Why can't I get values from depth texture using FBO?

Question

I can't get depth values from FBO's depth texture.. Program's workflow is like below :

Setup
1. Make pass1 FBO (color texture + depth texture)
2. Make pass2 RBO (color renderbuffer + depth renderbuffer)

Rendering
1. only vertex processing (pass1)
2. get depth values from pass0 FBO's depth texture
3. vertex and fragment processing using a previous depth values

Setup.1 code (create pass0 fbo)

glActiveTexture(GL_TEXTURE0);
glGenTextures(2, &pass1_tex);
glBindTexture(GL_TEXTURE_2D, pass1_tex[0]);
glTexImage2D(GL_TEXTURE_2D, GL_RGBA8, w, h, 0, GL_RGBA, GL_UNSIGNED_BYTE, 0);
glActiveTexture(GL_TEXTURE1);
glBindTexture(GL_TEXTURE_2D, pass1_tex[1]);
glTexImage2D(GL_TEXTURE_2D, GL_DEPTH_COMPONENT, w, h, 0, GL_DEPTH_COMPONENT, 0);
glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_MIN_FILTER, GL_NEAREST);
glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_MAG_FILTER, GL_NEAREST);
glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_WRAP_S, GL_CLAMP);
glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_WRAP_T, GL_CLAMP);
glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_COMPARE_MODE, GL_COMPARE_R_TO_TEXTURE);
glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_COMPARE_FUNC, GL_LEQUAL);
glTexParameteri(GL_TEXTURE_2D, GL_DEPTH_TEXTURE_MODE, GL_INTENSITY);
glGenFramebuffers(1, pass1_fbo);
glBindFramebuffer(GL_FRAMEBUFFER, pass1_fbo);
glBindFramebuffer(GL_FRAMEBUFFER, GL_COLOR_ATTACHMEMNT0, GL_TEXTURE_2D< pass1_tex[0], 0);
glBindFramebuffer(GL_FRAMEBUFFER, GL_DEPTH_ATTACHMEMNT, GL_TEXTURE_2D< pass1_tex[0], 0);

And to get depth values in rendering, I did like below :

glBindFramebuffer(GL_FRAMEBUFFER, pass1_fbo);
// vertex processing..
glActiveTexture(GL_TEXTURE1);
glGetTexImage(GL_TEXTURE_2D, 0, GL_DEPTH_COMPONENT, GL_FLOAT, &debug_buf[0]);

But, there are only 1.0 or 0.0 in debug_buf array. Of course, all objects are between 1.0 and -1.0 distance (NDC), a near and far value is 1.0 and 2.0 in gluPerspective().

Why can't I get proper depth values from FBO's depth texture?

Um, could you post code that actually compiles? glBindFramebuffer binds the framebuffer to the context; it only takes two parameters. Also, if that was supposed to be glFramebufferTexture, you put the same texture as both the color and depth, which I'm pretty sure is not legal. Are you checking framebuffer completeness? Lastly, you never bound the depth texture when you tried to call glGetTexImage — Nicol Bolas
The depth buffer will always have a value range from [-1.0 .. 1.0], regardless of the near and far values (at least as long as near < far and both are reasonable). That's what normalizing does here. — Razzupaltuff

Razzupaltuff Razzupaltuff · Accepted Answer · 2011-08-21T22:48:27

You can find the answer to your problem in this article. When using GL_COMPARE_R_TO_TEXTURE, sampling the depth texture returns a comparison result of the depth texture values against a reference value (which is the interpolated r texture coordinate). What you want to do is to specify the following texture parameters instead:

glTexParameteri (GL_TEXTURE_2D, GL_TEXTURE_COMPARE_MODE, GL_NONE);
glTexParameteri (GL_TEXTURE_2D, GL_DEPTH_TEXTURE_MODE, GL_INTENSITY);

Why can't I get values from depth texture using FBO?

2 Answers