

Since iOS >= 4.1 supports GL_OES_depth_texture, I'm trying to use GL_DEPTH_COMPONENT24 or GL_DEPTH_COMPONENT16 for the depth texture. I'm using the following calls to create the texture:

glTexImage2D(GL_TEXTURE_2D, 0, GL_DEPTH_COMPONENT, width, height, 0, GL_DEPTH_COMPONENT, GL_UNSIGNED_INT, 0);
glFramebufferTexture2D(GL_FRAMEBUFFER, GL_DEPTH_ATTACHMENT, GL_TEXTURE_2D, textureId, 0);

The framebuffer creation succeeds, but I don't know how to proceed. I'm lacking some fundamental understanding of depth textures attached to framebuffers.

  • What values should I output in the fragment shader? I mean, gl_FragColor is still an RGBA value, even though the texture is a depth texture. I cannot set the depth in the fragment shader, since gl_FragDepth is missing in OpenGL ES 2.0
  • How can I read from the depth texture in the final rendering pass, where the depth texture is attached as a sampler2D?
  • Why do I get an incomplete framebuffer if I set the third argument of glTexImage2D to GL_DEPTH_COMPONENT16, GL_DEPTH_COMPONENT16_OES or GL_DEPTH_COMPONENT24_OES?
  • Is it right to attach the texture to the GL_DEPTH_ATTACHMENT? If I'm changing that to GL_COLOR_ATTACHMENT0, I'm getting an incomplete framebuffer.
1
opengl-esopengl-es-2.0framebufferdepth-buffer

1 Answers

5
votes

Depth textures do not affect the output of the fragment shader. The value that ends up in the depth texture when you're rendering to it will be the fixed-function depth value.

So without gl_FragDepth, you can't really "set the depth in the fragment shader". You can, however, do what you describe, i.e., render depth to a texture in one pass and then read access that value in a later pass.

You can read from a depth texture using the texture2D built-in function just like for regular color textures. The value you get back will be (d, d, d, 1.0).

According to the depth texture extension specification, GL_DEPTH_COMPONENT16_OES and GL_DEPTH_COMPONENT24_OES are not supported as internal formats for depth textures. I'd expect this to generate an error. The incomplete framebuffer status you get is probably related to this.

It is correct to attach the texture to the GL_DEPTH_ATTACHMENT.