1 / 34

Normal Map Compression with ATI 3Dc™

Normal Map Compression with ATI 3Dc™. Jonathan Zarge jzarge@ati.com ATI Research Inc. Overview. Normal mapping Normal map compression techniques DXT compression ATI 3Dc™ Swizzled DXT5 compression. Normal Mapping. Increase visual realism Reduce geometric size of models

salena
Download Presentation

Normal Map Compression with ATI 3Dc™

An Image/Link below is provided (as is) to download presentation Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author. Content is provided to you AS IS for your information and personal use only. Download presentation by click this link. While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server. During download, if you can't get a presentation, the file might be deleted by the publisher.

E N D

Presentation Transcript

  1. Normal Map Compression with ATI 3Dc™ Jonathan Zarge jzarge@ati.com ATI Research Inc.

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

  3. 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

  4. Car Paint Demo

  5. 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

  6. nx nx nz (nx,nx,nz) Lighting values calculated at each pixel using normal read from normal map Normal Map Lighting 3 component normal map texture

  7. Triangle normal = (-0.6,-0.3,0.74) Triangle normal = (0,0,1) z y x z y x Coordinate space of normal defined by triangle orientation z component of normal map texel must be positive Triangle normal can be any normalized vector World Space vs. Tangent Space

  8. Example Tangent Space Normal Maps Tangent space normal maps can be efficiently compressed

  9. Generating Normal Maps

  10. 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

  11. Benefits of Normal Map Compression • More detailed normal maps • More normal maps • Memory and bandwidth can be diverted to other resources • Smaller media size

  12. Normal Map Compression Techniques • Lower precision texture formats • Texture formats with fewer channels • Denormalization • Palletization • Direct DXT compression • ATI 3Dc™ • Swizzled DXT5

  13. 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

  14. 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

  15. Color1 Interpolated Colors Color0 DXTC Color Compression WHITE CYAN BLUE MAGENTA GREEN YELLOW BLACK RED

  16. 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]]

  17. 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

  18. 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]]

  19. 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

  20. 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

  21. Ruby Demo

  22. 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_compression_3dc extension

  23. ATI 3Dc™ Texture Format

  24. 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

  25. 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)

  26. 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)); . .

  27. ATI 3Dc™ Compression Comparison

  28. ATI 3Dc™ Normal Compression Sample

  29. 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

  30. 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™

  31. 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)

  32. DXT5 Normal Map Compression RenderMonkey Example

  33. 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

  34. QUESTIONS? Email me jzarge@ati.com or devrel@ati.com More information available at ati.com/developer Please go see Chris Oat’s talk on “Advanced Character Rendering” Saturday, 10:25am

More Related