Depth texture tiling artefacts on T6xx?

Hi Stephane,

> The depth of field pass was sampling from and rendering to the same render target which seems to be what was introducing the corruption

I'll need to reread the spec when I get back in the office next week, but from memory reading and writing to the same target in the same FBO pass will result in undefined behavior in OpenGL ES. You seem to have the right solution - use a different target.

Brief reason for the artefacts: as we are tile based you will be reading back part of the image which is rendered from this frame (completed tiles), and part of the image from the previous frame (tiles still pending). I assume your DoF effect samples over the edge of a tile boundary, so "edge pixels" within a tile are loading texture data from neighboring tiles, which are in an undefined state (within a tile you will always get the old data which is why you don't get artifacts within the tile).

What I can't explain is why this appears worse on the right-hand side of the screen - this problem should effect the whole screen - so I still wonder if there is a precision issue lurking somewhere which gives different behavior depending on position. My best guess is that the sample point calculated by the DoF shader shifts slightly further out of the tile the further you get across the screen (bigger exponent needed => fewer mantissa decimal places => more coarse grained steps), whereas samples less than 1024 pixels across in your initial test image only sample within the tile being rendered.

Cheers,
Pete

Hi Stephane,

> The depth of field pass was sampling from and rendering to the same render target which seems to be what was introducing the corruption

I'll need to reread the spec when I get back in the office next week, but from memory reading and writing to the same target in the same FBO pass will result in undefined behavior in OpenGL ES. You seem to have the right solution - use a different target.

Brief reason for the artefacts: as we are tile based you will be reading back part of the image which is rendered from this frame (completed tiles), and part of the image from the previous frame (tiles still pending). I assume your DoF effect samples over the edge of a tile boundary, so "edge pixels" within a tile are loading texture data from neighboring tiles, which are in an undefined state (within a tile you will always get the old data which is why you don't get artifacts within the tile).

What I can't explain is why this appears worse on the right-hand side of the screen - this problem should effect the whole screen - so I still wonder if there is a precision issue lurking somewhere which gives different behavior depending on position. My best guess is that the sample point calculated by the DoF shader shifts slightly further out of the tile the further you get across the screen (bigger exponent needed => fewer mantissa decimal places => more coarse grained steps), whereas samples less than 1024 pixels across in your initial test image only sample within the tile being rendered.

Cheers,
Pete

Thanks much gents,

Peter, we initially thought the same thing initially, however, after isolating the DOF shader and reducing it to just a simple blit, the issue persisted. This made it clear that the issue was outside the realm of shader numerical precision. As Sean observed, the artefacts do seem to appear screen wide, however, happened to be more apparent on the water where the color contrast was higher. Again thanks for your help on this. I'm happy to provide a reproducer if you need to further investigate internally.

Best,

Stephane