Which parts of the GPU exactly process 2x2 pixels at once?

Discussion in 'Beginners Zone' started by Kaotik, Nov 23, 2010.

  1. Kaotik

    Kaotik Drunk Member
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    Is it the ROPs or which?

    Trying to get 100% clear on the "under 4 pixel polygons get wasted" (Huddys statement about 75% of work going to waste at max settings tesselation in StoneGiant)
     
  2. bmcnett

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    a quad is 2x2 pixels

    The pixel shader always processes 2 X 2 groups of pixels (called "quads") but there is no other part of a GPU that necessarily does. Quads exist for the purpose of mip mapped texture filtering; if you compute texture coordinates for each pixel in a 2 X 2 quad, you can do a little bit of math on the four coordinates to determine which mip maps should be sampled.

    This is one of many reasons why a GPU is poor at rendering tiny triangles. A one-pixel triangle is shaded as a 2 X 2 quad, which wastes 75% of pixel shader resources.
     
  3. ccanan

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    nice reply
     
  4. Kaotik

    Kaotik Drunk Member
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    Cheers, that cleared it up perfectly
     
  5. Grall

    Grall Invisible Member
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    If extensive tesselation support starts catching hold, what's the chance of future GPUs dropping the pixel quad concept entirely and shading only individual pixels?
     
  6. AlexV

    AlexV Heteroscedasticitate
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    It's more likely that they'll do something along the lines of quad-fragment-merging.
     
  7. sebbbi

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    If most rendered triangles are one pixel (or sub-pixel) sized, it's better just to render the points as a point cloud, and drop the triangles (polygons) entirely.
     
  8. ebola

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    i heard they looked into this sort of thing during the ps3's development - and you still need a triangle concept for filtering "or it looks like a mosaic.."
     
  9. fellix

    fellix Hey, You!
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    Other factors in the fragment pipeline would make shading of tiny primitives inefficient long before hitting the quad boundary, like rasterization granularity and batch-size limitations.
     

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