Reading a texture from multisample back-buffer

[Devdept2]

Hi,

I have a multisample OpenGL viewport (when creating the viewport I ask for a pixel format with multisample to wglChoosePixelFormatARB) and my geometry is displayed antialised.

Now, I read the backbuffer inside a texture that I later use use for a quick repaint of my scene in some situations.

The texture is createad with NEAREST min and mag filters and has the same size of the viewport, so no magnification and minification is involved.

On ATI cards the texture is exactly equal to the scene on screen (so when I use it for the repaint I don't notice any change on screen), but on NVidia cards they are not the same: the lines colors become a bit more intense in the texture.

What could be the problem and how could I fix it?

If I use a pixel format without multsampling there's no difference from the image on screen in both ATI and NVidia.

[BionicBytes]

Perhaps some screen images might help others see your problem?

[Devdept2]

This is the screen capture of the rendering of the lines:



and this is the screen capture of the texture generated reading the back buffer

[Dark Photon]

I have a multisample OpenGL viewport...Now, I read the backbuffer inside a texture that I later use use for a quick repaint of my scene in some situations....but on NVidia cards they are not the same: the lines colors become a bit more intense in the texture.

What could be the problem and how could I fix it?

Have noticed this. Seems that the NVidia driver isn't using the same downsample filter as the GPU for the ReadPixels path vs. the SwapBuffer path.

You could try doing a manual resolve/downsample via glBlitFramebuffer to a single sample texture (which should ideally happen on the GPU), and read back the texels from that. That might produce the desired results.

[ZbuffeR]

It *looks* like only a difference in gamma-correction.

[Devdept2]

I'm currently using

glCopyTexSubImage2D

to copy the BackBuffer to texture.

I'll give it a try using glBlitFramebuffer...

[Devdept2]

Hi,

I tried to use the glBlitFramebuffer but I still get the difference between the texture and the rendered image on NVidia cards.

Maybe I'm not using it the right way?

Here's my code.

// Draw the scene on the backbuffer
...

//Generate the FBO
rb = gl.GenRenderbuffersEXT();
gl.BindRenderbufferEXT(gl.RENDERBUFFER_EXT, rb);
gl.RenderbufferStorageEXT(gl.RENDERBUFFER_EXT, gl.RGB, width, height);


rbDepth = gl.GenRenderbuffersEXT();
gl.BindRenderbufferEXT(gl.RENDERBUFFER_EXT, rbDepth);
gl.RenderbufferStorageEXT(gl.RENDERBUFFER_EXT, gl.DEPTH_COMPONENT, width, height);


fbo = gl.GenFramebuffersEXT();

gl.BindFramebufferEXT(gl.FRAMEBUFFER_EXT, fbo);


gl.FramebufferRenderbufferEXT(gl.FRAMEBUFFER_EXT, gl.COLOR_ATTACHMENT0_EXT, gl.RENDERBUFFER_EXT, rb);
gl.FramebufferRenderbufferEXT(gl.FRAMEBUFFER_EXT, gl.DEPTH_ATTACHMENT_EXT, gl.RENDERBUFFER_EXT, rbDepth);

int status = gl.CheckFramebufferStatusEXT(gl.FRAMEBUFFER_EXT);


CheckFboStatus(status);

// Copy the Back Buffer to the FBO
gl.BindFramebufferEXT(gl.READ_FRAMEBUFFER_EXT, 0); // IS IT RIGHT???????????????
gl.BindFramebufferEXT(gl.DRAW_FRAMEBUFFER_EXT, fbo);
gl.BlitFramebufferEXT(0, 0, Width, Height, 0, 0, Width, Height, gl.COLOR_BUFFER_BIT, gl.NEAREST);

// Read the FBO image in 9 textures (3x3 grid)

for (int j = 0; j < 3; j++)


for (int i = 0; i < 3; i++)
{


gl.BindTexture(gl.TEXTURE_2D, texName[i + j * 3]);


// Debug.WriteLine("Grabbing tex " + texName[i + j * 2] + " at " + (texSize * i) + " , " + (texSize * j));
gl.CopyTexSubImage2D(gl.TEXTURE_2D, 0, 0, 0, texSize * i, texSize * j, texSize, texSize);
}

[Devdept2]

Anybody could help, please?

[tonyo_au]

Just a thought,
why use RenderbufferStorage. Render to an offscreen texture buffer that you can blit any time you like to the back buffer. Now even if there is a difference between AMD and nVidia versions, at least it is consistent on a card whether it is the first render or the "fake" blit.

[Dark Photon]

Please use \[code\] blocks to mark code.

// Copy the Back Buffer to the FBO
gl.BindFramebufferEXT(gl.READ_FRAMEBUFFER_EXT, 0);  // IS IT RIGHT???????????????
gl.BindFramebufferEXT(gl.DRAW_FRAMEBUFFER_EXT, fbo);
gl.BlitFramebufferEXT(0, 0, Width, Height, 0, 0, Width, Height, gl.COLOR_BUFFER_BIT, gl.NEAREST);
 
...
 
gl.CopyTexSubImage2D(gl.TEXTURE_2D, 0, 0, 0, texSize * i, texSize * j, texSize, texSize);

glCopyTexSubImage2D reads from the READ_BUFFER of the READ_FRAMEBUFFER. It doesn't look like you're reading from the framebuffer I think you intended to (see glBindFramebuffer() and glReadBuffer()). Looks like you're copying from the MSAA framebuffer as before here.

And agree with tonyo_au. Just blit to an FBO bound to a texture. Then you don't need the CopyTexSubImage to get it into another texture.

Also yes, binding FBO 0 reverts to the system framebuffer. I'm presuming that's the one you've got that's multisampled.

If you still have issues, you could see if the same happens when rendering to an off-screen MSAA FBO. There may be something about your system framebuffer that's "special".

I'm assuming you're not using sRGB framebuffers.