1. Non-photorealistic Rendering Pablo Picasso - The Bird Cage No electrons were harmed during the production of this presentation.

2. Non-photorealistic Rendering Interactive Technical Illustration –Bruce Gooch, Peter-Pike J.Sloan, Amy Gooch, Peter Shirley, Richard Riesenfeld (CS Dept, University of Utah) Image Precision Silhouette Edges –Ramesh Raskar(UNC @ Chapel Hill) –Michael Cohen(Microsoft Research)

3. Interactive Technical Illustration Using NPR techniques to clarify an illustration. http://www.cs.utah.edu/~bgooch/ITI

4. Interactive Technical Illustration Technical documentation illustrations are typically 2D, static and hand-drawn. Uses abstraction to convey information on shape, structure and material composition. How can technical illustration techniques be applied to an interactive 3D display ?

5. 2D Principles Applied Use line weight to suggest structure : –single line weight. –heavy edges, thin inner lines. –vary weight to improve perspective.

6. Three common shading models : –diffuse interpolate from cool to warm color –metallic –banded ( a |cos t| + (1-a) ) ^ p produces a “back-splash” of light. Model’s inner lines can be used as highlights. 2D Principles Applied

7. Shadowing can hint at 3D structure. –Used only when doesn’t mask details. –Will use cast shadow on ground plane. –High accuracy is not really important.

8. Interactive Principles Applied The object moves, not the viewer: –Moving highlights convey more information. –Each object is lit independently. Multiple highlights would be confusing.

9. Implementation Silhouettes and important edges –Hardware vs. software –Line style. Shading –Metal shading Shadowing –Soft vs. hard shadows.

10. Silhouettes & Important Edges “A silhouette is an edge that is connected to both a front-facing and a back-facing polygon.” Hardware Method GL & the stencil buffer. Draw shaded front faces Draw front faces in line mode in stencil buffer Draw back faces in line mode Setting color if stencil buffer was set, decrementing stencil buffer if drawn. Environment map.

11. Silhouettes & Important Edges Software Method - Math and bottlenecks : –Equation to determine silhouette edges. –Tested faster using a hierarchical Gauss map. –Parallelizable. Not a silhouette edge.A silhouette edge.

12. Silhouettes & Important Edges Line weight and creases: –Use shading or texture mapping to vary weight. –Extract creases and draw as white highlights. Can be covered by silhouette or shading. Doesn’t state HOW the creases are extracted.

13. Silhouettes & Important Edges Line weight and creases: –Use shading or texture mapping to vary weight. –Extract creases and draw as white highlights. –Draw thick boundary lines. Draw only over the object (stencil buffer). –Draw boundary silhouettes. Software edge list limits redrawing. –Fill gaps with large points at edge ends.

14. Shading Hardware environment mapping is used for its flexibility. Metal shading –Create a metal texture. –Add Phong highlights. –Sufficient to convey material information.

15. Shadowing Cast only shadows on the ground plane. Soft shadows : –Project shadow on a stack of slightly offset planes and accumulate them. –Assumes the silhouette remains the same. –Not exact, but faster.

16. Image Precision Silhouette Edges A hack that does the job quickly. http://www.cs.unc.edu/~raskar/Sil/

17. Image Precision Silhouette Edges The problem: –Find all silhouette edges. –Render only those which are not occluded.

18. Image Precision Silhouette Edges The problem: –Find all silhouette edges. –Render only those which are not occluded. Gooch’s answer : –Find them all. –Render them all. Several times if needs be.

19. Image Precision Silhouette Edges The problem: –Find all silhouette edges. –Render only those which are not occluded. Raskar’s answer : –Cheat.

20. The method First render the front-facing polygons. Render the back-facing polygons in black. –If needed, BF polygons can be found by reading back the frame buffer. Problem: This produces silhouettes at most 1 pixel wide. We need some preprocessing to be done.

21. How to render the BF polygons Render them as thick wireframe. –Easy and fast. –May leave gaps when lines get too thick. Translate the back-facing polygons. –e.g. using glPolygonOffset(). –May not give uniform outlines. Use view dependent modifications. –Can have ‘artistic’ results.

22. Results WireframeTranslationFattening 40 fps 50 fps 11.5 fps Original Venus model : 5672 triangles, 66 fps

23. Conclusion...