Displacement Maps as a New Geometry Primitive

1
Displacement Maps as a New Geometry Primitive

• Juan Guardado
• 3D Architecture Analysis Engineer
• Meltdown 2001

2
Agenda

• Motivation
• Terminology
• Algorithm
• Tessellation
• Displacement
• Lighting
• Results
• API Considerations
• Content Creation Tools
• Conclusion

3
Motivation

• Curved surfaces nice and smooth, but no detail
• Continuous LOD with nice silhouettes
• Bandwidth is crucial
• 40MVtx (32 bytes, small!) 1.3GB/s
• AGP4X at 1GB/s
• System memory BW shared with CPU
• Compression is enormous
• Generate vertices on chip
• Mesh-independent displacement maps

4
Motivation
?
5
Terminology

• Displacement map
• Medium resolution scalar height data in 2D
  texture
• Normal map
• High resolution map of tangent-space lighting
  normals
• Base mesh
• Low resolution mesh describing general shape of
  object
• Tessellation
• Amount of subdivision per base triangle, can vary
  by edge
• Tangent space
• Flatten geometry, interpolate incident vectors
  instead

6
Algorithm

• Generate new vertex
• Tessellate triangle
• Interpolate across end points
• Displace new vertex
• Sample and filter displacement map
• Displace along on interpolated normal

7
Algorithm

• Tessellation
• Interpolation
• Tessellation Rate
• Tessellation Type
• Displacement
• Pixel Lighting

8
Crack Prevention Notes

• No cracks!
• Ensure adjacent edges have identical
• Vertex normals
• Displacement direction
• N patch interpolation
• Vertex positions
• Interpolation direction
• Morphing direction
• Z for distance-based LOD
• Displacement data
• Watch out for mip levels and borders
• Only horizontal/vertical UV discontinuities

9
Interpolation

• Linear or N Patch
• Linear
• No curvature, use on semi-flat surfaces (terrain)
• Hard edges must have zero displacement
• N Patch
• Vertex positioned on cubic curve
• Nice silhouettes for rounded objects
• Adjacent vertices must have identical normals!
• No hard edges, yet

10
Tessellation Rate

• Each edge has distinct tessellation rate
• Independent of each other
• Adjacent edges tessellated identically
• User defined or
• Dependant on camera space
• Identity rate at Z1
• Displacement data should match tessellation
  resolution
• Equilateral base triangles give best results

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Discrete Tessellation

• Allows for pre-computed displacements
• Popping artifacts
• Regeneration for dynamic displacement maps
• Rate specified at integer levels
• Usage scenario
• Simple, small meshes
• Popping doesnt matter

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Adaptive Tessellation

• Geomorphing, avoids popping
• Floating point rates
• Free LOD system
• Dependent on camera space Z
• Works on N patches
• Can dynamically update maps
• Usage scenario
• Meshes benefiting from continuous LOD
• Especially useful on abutted meshes

13
Tessellation Insights

• Can control local tessellation through base mesh
• Put more triangles in detailed areas
• Also useful for N patch creases
• Small displacement map size
• Tessellation Rate Triangles spanned
• Generate 1 vertex from 1 displacement value
• Can offset tessellation rate and filtering per
  vertex

14
Sampling Displacements

• Pre-computed samples
• 8- or 16-bit integer data and 32-bit floating
  point
• Dynamic sampling
• 16-bit integer data preferred for memory
  footprint
• UV coordinates
• Linearly interpolated from base triangle
• Best if continuous across mesh
• Or ensure identical samples at different
  coordinates
• Also across mip levels
• Discontinuities must occur on horizontals/vertical
  s
• Trilinear filtering
• Mip mapping is inherently broken for borders
• Can fix with addressing modes and border texels

15
Displacement Addressing Modes

• Wrapping/UV wrapping avoids texel discontinuities
• Mirroring fine except for vector data
• Normal maps do not mirror
• Clamping
• Incorporate texture border along mip pyramid
• Border color
• Careful, may introduce ridges or valleys
• Border texture
• Can clamp to 100

16
Displacement

• Vertex shader receives displacement value
• Displace a new vertex along interpolated normal
• Scale along direction as well

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Displacement Insights

• Works on a general mesh
• Base mesh should preserve general shape
• No cubes into starships, except for Borg
• Displacements applied like textures
• Follows texture addressing conventions
• Mip level generation
• Preserve volume (e.g. avoid greying peaks)
• Multi-displacement maps by render texture
• At vertex level in future

18
Lighting

• Interpolated normal ? displaced normal
• Cant really displace a normal anyway
• Displaced lighting
• Pixel lighting with high-resolution normal map
• Detail normal maps
• Receive tangent space normal
• Pre-computed or procedurally generated
• Perturb tangent space basis

19
Lighting Insights

• Keep your normal map rotated properly
• Vertex lighting produces color irregularities on
  CLOD
• Normal maps generated from displacements
• Beware of stretching/squishing
• Apply displacement scale factor
• Requires high resolution displacement map
• Generate normal mip map from displacement mip map
• Tangent space discontinuities will produce
  lighting artifacts
• Smooth basis and normal differently

20
Crack Prevention Summary

• Tessellation and interpolation guarantee no
  cracks
• Work with us
• Ensure adjacent edges have identical
• Vertex positions to dictate interpolation
  direction
• Vertex normals for N patch interpolation and
  displacement direction
• UV discontinuities
• Inner border texels
• Horizontal/vertical discontinuities

21
Results Compression

• Varies with
• Vertex size
• Stripification
• Tessellation/displacement/normal map resolution
• Bytes per triangle, stripified mesh
• 44 bytes per vertex
• Displacement value 1 byte
• Displacement value Tessellated vertex
• Recall displacement-map-to-tessellation relation
• 1 byte ? 44 bytes

22
Results Demo
23
API Considerations

• Texture stage state to control sampling and
  filtering
• Programmable pipeline
• Tessellator samples and filters
• Displacement and normal available in vertex
  shader
• Displace tessellated vertices at will!

24
Content Creation Tools

• 3DStudio MAX 4 and Maya
• Preview plug-in using this algorithm
• Tessellation linkable library
• Plug-in source available
• Crack checker and corrector
• D3DX has d-map generator
• P-meshes to base mesh
• Ray casts to resolve displacements
• Others?

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Conclusion

• Very general algorithm
• Analogous to texture mapping
• No special vertex data required
• Mesh independent
• Works on N patches
• Minimizes impact on content creation
• Reuse bump map data (height, normal maps)
• Compression rate 441

26
Call to Action

• Get ready for displacement maps
• Must educate artists
• Low detail base mesh
• Preserve general shape
• Medium detail displacement map
• No bigger than tessellationtriangles
• High detail normal map
• Generated from high detail displacement map

27
Questions

• ?
• Juan Guardado
• jguardad_at_matrox.com