Questions about GPUs

1
Questions about GPUs

• AS-Temps réel Bordeaux

Philippe Decaudin Fabrice Neyret
Stéphane GuySylvain Lefebvre
2
Overview

• Suspicious behaviors of GPUs.
• What could be improved quickly.
• Our wishes.

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1. Suspicious behaviors

• 3D Textures, MIP-mapping and anisotropy
• Clamp to border and MIP-mapping
• Deferred shading for conditions

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3D textures and MIP-mapping

• Filtering of 3D textures is isotropic (same
  MIP-map level for u, v and w)
• When texturing a slice of volume with a 3D
  texture, the slope of the slice controls the
  blur sloppy gt blurry
• With 2D textures, this problem is solved by
  anisotropic filtering, but
• - it is not available for 3D textures
• - no dsdw

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Clamp_to_border and MIP-mapping

• Using non-square textures (2a ? 2b, a ? b)
• CLAMP_TO_BORDER_ARB enabled
• MIP-map enabled
• at MIP-map levels gt log2(a) (with a lt b), the
  texture becomes 1D
• clamp doesnt seem to work correctly

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Clamp_to_border and MIP-mapping

• a quad mapped with a white texture 256?64
• (u,v) (0,0) to (6,6)
• mapping clamped to 0,1?0,1
• border color is black
• MIP-map level forced with GL_TECTURE_MIN_LOD

(6,6)
Square texture (64?64)? correct
Clamp problem? bug?
MIP-map level 1 (256?64)
MIP-map level 8 (1?1)
(0,0)
Texture 256?64
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Clamp_to_border and MIP-mapping
? all MIP-map levels
128x32 64x16 32x8 16x4 8x2 4x1 2x1
1x1
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Deferred shading for conditions
Theory

• Custom shader
• costly particular case for some pixels
• general case fast.

1
fast, but a few pixels are wrong
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Deferred shading for conditions
Theory
1
2
flag wrong pixels in stencil
fast, but a few pixels are wrong
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Deferred shading for conditions
Theory
1
2
flag wrong pixels in stencil
fast, but a few pixels are wrong
3
use slow correction shader only where needed
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Deferred shading for conditions
1
2
flag wrong pixels in stencil
fast, but a few pixels are wrong
3

• Practice
• Does not work !?
• cost the same as full rendering
• stencil after fragment program ?

use slow correction shader only where needed
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2. What could be improved quickly

• Front to back rendering with alpha
• Fog and pre-multiplied alpha
• Scale / Bias not limited to 0,1
• Tiled textures storage

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Front to back rendering with alpha

• Theory front to back (to benefit Z-test
  culling)
• Practice with alpha must disable depth_test
  ! ? landscapes, billboards, volumes even more
  costly
• Suggested solution glEnable(ALPHA_DEPTH_TEST)do
  ing regular blending, then if (AscreengtAthreshold
  ) ZscreenZfrag

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Fog and pre-multiplied alpha

• Premultiplied alpha textures (aR,aG,aB,a) to
  avoid interpolation artefacts.
• Blend mode is then set to glBlendFunc(GL_ONE,GL_O
  NE_MINUS_SRC_ALPHA)

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Fog and pre-multiplied alpha

• Problem not compatible with standard Fog
  equation (glEnable(GL_FOG))Fog equation
  Cfrag (1-f) Cfrag f Cfog In our case,
  Cfrag aC (premultiplied
  alpha) Blend equation CdestCfrag
  (1-a)Cdest? Cdest (1-f)
  aC f Cfog (1-a)Cdest

missing a

• Suggestion
• if SRC_ALPHA GL_ONE then
• fog equation (1-f) C f a Cfog

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Scale / Bias not limited to 0,1

• Theoryscale, bias to tune transparency, light,
  color
• Practicecoef must lies in 0,1 !
• ? applications which need coef gt 1 must
  multipass (volume rendering, light maps)
• Suggested SolutionFloat operation in texture
  units for scaling.

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Tiled textures storage

• Principle
• Use a set of tiles to create a larger texture

fragment program
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Tiled textures storage

• Numerous new advanced usage of tiles
• Antialiased Parameterized Solid Texturing
• Hart et al ACM Transactions on Graphics 2002
• Adaptive Texture Maps
• Kraus and Ertl Graphics Hardware 2002
• Pattern Based Procedural Textures
• Lefebvre and Neyret I3D 2003
• and more !

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Tiled textures storage

• Storage requirements
• Efficient selection from FP
• No waste of memory
• Ease filtering / interpolation of tiles
• per-tile border color
• per-tile MIP-mapping

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Tiled textures storage

• Current solutions
• Store as separate textures
• limited bindings
• no real selection in FP
• Store in an atlas
• false neighborhood and no border color
• waste space if filtering on tiles
• limited to 4096x4096
• Store as a stack in a 3D Texture
• waste a lot of space (2n)
• ill-defined filtering

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Tiled textures storage

• Our proposal Texture stack
• 3D Texture with no filtering on R
• per-tile border colors
• per-tile filtering
• easy selection from fragment program

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3. Our wishes

• Having features more coherent
• programable blending
• feedback buffer
• read access to all state variables (ex
  MIP-mapping)
• r/w stencil from FP
• Native support of tiled textures
• Fragment culling order (stencil, A, depth)
• Tools for procedural textures

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Having features more coherent

• Programable blending
• multi-pass algorithms with intermediate results
• needs complex blending
• current blending very limited compared to
  programable pipeline

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Having features more coherent

• Feedback buffer
• lot of vertex attributes
• few can be read back

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Having features more coherent

• Perspective correction
• rasterization in object space not always desired
• ?
• need control on variables interpolated by
  rasterizer
• glEnable/Disable(param,GL_PERSPECTIVE_CORRECTION)

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Native support of tiled textures

• Difficulties
• Storage (borders, repeat, )
• MIP-mapping
• Linear interpolation

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Fragment culling order

• Early culling essential for performance
• Would let the user specify test order between
• alpha test
• stencil
• depth
• discard (from FP)

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Tools for procedural textures

• New texture types (envmap, indir., N, shaders, )
• custom filtering
• MIP-mapping levels
• have to be infered from ddx ddy
• while computed for standard textures
• needs to be accessible (read)
• resolution specifiable by user

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Questions about GPUs

• AS-Temps réel Bordeaux

Philippe Decaudin Fabrice Neyret
Stéphane GuySylvain Lefebvre