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Propagating the VLF - Problems and Solutions II

Propagating the VLF - Problems and Solutions II. Insu Yu i.yu@cs.ucl.ac.uk. Overview. Specular Transfer Determination of reflected Direction Forward/Backward Transfer Discontinuity and Resampling Caustics What is caustics Holes & Jittering Propagation Issues Progressive propagation

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Propagating the VLF - Problems and Solutions II

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  1. Propagating the VLF - Problems and Solutions II Insu Yu i.yu@cs.ucl.ac.uk

  2. Overview • Specular Transfer • Determination of reflected Direction • Forward/Backward Transfer • Discontinuity and Resampling • Caustics • What is caustics • Holes & Jittering • Propagation Issues • Progressive propagation • Effect of Parameter Adjustment

  3. Specular Reflection • What is Ideal specular reflection model ? • All the incoming Energy (L) is reflected off the direction(R) of a surface • Incidence angle (θi) equals the angle of specular reflection (θr) • A Mirror Reflection • How can we adapt this model on VLF ?

  4. Determination of reflected direction • Discrete ‘Uniformly Sampled Directions’ • Due to the Representation of Finite Directions, an Ideal reflected PSF direction can hardly be found VLF Specular Reflection PSF Directions (Quadrant)

  5. Nearest direction Select closest direction to the reflected direction Works better on High Resolution Directions Nearest Vs Tri-linear Directions Finding Nearest PSF Direction

  6. Nearest Vs Tri-linear Directions (Cont’) • Tri-linear • Select Three PSF direction which enclose the reflected Direction • Transfer Incoming Energy to three Directions according to barycentric weights of each direction • Glossy not perfect specular • Tri-linear interpolated transfer introduce excessive blurring Tri-linear Transfer

  7. Forward Vs Backward Mapping • Forward mapping • Diffuse to Specular Transfer • Shooting Energy from Sender to Receiver • Miss-alignment of source to destination leads holes Holes Forward Mapping

  8. Forward Vs Backward Mapping(Cont’) • Backward mapping • Trace a ray in Reflected Direction in back order • One-to-One mapping • UV uniform subdivision on receiver planes • Need to use bilinear interpolation scheme to pull the radiance values Backward Mapping

  9. Discontinuities & Resampling • Specular Reflection is not jittered • Diffuse Surfaces are jittered multiple time • Due to Directional Energy Transfer • URM is updated only once where Diffuse to Specular Transfer occurs • Blocky discontinuities appear despite Tri-linear transfer • Resample all TRM at end of propagation by backwards ray tracing Resampling

  10. Relation to Directions • Accuracy of Specular Transfer is proportional to the number of directions Ray Tracing Light Field Specular Reflection (ES*D)

  11. Caustics

  12. Caustics • Caustics comes free as in Specular to Diffuse Transfer

  13. Specular to Diffuse Transfer • Caustics present where Diffuse surface gathers energy from specular senders

  14. Holes and Jittering • Holes Artefact • Due to representation of Discretisation Directions • Transfer from small reflector to large receiver • Long distance between reflector and receiver • Increase sampling density of directions • More jittered samples for caustic

  15. Filtering • Filtering to achieve accuracy and avoid aliasing • Caustics on Diffuse map can be excessively Blurred & introduce light leaking • Require Adjustment of Gaussian Kernel Sigma • Possible to have higher resolution diffuse maps on caustic receivers • Can exploit additional map for caustics

  16. Propagation • Propagation Process over various iterations

  17. Progressive Propagation • Progressive propagation framework • Estimating unshot radiance • Selecting shooting sources & managing swapping of maps • Purging of unshot energy • Zero-energy surfaces are never senders • Other issue • Capping of scenes

  18. Various Polygons Various PSF Directions Various PSF Size & Tile Size Propagation time and on memory Dual Xeon 1.7Ghz Scalability Test (Effect of parameters)

  19. Propagation time varies quadratically with the number of polygons (513 Direction, 8x8 Tiles, 64x64 Cells) The memory grows linearly with the number of polygons TEST Scene One Emitter polygons 224 to 1736 5:1 ratio of diffuse to specular surfaces Effect of parameters (Polygons)

  20. Effect of parameters (Directions) • Increasing PSF Directions • Pros • The greater accuracy is achieved • Less jittering is necessary (overcome missing holes) • Cons • More memory usage • Longer Propagation Time • Linear Relationship between the number of direction and propagation time/memory Office Test Scene

  21. Effect of parameters (PSF/TileResolution) • Size of Tile/PSF resolution determine the speed of propagation & rendering • Increasing Resolution results in faster intersection searching but more memory & propagation time

  22. ? Question

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