normal map compression with ati 3dc™ jonathan zarge [email protected] ati research inc

Normal Map Compression with ATI 3Dc™Jonathan [email protected] Research Inc.

Normal Map Compression with ATI 3Dc™

Overview

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


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


Car Paint Demo


Traditional Lighting Model

n0

n1 n2

Lighting values calculated at vertices and then interpolated across triangle

Normals interpolated across triangle andlighting values calculated at each pixel


Normal Map Lighting

3 component normal map texture

Lighting values calculated at each pixel using normal read from

normal map

(nx,nx,nz)

nx nx nz


World Space vs. Tangent Space

Triangle normal = (-0.6,-0.3,0.74)

z

y

x

Triangle normal can be any normalized vector

xy z

Triangle normal = (0,0,1)

Coordinate space of normaldefined by triangle orientation

z component of normal maptexel must be positive


Example Tangent Space Normal Maps

Tangent space normal maps can beefficiently compressed


Generating Normal Maps


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


Benefits of Normal Map Compression

• More detailed normal maps• More normal maps• Memory and bandwidth can be diverted

to other resources• Smaller media size


Normal Map Compression Techniques

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


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


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 (2*16 +16*2) / 16 = 4 bits per pixel


DXTC Color Compression

BLACK

CYAN WHITE

MAGENTABLUE

GREEN YELLOW

RED

Color0

Color1

Interpolated Colors


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]]


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 (2*8 +16*3) / 16 = 4 bits per pixel


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


..


// decompressed texel values

for n = 0 to 15

texel[n] = alpha[index[n]]


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


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:

– 4:1 for 32 bit textures

– 2:1 for 16 bit textures


Ruby Demo



• 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_compression_3dc extension


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


Using ATI 3Dc™

Pixel shader 2.0 code:

; scale, bias, half, onedef c0, 2.0f, -1.0f, 0.5f, 1.0f..texld r1, t1, s1 ;normal mapmad r1, r1, c0.x, c0.y ; scale/bias to -1,1

; Compute third componentdp2add r1.z, r1, -r1, c0.w ; 1-x*x-y*yrsq r1.z, r1.z ; 1 / sqrt(1-x*x-y*y)rcp r1.z, r1.z ; sqrt(1-x*x-y*y)


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)); . .


ATI 3Dc™ Compression Comparison


ATI 3Dc™ Normal Compression Sample


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


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™


Using DXT5 Compression

Pixel shader 2.0 code:

;scale, bias, half, onedef 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 componentdp2add r1.z, r1, -r1, c0.w ; 1-x*x-y*yrsq r1.z, r1.z ; 1 / sqrt(1-x*x-y*y)rcp r1.z, r1.z ; sqrt(1-x*x-y*y)


DXT5 Normal Map Compression RenderMonkey Example


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


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™ jonathan zarge [email protected] ati research inc

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