I'm trying to write to an OpenGL depth buffer using a slight modification of the default fragment shader. The idea is that I can just set the uniform depth for a sprite (not sure how efficient this will be, but trying to get it to work first.)

Shader:

Setting up the depth buffer: (NOT using allegro's display options to specify a buffer, but I've done this manually before and it worked fine.)

        glGenTextures(1, &depthTexture_GL);
  glBindTexture(GL_TEXTURE_2D, depthTexture_GL);


  glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_MIN_FILTER, GL_LINEAR);
  glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_MAG_FILTER, GL_LINEAR);
  glTexParameterf(GL_TEXTURE_2D, GL_TEXTURE_WRAP_S, GL_CLAMP);
  glTexParameterf(GL_TEXTURE_2D, GL_TEXTURE_WRAP_T, GL_CLAMP);


  glTexImage2D(GL_TEXTURE_2D, 0, GL_DEPTH_COMPONENT, W, H, 0, GL_DEPTH_COMPONENT, GL_UNSIGNED_BYTE, 0);
  

  glBindTexture(GL_TEXTURE_2D, 0);

And binding the depth buffer like this:

  glBindFramebuffer(GL_FRAMEBUFFER, al_get_opengl_fbo(al_get_target_bitmap()));
  glFramebufferTexture2DEXT(GL_FRAMEBUFFER_EXT, GL_DEPTH_ATTACHMENT_EXT, GL_TEXTURE_2D, depthTexture_GL, 0);
  GLenum err = glCheckFramebufferStatus(GL_FRAMEBUFFER);
  ...
 glDepthMask(true);

But the fragment shader does not appear to be writing to the buffer when i set gl_fragDepth.

If I pre-load the depth buffer with pixel data I can display it fine (if bind the buffer as a texture at a later draw call) so I know the buffer is being setup correctly, it just does not appear to be binding correctly even though I don't get any errors when I bind. gl_fragDepth just does not appear to be writing to the buffer.

Is allegro doing something behind the scenes that would be unbinding the depth buffer when I do a simple al_draw_bitmap() call? (binding a different FBO?)

HALP!?

You could check with an OpenGL debugger (I've found "apitrace" for Linux, which is a rather hard to use one...). As far as I know Allegro should not do anything like that. Did you call glEnable(GL_DEPTH_TEST)?

I've tried glEnable(GL_DEPTH_TEST) but afaik that only discards the fragment routine if it fails the criteria for the depth test which at this point, I don't even care about.

I know its not writing to it, so enabling the actual depth test wont help.

I'll check out apitrace. I didn't know anything like that even existed...

Edit: apitrace basically just shows a log of the APi calls which isn't all that useful. it doesnt look like allergo is unbinding anything... and I already know the framebuffer is complete and there are no errors after attaching the depth buffer.

From: https://www.opengl.org/sdk/docs/man/html/glDepthMask.xhtml

I had the following:

    glDepthFunc(GL_ALWAYS);
    glEnable(GL_DEPTH_TEST);
    glDepthMask(true);

But that just seemed to cause the buffer to be full of 0.5 even though I had never cleared the buffer and had pre-loaded it with randomized pixel values.

Good to know that it has to be enabled for the mask to be written but doesn't seem to help, and in fact made things more difficult to debug because it was clearing the buffer.

Even if my fragment shader specifically sets gl_fragDepth=0 or gl_fragDepth=1, the buffer is still a neutral grey when GL_DEPTH_TEST is enabled.

How are you setting your value for depth?

Guess who was changing the target bitmap AFTER setting the shader?

<--- this guy.