FBO, PingPong, Shared Depth Render Buffer Problem

OpenGL OpenGL: Advanced Coding
1

Hi all!

I try to do some -very simple- fluid simulation on the GPU [currently NV30, Linux, NVIDIA 76.67 Driver].

I use OpenGL FBO’s and use 3 textures, 2 for input, 1 for output. Then textures are cycled, to achieve feedback.

Frame #1: RTT to texture 0, reading from 1 and 2
Frame #2: RTT to texture 2, reading from 0 and 1
Frame #3: RTT to texture 1, reading from 2 and 0
Frame #4: RTT to texture 0, reading from 1 and 2

This works quite well, using one FBO with a shared depth buffer and the three textures [GL_FLOAT_R32_NV, GL_TEXTURE_RECTANGLE_NV] which are attached in this round-robin style.

Now I wanted to mask out some part of the computation using the depth test. So I bind the FBO with a dummy texture and a DEPTH_ATTACHMENT renderbuffer. I clear the depth buffer to 1.0, initially. Then I draw the mask with depth test disabled, depth write enabled, setting depth values to 0.0 where NO computation should be done.

In the simulation loop I leave the depth buffer unchanged (glDepthMask(GL_FALSE), glDepthFunc(GL_LESS), no more glClear). I enable the depth test and draw a fullscreen quad with depth values 0.75 into the FBO. I thought I would get updates only where there is no 0.0 in the depth buffer. But somehow this does not work, I only get strange results. This all works o.k. if I always clear the depth buffer and render the mask every time in the loop -> something I try to avoid, because the mask is static.

The question is: What is going to happen with the DEPTH_ATTACHMENT of the FBO, when the color attachments are switched? Is it kept as it was?
I could not find anything in the FBO spec.

Any hints?

Thanks in advance!

GordonS

2

Now I wanted to mask out some part of the computation using the depth test. So I bind the FBO with a dummy texture and a DEPTH_ATTACHMENT renderbuffer. I clear the depth buffer to 1.0, initially. Then I draw the mask with depth test disabled, depth write enabled, setting depth values to 0.0 where NO computation should be done.
Maybe its because the depth buffer isn’t written to, when the depth test is disabled. Try enabling depth test while setting depth test to GL_ALWAYS.

3

Thanks for the answer.
Well, I verified the depth buffer was written to with correct values.
Also, when clearing/writing to it every frame in the loop, everything was correct.
But, I kept on searching and the solution is:
When attaching a new texture as render target also re-attach the already attached depth buffer.

<generate fbo, renderbuffer, 3 textures texId[3]>

<RenderLoop Start>

// attach new texture and re-attach renderbuffer
BindFramebufferEXT(GL_FRAMEBUFFER_EXT, fboId);
FramebufferTexture2DEXT(GL_FRAMEBUFFER_EXT, GL_COLOR_ATTACHMENT0_EXT, GL_TEXTURE_RECTANGLE_NV, texId[0], 0);
FramebufferRenderbufferEXT(GL_FRAMEBUFFER_EXT, GL_DEPTH_ATTACHMENT_EXT, GL_RENDERBUFFER_EXT, depthId);

<render to texture and shared depth buffer, read from texId[1] and texId[2]>

// unattach texture and render buffer -> why???
FramebufferTexture2DEXT(GL_FRAMEBUFFER_EXT, GL_COLOR_ATTACHMENT0_EXT, GL_TEXTURE_RECTANGLE_NV, 0, 0);
FramebufferRenderbufferEXT(GL_FRAMEBUFFER_EXT, GL_DEPTH_ATTACHMENT_EXT, GL_RENDERBUFFER_EXT, 0);
BindFramebufferEXT(GL_FRAMEBUFFER_EXT, 0);

<render other stuff>

<cycle texId array temp=texId[2], texId[2]=texId[1], texId[1]=texId[0], texId[0]=temp>

<RenderLoop End>

So this did the trick. I don’t know, why
(1) I have to attach/unattach the depth buffer every time.
(2) I also have to unattach the texture every time, altough I attach a different texture to the same COLOR_ATTACHMENT. I thought this would automatically unattach the old texture, but this does not work.

Thanks again for any hints.

GordonS

4

Thanks for the answer.
Well, I verified the depth buffer was written to with correct values.
Also, when clearing/writing to it every frame in the loop, everything was correct.
But, I kept on searching and the solution is:
When attaching a new texture as render target also re-attach the already attached depth buffer.

<generate fbo, renderbuffer, 3 textures texId[3]>

<RenderLoop Start>

// attach new texture and re-attach renderbuffer
BindFramebufferEXT(GL_FRAMEBUFFER_EXT, fboId);
FramebufferTexture2DEXT(GL_FRAMEBUFFER_EXT, GL_COLOR_ATTACHMENT0_EXT, GL_TEXTURE_RECTANGLE_NV, texId[0], 0);
FramebufferRenderbufferEXT(GL_FRAMEBUFFER_EXT, GL_DEPTH_ATTACHMENT_EXT, GL_RENDERBUFFER_EXT, depthId);

<render to texture and shared depth buffer, read from texId[1] and texId[2]>

// unattach texture and render buffer -> why???
FramebufferTexture2DEXT(GL_FRAMEBUFFER_EXT, GL_COLOR_ATTACHMENT0_EXT, GL_TEXTURE_RECTANGLE_NV, 0, 0);
FramebufferRenderbufferEXT(GL_FRAMEBUFFER_EXT, GL_DEPTH_ATTACHMENT_EXT, GL_RENDERBUFFER_EXT, 0);
BindFramebufferEXT(GL_FRAMEBUFFER_EXT, 0);

<render other stuff>

<cycle texId array temp=texId[2], texId[2]=texId[1], texId[1]=texId[0], texId[0]=temp>

<RenderLoop End>

So this did the trick. I don’t know, why
(1) I have to attach/unattach the depth buffer every time.
(2) I also have to unattach the texture every time, altough I attach a different texture to the same COLOR_ATTACHMENT. I thought this would automatically unattach the old texture, but this does not work.

Thanks again for any hints.

GordonS

5

Thanks for the answer.
Well, I verified the depth buffer was written to with correct values.
Also, when clearing/writing to it every frame in the loop, everything was correct.
But, I kept on searching and the solution is:
When attaching a new texture as render target also re-attach the already attached depth buffer.

<generate fbo, renderbuffer, 3 textures texId[3]>

<RenderLoop Start>

// attach new texture and re-attach renderbuffer
BindFramebufferEXT(GL_FRAMEBUFFER_EXT, fboId);
FramebufferTexture2DEXT(GL_FRAMEBUFFER_EXT, GL_COLOR_ATTACHMENT0_EXT, GL_TEXTURE_RECTANGLE_NV, texId[0], 0);
FramebufferRenderbufferEXT(GL_FRAMEBUFFER_EXT, GL_DEPTH_ATTACHMENT_EXT, GL_RENDERBUFFER_EXT, depthId);

<render to texture and shared depth buffer, read from texId[1] and texId[2]>

// unattach texture and render buffer -> why???
FramebufferTexture2DEXT(GL_FRAMEBUFFER_EXT, GL_COLOR_ATTACHMENT0_EXT, GL_TEXTURE_RECTANGLE_NV, 0, 0);
FramebufferRenderbufferEXT(GL_FRAMEBUFFER_EXT, GL_DEPTH_ATTACHMENT_EXT, GL_RENDERBUFFER_EXT, 0);
BindFramebufferEXT(GL_FRAMEBUFFER_EXT, 0);

<render other stuff>

<cycle texId array temp=texId[2], texId[2]=texId[1], texId[1]=texId[0], texId[0]=temp>

<RenderLoop End>

So this did the trick. I don’t know, why
(1) I have to attach/unattach the depth buffer every time.
(2) I also have to unattach the texture every time, altough I attach a different texture to the same COLOR_ATTACHMENT. I thought this would automatically unattach the old texture, but this does not work.

Thanks again for any hints.

GordonS