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Non-Photorealistic Rendering Painting, Drawing, Sketching

Non-Photorealistic Rendering Painting, Drawing, Sketching. Admin. Reminder: no class Monday Optional review/Q&A session: Thursday May 1, 3 PM Reminder: final exam Monday, May 5, 9 AM Reminder: assn4 due Monday Late days okay, but not beyond 11:59 PM Wednesday. Recap: Stochastic Sampling.

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Non-Photorealistic Rendering Painting, Drawing, Sketching

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  1. Non-Photorealistic RenderingPainting, Drawing, Sketching David Luebke 16/7/2014

  2. Admin • Reminder: no class Monday • Optional review/Q&A session: Thursday May 1, 3 PM • Reminder: final exam Monday, May 5, 9 AM • Reminder: assn4 due Monday • Late days okay, but not beyond 11:59 PM Wednesday David Luebke 26/7/2014

  3. Recap: Stochastic Sampling • Sampling theory tells us that with a regular sampling grid, frequencies higher than the Nyquist limit will alias • Q: What about irregular sampling? • A: High frequencies appear as noise, not aliases • This turns out to bother our visual system less! David Luebke 36/7/2014

  4. Recap: Stochastic Sampling • Poisson distribution: • Completely random • Add points at random until area is full. • Uniform distribution: some neighboring samples close together, some distant David Luebke 46/7/2014

  5. Recap: Stochastic Sampling • Poisson disc distribution: • Poisson distribution, with minimum-distance constraint between samples • Add points at random, removing again if they are too close to any previous points • Jittered distribution • Start with regular grid of samples • Perturb each sample slightly in a random direction • More “clumpy” or granular in appearance David Luebke 56/7/2014

  6. Recap: Stochastic Sampling • Spectral characteristics of these distributions: • Poisson: completely uniform (white noise). High and low frequencies equally present • Poisson disc: Pulse at origin (DC component of image), surrounded by empty ring (no low frequencies), surrounded by white noise • Jitter: Approximates Poisson disc spectrum, but with a smaller empty disc. David Luebke 66/7/2014

  7. Stochastic Sampling:The Exciting Conclusion • Distributed ray tracing is an elegant technique that tackles many problems at once • Stochastic ray tracing: distribute rays stochastically across pixel • Distributed ray tracing: distribute rays stochastically across everything David Luebke 76/7/2014

  8. Distributed Ray Tracing • Distribute rays stochastically across: • Pixel for antialiasing • Light sourcefor soft shadows • Reflection function for soft (glossy) reflections • Time for motion blur • Lens for depth of field • Cook: 16 rays suffice for all of these David Luebke 86/7/2014

  9. Distributed Ray Tracing • Distributed ray tracing is basically a Monte Carlo estimation technique • Practical details: • Use lookup tables (LUTs) for distributing rays across functions • See W&W Figure 10.14 p 263 • Learn lots more about this in Image Synthesis David Luebke 96/7/2014

  10. Non-Photorealistic Rendering • Next topic: Non-Photorealistic Rendering David Luebke 106/7/2014

  11. Non-Photorealistic Rendering “Using a term like ‘nonlinear science’ is like referring to the bulk of zoology as ‘the study of nonelephant animals’” • Photorealism: age-old goal of graphics • Non-photorealistic rendering • Broadly, techniques for rendering that don’t strive for realism, but style, expressiveness, abstraction, uncertainty, etc • A terrible, terrible term that we’re probably stuck with • Better terms: stylized rendering, artistic rendering, abstract rendering David Luebke 116/7/2014

  12. Stylized Rendering • NPR is most commonly used to refer to graphics techniques that emulate a particular artistic style or medium • Impressionistic painting • Pen-and-ink (cross hatching, outlining, etc) • Colored-pencil, copperplate engraving, you name it • Cartoon drawing David Luebke 126/7/2014

  13. Toon shading • Toon shading is the term that we use to refer to cartoon-like rendering effects • Tendencies in cartoon/comic style: • Simple, flat shading (cel shading) • Two-tone (light/shadow) or three-tone (light/shadow/highlight) • Edge highlighting • Boundary (border edge) • Crease (hard edge) • Material edge • Silhouette edge David Luebke 136/7/2014

  14. Silhouette Rendering • Boundary (border edge) • Not shared by two polygons • E.g. the edge of a sheet of paper • Solid models usually have no boundaries • Crease (hard edge) • Shared by two polygons at a dihedral angle greater than some threshold (often 60°) • Or a vertex with two normals/two colocated vertices • Ridge or valley edges David Luebke 146/7/2014

  15. Silhouette Rendering • Material edge • Triangles sharing edge have different materials/texture maps/etc • Or just an edge that the artist wants to emphasize • Silhouette edge • Triangles sharing edge face different directions (towards/away from viewer) • Lots of techniques to find at runtime! David Luebke 156/7/2014

  16. Summary: Edge Highlighting • Toon shading (and other NPR techniques based on drawing) requires some edges be drawn or highlighted: • Silhouette edges • Mesh boundaries (always on silhouette) • Creases (ridge and valley) • Material boundaries • Find first at run-time, precalculate the others (unless object is deformable) David Luebke 166/7/2014

  17. Recap: Silhouette Edges • Surface angle silhouetting • Calc N●V, if below threshold  draw black • Best as a per-pixel routine • E.g., Assn4c • Also can do with a spheremap, or use a mip-map with top-level textures dark • Pros: • Uses the texture hardware  fast • Can antialias the resulting lines • Cons: • Line width depends on curvature • Doesn’t work for some models (e.g., a cube) David Luebke 176/7/2014

  18. Recap: Silhouette Edges • Procedural Geometry Silhouetting • Idea: render the geometry in such a way that the silhouettes “fall out”, e.g.: • Draw frontfacing polygons • Draw backfacing polygons • But draw them in (possibly thick) wireframe • Or draw them z-biased forward a bit • Or “fatten” them • Or displace them along their normals (“halo” effect) • Flip normals • Amount of displacement varies w/ distance (why?) • Perfect task for vertex shader! • Pros: relatively robust, doesn’t need connectivity info • Cons: Wastes some fill & some polys, needs antialiasing David Luebke 186/7/2014

  19. Results Wireframe Translation Fattening 40 fps 50 fps 11.5 fps Original Venus model : 5672 triangles, 66 fps David Luebke 196/7/2014

  20. Recap: Silhouette Edges • Image Processing Silhouetting • Idea: analyze the image after it’s rendered, and extract silhouettes (i.e., edge detection) • Perfect for fragment program! • Can help by rendering e.g. depth image, object-ID image, normal image David Luebke 206/7/2014

  21. Recap: Silhouette Edges • Silhouette Edge Detection • Idea: find silhouette edges geometrically on the CPU and render them explicitly • Brute force: test every edge to see if its adjoining polygons face opposite directions in eye space • Can speed this up with randomized coherent search • Most work, but gives the most flexibility in how silhouettes are drawn • GPU variant: • Draw degenerate quadrilateral at each edge • Use vertex shader to “fatten” quad into a “fin” when edge is on silhouette • Fin thickness based on distance to eyepoint David Luebke 216/7/2014

  22. Highlighting Ridge Edges • Clever related technique by Raskar: • Add “fins” to every edge at dihedral angle • Size fins according to distance to viewer • Again, perfect for vertex shader • Similar but more complicated technique for highlighting valley edges David Luebke 226/7/2014

  23. Drawing Lines:Outlining Polygons • Surprisingly hard to draw polys as filled outlines • Problem: depth buffer values of edge & polys same • 2-pass technique: draw polys, then draw edges • Z-bias edges forward or polygons back (glPolygonOffset) • Works okay, but has occasional problems • 3-pass technique: • Render filled polygon • Disable depth buffer writes (leave depth test on) • Enable color buffer writes • Render polygon edges polygon • Normal depth & color buffering • Render filled polygon again • Enable depth buffer writes • Disable color buffer writes David Luebke 236/7/2014

  24. Drawing Lines:Hidden-Line Rendering • Hidden-line vs. obscured line vs halos • Hidden-line • Draw polygons to depth buffer (not color buffer) • Draw edges using previous technique • Obscured (light, dotted, dashed) line • Draw all edges in obscured style • Draw polygons to depth buffer (not color buffer) • Draw edges using previous technique • Haloed line • Draw all edges as thick background-color lines • Draw edges using biasing, foreground-color David Luebke 246/7/2014

  25. Other Styles • Impressionistic or “painterly” rendering: • Sprinkle particles on object surface • Draw particles as brushstrokes • Can render images to encode normals, surface curvature, depth, color/tone info David Luebke 256/7/2014

  26. Painterly Rendering • More info if time permits… David Luebke 266/7/2014

  27. Other Styles • Hatching: • Store different cross-hatch patterns representing different tones as textures • Clever ways to use texture hardware to blend between tones at run-time • More info if time permits… David Luebke 276/7/2014

  28. Other Styles • “Graftals” are a general term used for strokes, decals, little bits of geometry • Dynamic placement of graftals to achieve certain effects/styles: David Luebke 286/7/2014

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