Single Pass Point Rendering and Transparent Shading

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Single Pass Point Rendering and Transparent
Shading

• Paper by Yanci Zhang and Renato Pajarola
• Presentation by
• Harmen de Weerd and Hedde Bosman

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The standard 21 algorithm

• Visibility Pass
• Smooth point interpolation and shading Pass
• Normalization/shading Pass
• Well known from the lab sessions

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The standard 21 algorithm cons

• 2 passes over the dataset with computational
  intensive shaders
• Transparency hard to achieve
• Back to front alpha blending, z-buffer is turned
  off
• Interpolation between overlapping splats in one
  layer uses the z-buffer to cull fragments that do
  not belong in this one layer (i.e. are not
  visible).
• Possible with depth-peeling seen in previous
  presentation, but uses multiple geometry passes
• One other algorithm proposed for alpha blending
  which cannot use the GPU

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A new algorithm

• Idea
• Create multiple groups of the point set that do
  not need a separate visibility pass
• Render an image for each group
• postpone e-z-buffer test and smooth point
  interpolation to an image composition pass

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11 algorithm
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Deferred Blending
Divide point set S in K Groups
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11 Algorithm Grouping
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11 Algorithm Grouping
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Largest First grouping algorithm
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Largest First grouping algorithm
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Largest First grouping algorithm
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Largest First grouping algorithm
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Largest First grouping algorithm
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11 Algorithm Images...

• Create K images from K groups
• Add depth info to images.
• Since splats do not overlap in object space
  (because of grouping) we do not need to worry
  about visibility culling
• Do use z-buffers since splats might overlap in
  image space

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11 Algorithm Image composition

• For each resulting fragment
• Determine the minimum depth of the fragment
• Determine the sum of the color components if the
  depth is within e of the minimum depth
• Determine the sum of all alpha components if the
  depth is within e of the minimum depth
• Normalize the colors using the alpha RGBRGB/A

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11 Algorithm Transparency

• Previous algorithm does not account for
  transparency, so...
• Adapt grouping algorithm to make sure no holes
  exists in surfaces of one group.
• Render each group image using alpha blending

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Optimal grouping
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Transparency Problems

• cannot account weight and alpha simultaneously
• Need full surface coverage to look 'through' a
  material layer
• Should not have overlap between splats

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Transparency Problems 2,3
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Transparency Solutions

• cannot account weight and alpha simultaneously
• Solution
• Do not account the weight.
• Artifacts are reduced dramatically by multiple
  transparent surface layers
• Small errors aren't even visible because color is
  only 8bit.

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Grouping algorithm extensions

• Relaxation of some edge definitions
• Virtually co-planer overlapping splats can go in
  the same group
• If normals of splats are in opposite directions
  (n1 . N2 lt 0) the splats can be in the same
  group.
• Relax overlap condition with user defined
  parameter
• Add points to multiple groups

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Grouping algorithm extensions
Grouping results. a) Splats have smaller overlaps
but less surface coverage for K 8. b) Splats
have bigger overlaps but better surface cover for
K 4.
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Fragment culling

• Optimally in each transparent surface layer there
  is exactly one fragment contributing to
  alpha-blending per pixel
• Two methods to create this situation
• Reduce z-buffer precision
• increase surface coverage in a group image
• Increased splat radius
• Add splats to multiple groups
• Decrease number of groups

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Basic 11 Algorithm (1)?

• For each group
• Render splat with color, depth and kernel weight
• Voronoi enhanced depth buffer (lower kernel
  weight is culled)?
• For each pixel
• Determine the minimum depth
• For each group image
• Sum the color and weight attributes if the
  fragment is in the nearest layer.
• Average the sum of color by the sum of weights.

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Voronoi enhance
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Basic transparency algorithm (2)?

• Use BSP-tree for efficient back-to-front ordering
• Transparency blending pass (using a-blending)
• Render all splats pi of each group Sk using
  modified radii r into separate target images Ik
• Perform back-to-front a-blending using material
  transparency afrag
• cnew afragcold (1 - afrag) cfrag
• Compositing pass (PBR blending)
• Average color of all images Ik into final
  framebuffer

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Reduction in z-buffer accuracy

• Overlapping splats cause too much attenuation
• Solution cull fragments that are too close to
  rendered fragments
• Report the depth of each fragment in steps of
  size e

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High quality transparency algorithm (3)?

• Geometry pass for nearest layer
• Use basic algorithm 1 to get depth and kernel
  information about the nearest layer
• Geometry pass for other layers
• Use transparency algorithm 2, but cull all
  fragments from nearest layer using depth mask the
  first pass
• Composition pass
• Perform smooth point interpolation for nearest
  layer
• Average the color over all images for other
  layers
• Combine the nearest layer with the other layers

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Reflections and refractions

• Refraction effects and specular reflection
  improve rendering realism
• Both are derived from incident viewing vector,
  surface normal, and environment mapping
• Can be added to the nearest layer pass of
  algorithm 3
• Downside only reflections and refractions for
  nearest visible layer

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Multi-layer reflection/refraction

• Accumulate opacity over all layers for each group
  Sk separately
• Assuming constant material opacity a, approximate
  number of layers l atotal / a
• Ratio of light absorption (1 a)l
• Transmitted total refraction sin qT hl sin qI

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Single vs Multi-layer refraction
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Per fragment shading

• As with our lab sessions, all needed attributes
  per fragment are interpolated
• Deferred shading approach can be used for any
  attribute other than color
• Phong lighting
• Environment map reflection
• Multi-layer refraction
• Attenuation

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Visual results
Combining opaque and transparent objects
Single-pass (a) versus two-pass (b) algorithm
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Visual results
Depth Peeling vs Algorithm 3 vs Algorithm 2
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Speed Comparison
Standard 21 PBR algorithm vs the novel 11 pass
PBR algorithm
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Visual Comparison
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Ball Joint
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David Head
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Female model
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Q / A