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Google Summer of Code 2017

Weekly Report on Many-light Sampling Method

Goal of the project is to integrate the new many-light sampling algorithm proposed by Nathan Veghdal

1. May 15^th - May 30^th (Community Bonding Period)

2. May 31^st – June 2^nd

3. June 5^th - June 9^th

4. June 12^st – June 16^th

5. June 19^th – June 23^rd

6. June 26^th – June 30^th

7. July 3^rd – July 7^th

8. July 10^th – July 14^th

9. July 17^th – July 21^st

10. July 24^th – July 28^th

11. July 31^st – August 4^th

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Solid Angle Light Evaluation

• Psychopath doesn't have MIS implemented

• Shading point in pathvertex can only provide point on the surface to be evaluated, but not the ShadingPoint hit before

  • both DirectLightIntegrator and PathTracer have knowledge about the parent shading point, we just need to expose it to the PathVertex
  • I am currently working on it, but it seems that I am missing a condition when assigning it so I get an assertion error right now

• Store all shading points of a single path into a memory arena within the PathTracer

• Store last parent shading point

• Expose the parent shading point to the PathVertex

• use ShadingPoint instead of a regular point in node evaluation

• Use parent shading point for evaluate_pdf both in DirectLightIntegrator and PathTracer

• added projected solid angle light evaluation

• debugged edge case output from sub_hemispherical_light_source_contribution() when node probabilities p1 and p2 are of similar values with different signs which is illegal! e.g.

approx_contribution p1:   -5.144703
approx contribution p2:    5.185083
                 total:    0.040380
                    p1:   -127.405693
                    p2:    128.405701

• The bug was introduced with the solid angle eval approach
• FIX: handle an edge case where a negative 0 is returned

Light tree with hemispherical light source approximation
converging 30s
CDF	LightTree
converging 1min

Light Tree node evaluation comparison
Squared distance	Hemispherical light approximation
25spp

NPL-EMT mixed evaluation
CDF	lightTree
converging

The new hemispherical approximation produces a defect in rendering with the EMT-NPL scene. (shadow to light transition on the sphere)

• Built an many EMT light test scene containing 50,340 EMTs

  • use of LightTree reduces noise at 16spp but doesn't offer speed improvement

• Tested SAHPartitioner with the current LightTree

  • SAHPartitioner does not produce correct result because it doesn't force the split for overlapping bboxes
  • Such behavior results with multiple EMTs being stored in a single node and makes the rendering incorrect because some lights get completely ignored.

Appleseed basement workshop scene
CDF	LightTree
16spp
Converging 1 min

Currently working on:

• FIX the rendering artefact in the EMT-NPL scene
• Add reference in the code to the used article

LightTree seem to be working a bit slower than the CDF approach for small number of light sources.

• Produce many light test scenes
  • many NPL light sources
  • many EMT light sources
  • many mixed light sources

Ideas on Implementing Light Tree Sampling Based on the Light Orientation

• Cone of normals
• K-means of children vertices

Optimization TODO

• Store items within the LightTree nodes
• Store bbox node info within the node itself
• Reduce number of variables set by recursive_node_update() (i.e. variables which can be set during the building time)

Collection of useful papers

• A Spherical Cap Preserving Parameterization for Spherical Distributions, 2017
• Importance Sampling of Many Lights With Adaptive Tree Splitting