FBO + multisample + blit + multiple color buffers

OpenGL OpenGL: Advanced Coding
1

Hi all,

Using FBO alone is easy. Performing multisample antialiasing on a FP16 target is a bit harder. The procedure I’m using is:

  • Create FBO A, attach the renderable texture to COLOR_ATTACHEMENT_0, attach a depth buffer.
  • Create FBO B, attach a multisample render buffer to COLOR_ATTACHMENT_0, attach a multisample depth buffer.
  • Render scene into FBO B
  • Blit FBO B into FBO A

Now, my problem is that I want to use multiple FP16 render buffers AND antialiasing at the same time.

I felt that the procedure above should work “as is”, ie. with a single blit, the two color attachments get copied. Doesn’t seem that way. When I try to display the second color attachment, I get the data from the first one.

There’s not a single word in the specifications about mixing ARB_draw_buffers with EXT_framebuffer_multisample/blit.

I tried a slightly different approach. Instead of a single blit, I do:

  • glBindFramebufferEXT(GL_READ_FRAMEBUFFER_EXT, m_fboB);
  • glBindFramebufferEXT(GL_DRAW_FRAMEBUFFER_EXT, m_fboA);
  • glReadBuffer(GL_COLOR_ATTACHMENT0_EXT)
  • glDrawBuffer(GL_COLOR_ATTACHMENT0_EXT);
  • glBlitFramebufferEXT(0, 0, m_width - 1, m_height - 1, 0, 0, m_width - 1, m_height - 1, mask, GL_NEAREST);
  • glReadBuffer(GL_COLOR_ATTACHMENT1_EXT);
  • glDrawBuffer(GL_COLOR_ATTACHMENT1_EXT);
  • glBlitFramebufferEXT(0, 0, m_width - 1, m_height - 1, 0, 0, m_width - 1, m_height - 1, mask, GL_NEAREST);

I check for glGetError between each call, and no error happens. But the end result (when displaying the secondary texture) is still showing the first texture’s content.

I have a GF 8800 GTX under Vista 32 bits.

Did anybody succeed in mixing those extensions together ? Or has any idea ?

Y.

2

So what’s attached to GL_COLOR_ATTACHMENT1_EXT?
The depth buffer isn’t a color attachment if that’s what you’re looking for.

N.

3

There’s not a single word in the specifications about mixing ARB_draw_buffers with EXT_framebuffer_multisample/blit.

From the EXT_framebuffer_blit specs:

 12) Should we add support for multiple ReadBuffers, so that
    multiple color buffers may be copied with a single call to
    BlitFramebuffer?

Resolved: No, we considered this but the behavior is awkward
to define and the functionality is of limited use.

So, your multiple-blits-approach seems the only reasonable way. It might be a driver bug…

4

I’m rendering to 2 color textures and 1 depth buffer. So in COLOR_ATTACHMENT_1_EXT, there is the secondary color texture. There’s no problem related to the depth buffer.

5

Right… but that doesn’t tell me what the “correct” approach should be. The one I’ve tried is the “natural” one that came to my mind, not necessarily the good one…

6

Bump. Problem is still alive…

7

A nasty solution I know, but what about swapping the colour attachments around so that after each blit the next target is moved to attachment 0.

If that works then you can assume a driver bug/oversight wrt copying from different source attachments/destinations on an FBO.

8

Has this problem been resolved is some other way or is the above suggestion the correct way to do it?