Reading a texture from multisample back-buffer

[Devdept2]

Drawing directly on the FBO with texture I get a texture without antialiasing.

I will try to use a multisample FBO and see what happens, but I'm losing hope...

[tonyo_au]

You need to create multi-sample textures with glteximage2dmultisample, not normal textures and enable mutli-sampling

[Devdept2]

I tried drawing on a multisample FBO with the same number of Samples of my pixel format (that is 2) and then copying it to my back buffer.

The situation improved but it's still not pixel perfect yet (near horizontal and near vertical lines are the same, oblique lines are not)...

                int nSamples;
                nSamples = 2;
 
                //Now make a multisample color buffer
                rbColorMS= gl.GenRenderbuffersEXT();
                gl.BindRenderbufferEXT(gl.RENDERBUFFER_EXT, rbColorMS);
                gl.RenderbufferStorageMultisampleEXT(gl.RENDERBUFFER_EXT, nSamples, gl.RGBA8, fboTexSize, fboTexSize);
                gl.BindRenderbufferEXT(gl.RENDERBUFFER_EXT, 0);
 
                //Make a depth multisample depth buffer
                rbDepthMS = gl.GenRenderbuffersEXT();
                gl.BindRenderbufferEXT(gl.RENDERBUFFER_EXT, rbDepthMS);
                gl.RenderbufferStorageMultisampleEXT(gl.RENDERBUFFER_EXT, nSamples, gl.DEPTH_COMPONENT, fboTexSize, fboTexSize);
                gl.BindRenderbufferEXT(gl.RENDERBUFFER_EXT, 0);
 
                // Create FBO and attach the 2 render buffers
                fboMS = gl.GenFramebuffersEXT();
                gl.BindFramebufferEXT(gl.FRAMEBUFFER_EXT, fboMS);                           
                gl.FramebufferRenderbufferEXT(gl.FRAMEBUFFER_EXT, gl.COLOR_ATTACHMENT0_EXT, gl.RENDERBUFFER_EXT, rbColorMS);
                gl.FramebufferRenderbufferEXT(gl.FRAMEBUFFER_EXT, gl.DEPTH_ATTACHMENT_EXT, gl.RENDERBUFFER_EXT, rbDepthMS);
 
                status = gl.CheckFramebufferStatusEXT(gl.FRAMEBUFFER_EXT);
                CheckFboStatus(status);
                gl.BindFramebufferEXT(gl.FRAMEBUFFER_EXT, 0);
 
            // Enable Multisample FBO
            gl.BindFramebufferEXT(gl.FRAMEBUFFER_EXT, fboMS);
 
            gl.Clear(gl.COLOR_BUFFER_BIT | gl.DEPTH_BUFFER_BIT);
 
            // DRAW THE SCENE on the multisample FBO....
 
...
            // Later on, copy the Mutlisample FBO to the Backbuffer
 
            gl.BindFramebufferEXT(gl.READ_FRAMEBUFFER_EXT, fboMS);
            gl.BindFramebufferEXT(gl.DRAW_FRAMEBUFFER_EXT, 0);
            gl.BlitFramebufferEXT(0, 0, Width, Height, 0, 0, Width, Height, gl.COLOR_BUFFER_BIT, gl.NEAREST);
            gl.BindFramebufferEXT(gl.READ_FRAMEBUFFER_EXT, 0);
            gl.BindFramebufferEXT(gl.DRAW_FRAMEBUFFER_EXT, 0);

[Devdept2]

So is there no way to get a pixel perfect capture of a Multisampled framebuffer on NVidia cards?

If I was doing antialiasing by myself (through Accumulation buffer) would I still get this problem? (I'm afraid of the penalty of having to draw the scene multiple times, though...).

I could also always draw in the multisampled FBO and copy it to the backbuffer at every frame (instead of drawing directly to the backbuffer for standard frames and use the draw to FBO + copy to Backbuffer for the quick repaints at the end of a movement of the scene).
This way I would always pay the cost of the copy from FBO to BackBuffer (is it much?), but the frames would be consistent because I would be drawing them always in the same way.

[Xenomi]

thanks for ur posting. i used to try to draw in multisampled FBO but i cant to get perfect pixel on nivida card

[Dark Photon]

So is there no way to get a pixel perfect capture of a Multisampled framebuffer on NVidia cards?

To your question, obtaining exactly the same output relies on either 1) you replicating exactly the downsample filter that NVidia is using, or 2) just using NVidia's downsampling. #2 is probably the safest since #1 is potentially vendor-specific magic.

As far as obtaining the actual framebuffer samples, you can read back the actual subsamples from an MSAA FBO (possibly with help of a tiny frag shader). You can also read back the downsampled pixel values of course. So getting the actual data pre- and post- isn't a problem. You canalso query sample positions IIRC. But you still come back to needing to replicate the downsample filter being used (or just using the one that's built-in).

I could also always draw in the multisampled FBO and copy it to the backbuffer at every frame ... This way I would always pay the cost of the copy from FBO to BackBuffer (is it much?), but the frames would be consistent because I would be drawing them always in the same way.

Should be fairly cheap. I mean, the GPU has to downsample the image to render it anyway, and if you do timing in an MSAA video mode you'll see that this is rolled into your SwapBuffer time. So forcing your own downsample via glBlitFramebuffer and blitting from an MSAA FBO to a single-sample system FB should just be making this downsample explicit in your code.

[Devdept2]

Thank you very much.

I went down the route of drawing my scene on the Multisample FBO always and copying it at every frame on the Single Sample BackBuffer and it works nice.