Blurring the depth buffer in OpenGL - how to access mipmap levels in a fragment shader?

Question

I'm trying to blur a depth texture by blurring & blending mipmap levels in a fragment shader.

I have two frambuffer objects:
1) A color frambuffer with a depth renderobject attached.
2) A z framebuffer with a depth texture attached.

Once I render the scene to the color framebuffer object, I then blit to the depth buffer object, and can successfully render that (output is a GL_LUMINANCE depth texture).

I can successfully access any given mipmap level by selecting it prior to drawing the depth buffer, for example, I can render mipmap level 3 as follows:

// FBO setup - all buffer objects created successfully and are complete and the color
// framebuffer has been rendered to (it has a depth renderbuffer attached), and no
// OpenGL errors are issued:
glBindFramebuffer(GL_READ_FRAMEBUFFER, _fbo_color);
glBindFramebuffer(GL_DRAW_FRAMEBUFFER, _fbo_z);
glBlitFramebuffer(0,0,_w, _h, 0, 0, _w, _h, GL_DEPTH_BUFFER_BIT, GL_NEAREST);
glGenerateMipmap(GL_TEXTURE_2D);

glBindFramebuffer(GL_FRAMEBUFFER, 0);

// This works:
// Select mipmap level 3
glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_BASE_LEVEL, 3);
glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_MAX_LEVEL, 3);

draw_depth_texture_on_screen_aligned_quad();

// Reset mipmap
glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_BASE_LEVEL, 0);
glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_MAX_LEVEL, 1000);

As an alternative, I'd like to add the bias parameter to the texture2D() GLSL function, or use texture2DLod() and operate with a single texture sampler, but whenever I choose a level over than 0, it appears that the mipmap hasn't been generated:

// fragment shader (Both texture2DLod and texture2D(sampler, tcoord, bias)
// are behaving the same.
uniform sampler2D zbuffer;
uniform int mipmap_level;

void main()
{
    gl_FragColor = texture2DLod(zbuffer, gl_TexCoord[0].st, float(mipmap_level));
}

I am not sure how the mipmapping works with the glBlitFramebuffer(), but my question is what is the proper way to setup the program such that calls made to texture2D/texture2DLod give the expected results?

Thanks, Dennis

Answer 1

Ok - I think I've got it... My depth buffer didn't have the mipmap levels generated. I'm using multi-texturing, and during rendering, I am activating texture unit 0 for the color framebuffer texture, and texture unit 1 for the depth buffer texture. When I activate/bind the textures, I call glGenerateMipmap(GL_TEXTURE_2D) as follows:

glActiveTextureARB(GL_TEXTURE0_ARB);
glBindTexture(GL_TEXTURE_2D, _color_texture);
glGenerateMipmap(GL_TEXTURE_2D);

glActiveTextureARB(GL_TEXTURE1_ARB);
glBindTexture(GL_TEXTURE_2D, _zbuffer_texture);
glGenerateMipmap(GL_TEXTURE_2D);

When this is done, increasing the bias in the texture2D(sampler, coord, bias) gives fragments from the mipmap level as expected.