Rendering Geometry with Relief Textures

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Rendering Geometry withRelief Textures

• L.Baboud X.Décoret
• ARTIS-GRAVIR/IMAG-INRIA

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Height-field representation

• What can we represent?
• How to render it?
• Fast ?
• Exact?
• Previous work
• Our contributions

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Height-field

• Function from 0,12 to 0,1
• Represents a surface
• Sampled in a texture
• Memory/GPU efficient

(a) Height texture
(b) Reconstructed surface
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Height-field

• Function from 0,12 to 0,1
• Represents a surface
• Sampled in a texture
• Memory/GPU efficient

(a) nearest

• Theory How do you reconstruct?
• Practice Which interpolation for texture
  lookups?

(b) linear
Refer to the paper for details
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How to render?

• Polygonalize
• Simple natural
• Costly, not output sensitive
• Aliasing -gt LOD

Small coverage

• What if the object is not the main one?
• Bump
• Small objects

Large coverage
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How to render?

• Polygonalize
• Simple natural
• Costly, not output sensitive
• Aliasing -gt LOD
• Ray-tracing
• Slow? Feasible on GPU!
• Output sensitive

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Ray-tracing

• Existing solution VDM Wang 2003
• Sampling in all viewing directions
• Memory costly (4 Mb compressed for 128x128)
• Use GPU capabilities instead

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Principle (1/2)
Top view
2D slice
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Principle (2/2)

• Comparing heights
• Along ray
• In the heightfield
• Until we pass below

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Policarpo I3D 05
Binary search
How many iterations?
Is it correct?
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Policarpo I3D 05
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Policarpo I3D 05
Fixed size steps
Binary search
Better but still potentially false
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Policarpo I3D 05
Fixed size steps
Binary search
Missed intersection
Amounts to vertical slicing
Size of steps along the ray depends on ray tilt
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Policarpo I3D 05
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Tatarchuk I3D06

• Varying size steps (between two bounds)
• Depending on ray tilt

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Conclusion on existing methods

• Advantages speed
• Drawbacks missed intersections (at grazing
  angles)

Policarpo05
Tatarchuk06
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Being exact, what for?

• Intellectually rewarding ?
• Usability large scale geometry
• Terrain, buildings, architectural details
• Objects (cars, etc.)

How many polygons ?
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Being exact, what for?

• Intellectually rewarding ?
• Usability large scale geometry
• Terrain, buildings, architectural details
• Objects (cars, etc.)

Only 6 quads !
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Being exact, is it hard?

• Problem with constant steps
• Can always miss intersections

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Being exact, is it hard?

• Problem with constant steps
• Can always miss intersections
• Travel slowly above empty space

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Being exact, is it hard?

• Problem with constant steps
• Can always miss intersections
• Travel slowly above empty space
• Our contributions
• Amanatides based approach
• Failsafe approach
• But requires preprocess

solves
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Adapted Amanatides traversal

• We run along the ray on texel edges

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Adapted Amanatides traversal

• We run along the ray on texel edges
• Advantages
• very simple iterations
• exact intersection

fragment shader main loop
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Adapted Amanatides traversal

• We run along the ray on texel edges
• Advantages
• very simple iterations
• exact intersection
• Drawbacks
• potentially many iterations
• texture resolution dependent

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Adapted Amanatides traversal

• Texture resolution dependent
• Double resolution ? half speed
• Ideally should depend on variations of
  heightfield, not sampling frequency
• Binary search doesnt have this drawback
• Constant number of texture lookups

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Analysis of binary search

• At the moment we pass below
• At least one intersection
• But potentially several

?
2D slice
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Analysis of binary search

• At the moment we pass below
• At least one intersection
• But potentially several
• Ideally at most one intersection

?
2D slice
How can we guarantee this?
Precompute safety radius
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What is safety radius ?

• Local information

Which step ?
T
Precomputed radius
?
For any possible viewing ray passing above T
Top view
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What is safety radius ?

• Local information

Viewing direction
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What is safety radius ?

• Local information
• In 3D for each texel
• Scan every direction
• Keep the minimum radius

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Failsafe binary search
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Failsafe binary search
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Failsafe binary search
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Failsafe binary search
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Failsafe binary search
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Failsafe binary search
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Failsafe binary search
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Failsafe binary search
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Failsafe binary search
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Failsafe binary search
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Various details

• Textures
• Can have different resolutions
• Normal, color, height
• Can be packed/compressed
• Needs recent fragment shaders supporting dynamic
  loops

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Results

• Fast and exact
• Height-field regularity dependent
• Correct interaction with Z-buffer
• Output sensitive

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Height-field representation

• What can we represent?
• How to render it?
• Fast ?
• Exact?
• Previous work
• Our contributions

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More than bump

• Previous methods small scale bump
• Curved surfaces warping considerations
• VDM Wang 03
• Policarpo05
• We want to do large scale
• Problem limited expressiveness
• orthogonal height-fields

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Expressiveness of heightfields

• Lack of samples on vertical sides

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Projective height-fields
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Projective height-fields

• Similar to cubist images Hanson 98
• Projective transforms preserve lines
• Unmodified algorithm

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Projective height-fields

• Wasted fillrate
• Many eventually discarded fragments
• Easy to solve clip the bounding box

(a) not clipped
(b) clipped
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Multiple height-fields
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Conclusion

• Remember fast and exact is possible!
• Efficient representation
• Small memory footprint
• Automatic LOD
• Two useful methods (dynamic/static)

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Future

• Geometry representable with heightfields
• Because we are exact
• Model directly with heightfields
• Photoshop 3D Mok01
• Apply to judicious situations

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Future

• Thank you!
• Questions?