Transcript ATI - Normal Map Compression with ATI 3Dc

Normal Map Compression
with ATI 3Dc™
Jonathan Zarge
[email protected]
ATI Research Inc.
Overview
Normal mapping
• Normal map compression techniques
• DXT compression
• ATI 3Dc™
• Swizzled DXT5 compression
•
Normal Map Compression with ATI 3Dc™
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
•
Normal Map Compression with ATI 3Dc™
Car Paint Demo
Normal Map Compression with ATI 3Dc™
Traditional Lighting Model
n1
n0
n2
Lighting
calculated
at vertices
Normalsvalues
interpolated
across
triangle and
then interpolated
acrossattriangle
lighting
values calculated
each pixel
Normal Map Compression with ATI 3Dc™
Normal Map Lighting
nx nx nz
(nx,nx,nz)
Lighting values calculated at each
pixel using normal read from
normal map
3 component normal
map texture
Normal Map Compression with ATI 3Dc™
World Space vs. Tangent Space
Triangle normal = (-0.6,-0.3,0.74)
Triangle normal = (0,0,1)
y z
x
z
y
Coordinate space of normal
defined by triangle orientation
x
Triangle normal can be
any normalized vector
z component of normal map
texel must be positive
Normal Map Compression with ATI 3Dc™
Example Tangent Space
Normal Maps
Tangent space normal maps can be
efficiently compressed
Normal Map Compression with ATI 3Dc™
Generating Normal Maps
Normal Map Compression with ATI 3Dc™
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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Normal Map Compression with ATI 3Dc™
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 with ATI 3Dc™
Normal Map Compression
Techniques
Lower precision texture formats
• Texture formats with fewer channels
• Denormalization
• Palletization
• Direct DXT compression
• ATI 3Dc™
• Swizzled DXT5
•
Normal Map Compression with ATI 3Dc™
DXT Compression
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Designed to compress color data
No suited for arbitrary data
– One channel dependent on another
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Lossy format
– Image divided into 4x4 pixel blocks
– 16 possible colors in block
– Only 3 or 4 colors in compressed data
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Local similarity in small region enables
high quality color compression
Normal Map Compression with ATI 3Dc™
DXT Compression Color Block
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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 (2*16 +16*2) / 16 = 4 bits per pixel
Normal Map Compression with ATI 3Dc™
DXTC Color Compression
Color1
WHITE
CYAN
BLUE
MAGENTA
GREEN
BLACK
YELLOW
RED
Color0
Interpolated Colors
Normal Map Compression with ATI 3Dc™
DXTC Color Decompression
For the four color case, given color[0],
color[1], and index[1..15]:
// calculating interpolated colors
color[2] = (2*color[0] + color[1] + 1)/3
color[3] = (color[0] + 2*color[1] + 1)/3
// decompressed texel values
for n = 0 to 15
texel[n] = color[index[n]]
Normal Map Compression with ATI 3Dc™
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:
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– 64 (2*8 +16*3) / 16 = 4 bits per pixel
Normal Map Compression with ATI 3Dc™
DXTC Alpha Decompression
For the 8 alpha value case, given alpha[0],
alpha[1], and index[1..15]:
// calculating interpolated alpha values
alpha[2] = (6*alpha[0] + 1*alpha[1] + 3)/7
alpha[3] = (5*alpha[0] + 2*alpha[1] + 3)/7
..
alpha[7] = (1*alpha[0] + 6*alpha[1] + 3)/7
// decompressed texel values
for n = 0 to 15
texel[n] = alpha[index[n]]
Normal Map Compression with ATI 3Dc™
Using DXT Compression for
Normal Maps
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Not a good idea!
Block artifacts
– Normals describe curved surfaces, not lines
– Especially visible in diagonal gradients
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Colors stored in 5.6.5 format
– Fine detail lost
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Errors are amplified for specular lighting
Storage is inefficient because normalized
vectors can be stored in two values
Normal Map Compression with ATI 3Dc™
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:
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– 4:1 for 32 bit textures
– 2:1 for 16 bit textures
Normal Map Compression with ATI 3Dc™
Ruby Demo
Normal Map Compression with ATI 3Dc™
ATI 3Dc™ Texture Format
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Decompression accomplished in
hardware
– Almost no impact on performance
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Can be used for two unrelated scalars
– e.g. shininess, refractive index, transparency
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Library for compressing to 3Dc™
available from ati.com/developer
Available in OpenGL using the
GL_ATI_texture_compression_3dc
extension
Normal Map Compression with ATI 3Dc™
ATI 3Dc™ Texture Format
Normal Map Compression with ATI 3Dc™
Using ATI 3Dc™ with Normal
Maps
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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
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Single channel z component texture
Normal Map Compression with ATI 3Dc™
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-x*x-y*y
rsq r1.z, r1.z
; 1 / sqrt(1-x*x-y*y)
rcp r1.z, r1.z
; sqrt(1-x*x-y*y)
Normal Map Compression with ATI 3Dc™
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));
.
.
Normal Map Compression with ATI 3Dc™
ATI 3Dc™ Compression
Comparison
Normal Map Compression with ATI 3Dc™
ATI 3Dc™ Normal Compression
Sample
Normal Map Compression with ATI 3Dc™
Drawbacks of ATI 3Dc™
Can only represent tangent space normal
maps
• Cannot represent high precision normal
maps
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– Large areas of low curvature (car hood) may
require 16 bits per component
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More pixel shader instructions required
Not available on all hardware
Normal Map Compression with ATI 3Dc™
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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Normal Map Compression with ATI 3Dc™
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-x*x-y*y
rsq r1.z, r1.z
; 1 / sqrt(1-x*x-y*y)
rcp r1.z, r1.z
; sqrt(1-x*x-y*y)
Normal Map Compression with ATI 3Dc™
DXT5 Normal Map Compression
RenderMonkey Example
Normal Map Compression with ATI 3Dc™
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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Normal Map Compression with ATI 3Dc™
QUESTIONS?
Email me [email protected] or [email protected]
More information available at ati.com/developer
Please go see Chris Oat’s talk on “Advanced
Character Rendering” Saturday, 10:25am
Normal Map Compression with ATI 3Dc™