OpenGL: Use depth buffer from Gbuffer in default framebuffer

[Peteck]

So I am making a deferred rendering application with OpenGL. After doing the geometry and light passes I want to render some particles. But I need some depth testing on those. So what I want to do is to use the depth buffer binded to my GBuffer to do some depth testing when drawing to the default OpenGL framebuffer.

I make and bind the depth buffer/texture to the framebuffer/GBuffer like this

glGenTextures(1, &_depthTextureID);
glBindTexture(GL_TEXTURE_2D, _depthTextureID);
glTexImage2D(GL_TEXTURE_2D, 0, GL_DEPTH24_STENCIL8, _width, _height, 0, GL_DEPTH_STENCIL, GL_UNSIGNED_INT_24_8, nullptr);
glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_WRAP_S, GL_CLAMP_TO_EDGE);
glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_WRAP_T, GL_CLAMP_TO_EDGE);
glTexParameterf(GL_TEXTURE_2D, GL_TEXTURE_MIN_FILTER, GL_NEAREST);
glTexParameterf(GL_TEXTURE_2D, GL_TEXTURE_MAG_FILTER, GL_NEAREST);
glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_COMPARE_MODE, GL_NONE);
glTexParameteri(GL_TEXTURE_2D, GL_DEPTH_TEXTURE_MODE, GL_LUMINANCE);
glFramebufferTexture2D(GL_DRAW_FRAMEBUFFER, GL_DEPTH_STENCIL_ATTACHMENT, GL_TEXTURE_2D, _depthTextureID, 0);

My question then is: How do I bind the Gbuffer depth buffer to the default framebuffer for depth testing on the geometry pass?

[lolz123]

Short answer: You can't. Render to a special "composition" texture that is part of the GBuffer, and then blit to the main framebuffer.

Long answer: Depth testing only works when rendering to the framebuffer to which it is bound. Since you cannot bind a custom depth buffer to the main framebuffer, you cannot simply switch the depth buffer over to the main framebuffer as an attachment.
You could do manual depth testing in a shader if you want to, but this is slow. So the best solution is the render to a "composition" attachment part of the GBuffer, and when you are done with all rendering, blit that to the main framebuffer.

[Peteck]

I did some searching when I saw that I couldn't bind the depth buffer to another framebuffer.

glBindFramebuffer(GL_READ_FRAMEBUFFER, fbo);
glBindFramebuffer(GL_DRAW_FRAMEBUFFER, 0);
glBlitFramebuffer(0, 0, width, height, 0, 0, width, height, GL_DEPTH_BUFFER_BIT, GL_NEAREST);

For me it looks like I just can blit the depth bits into the default frammebuffer. Don't know if it will work, haven't tested it yet.