Normal%20Map%20Compression%20with%20ATI%203Dc

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Normal Map Compression with ATI 3Dc

• Jonathan Zarge
• jzarge_at_ati.com
• ATI Research Inc.

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Overview

• Normal mapping
• Normal map compression techniques
• DXT compression
• ATI 3Dc
• Swizzled DXT5 compression

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Normal Mapping

• Increase visual realism
• Reduce geometric size of models
• Create detail by simulating geometry
• More efficient than dense geometry
• Textures provide lighting (normal) information
• Used in Half-Life 2, Doom 3, and Far Cry

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Car Paint Demo
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Traditional Lighting Model
n2
n1
Lighting values calculated at vertices and then
interpolated across triangle
Normals interpolated across triangle and lighting
values calculated at each pixel
n0
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Normal Map Lighting
3 component normal map texture
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World Space vs. Tangent Space
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Example Tangent Space Normal Maps
Tangent space normal maps can be efficiently
compressed
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Generating Normal Maps
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Normal Mapping Drawbacks

• Low angle views distorted
• Silhouette edges do not have detail
• High texture bandwidth required
• Large amount of texture memory
• DXT compression not optimal for normal maps

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Benefits of Normal Map Compression

• More detailed normal maps
• More normal maps
• Memory and bandwidth can be diverted to other
  resources
• Smaller media size

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Normal Map Compression Techniques

• Lower precision texture formats
• Texture formats with fewer channels
• Denormalization
• Palletization
• Direct DXT compression
• ATI 3Dc
• Swizzled DXT5

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DXT Compression

• Designed to compress color data
• No suited for arbitrary data
• One channel dependent on another
• Lossy format
• Image divided into 4x4 pixel blocks
• 16 possible colors in block
• Only 3 or 4 colors in compressed data
• Local similarity in small region enables high
  quality color compression

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DXT Compression Color Block

• Image divided into 4x4 blocks
• Two 16-bit 5.6.5 colors stored representing
  endpoints of linear color-ramp
• One or two interpolated colors calculated
• 16 two-bit indices stored representing index into
  color ramp
• Effective bit-rate
• 64 (216 162) / 16 4 bits per pixel

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DXTC Color Compression
WHITE
CYAN
BLUE
MAGENTA
GREEN
YELLOW
BLACK
RED
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DXTC Color Decompression

• For the four color case, given color0,
  color1, and index1..15
• // calculating interpolated colors
• color2 (2color0 color1 1)/3
• color3 (color0 2color1 1)/3
• // decompressed texel values
• for n 0 to 15
• texeln colorindexn

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DXT5 Compression Alpha Block

• Like color, image divided into 4x4 blocks
• Two 8-bit scalars stored representing endpoints
  of linear alpha-ramp
• 4 or 6 intermediate alpha values calculated
• 16 three-bit indices stored representing index
  into alpha ramp
• Effective bit-rate
• 64 (28 163) / 16 4 bits per pixel

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DXTC Alpha Decompression

• For the 8 alpha value case, given alpha0,
  alpha1, and index1..15
• // calculating interpolated alpha values
• alpha2 (6alpha0 1alpha1 3)/7
• alpha3 (5alpha0 2alpha1 3)/7
• ..
• alpha7 (1alpha0 6alpha1 3)/7
• // decompressed texel values
• for n 0 to 15
• texeln alphaindexn

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Using DXT Compression for Normal Maps

• Not a good idea!
• Block artifacts
• Normals describe curved surfaces, not lines
• Especially visible in diagonal gradients
• Colors stored in 5.6.5 format
• Fine detail lost
• Errors are amplified for specular lighting
• Storage is inefficient because normalized vectors
  can be stored in two values

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ATI 3Dc Texture Format

• Two independent channel texture format
• Exposed in D3D via the FOURCC code ATI2
• Composed of 2 DXT5 alpha blocks
• Extremely useful for normal textures
• Compression
• 41 for 32 bit textures
• 21 for 16 bit textures

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Ruby Demo
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ATI 3Dc Texture Format

• Decompression accomplished in hardware
• Almost no impact on performance
• Can be used for two unrelated scalars
• e.g. shininess, refractive index, transparency
• Library for compressing to 3Dc available from
  ati.com/developer
• Available in OpenGL using the GL_ATI_texture_comp
  ression_3dc extension

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ATI 3Dc Texture Format
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Using ATI 3Dc with Normal Maps

• x and y (of normal) stored in the texture
• z value calculated by x2 y2 z2 1
• z sqrt(1 x2 y2)
• Only positive root used because normal is in
  tangent space
• Calculation done in pixel shader
• Can also be derived by a texture lookup
• Single channel z component texture

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Using ATI 3Dc

• Pixel shader 2.0 code

scale, bias, half, one def c0, 2.0f, -1.0f,
0.5f, 1.0f . . texld r1, t1, s1 normal map mad
r1, r1, c0.x, c0.y scale/bias to -1,1
Compute third component dp2add r1.z, r1, -r1,
c0.w 1-xx-yy rsq r1.z, r1.z 1 /
sqrt(1-xx-yy) rcp r1.z, r1.z
sqrt(1-xx-yy)
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Using ATI 3Dc

• Pixel shader code (HLSL)

sampler bump_map float4 ps_main(
PS_INPUT_STRUCT psInStruct ) COLOR0 //
Unpack two component bump map float3 bump
tex2D( bump_map, psInStruct.bump_map )
// Put x and y in -1..1 range and compute z
bump ( bump 2.0f ) - 1.0f bump.z sqrt(1
- dot(bump.xy, bump.xy)) . .
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ATI 3Dc Compression Comparison
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ATI 3Dc Normal Compression Sample
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Drawbacks of ATI 3Dc

• Can only represent tangent space normal maps
• Cannot represent high precision normal maps
• Large areas of low curvature (car hood) may
  require 16 bits per component
• More pixel shader instructions required
• Not available on all hardware

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Normal Map Compression Using Swizzled DXT5

• For any hardware with at least ps 1.4 support
• Store x and y in alpha and green channels of the
  texture map
• Shader similar to 3Dc with one extra pixel
  shader instruction
• Results better than using DXTC directly but not
  as good as 3Dc

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Using DXT5 Compression

• Pixel shader 2.0 code

scale, bias, half, one def c0, 2.0f, -1.0f,
0.5f, 1.0f . . texld r1, t1, s1 normal map mov
r1.x, r1.a restore x from alpha mad r1, r1,
c0.x, c0.y scale/bias to -1,1 Compute third
component dp2add r1.z, r1, -r1, c0.w
1-xx-yy rsq r1.z, r1.z 1 /
sqrt(1-xx-yy) rcp r1.z, r1.z
sqrt(1-xx-yy)
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DXT5 Normal Map Compression RenderMonkey Example
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Summary

• Normal Mapping great feature
• Directly using DXT compression not great for
  normal textures
• Compression with 3Dc lowers the cost normal
  mapping
• 3Dc available in D3D and OpenGL
• Swizzled DXT5 compression reasonable alternative
  for older hardware

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QUESTIONS?

• Email me jzarge_at_ati.com or devrel_at_ati.com
• More information available at ati.com/developer
• Please go see Chris Oats talk on Advanced
  Character Rendering Saturday, 1025am