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SI31 Advanced Computer Graphics AGR

SI31 Advanced Computer Graphics AGR. Lecture 17 Radiosity - Conclusion Non-PhotoRealistic Rendering. Radiosity - Substructuring. Dilemma: accuracy in radiosity demands many, small patches efficiency in radiosity demands a few, large patches Substructuring provides a solution

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SI31 Advanced Computer Graphics AGR

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  1. SI31Advanced Computer GraphicsAGR Lecture 17 Radiosity - Conclusion Non-PhotoRealistic Rendering

  2. Radiosity - Substructuring • Dilemma: • accuracy in radiosity demands many, small patches • efficiency in radiosity demands a few, large patches • Substructuring provides a solution • each patch divided into a number of subpatches

  3. Substructuring - Managing the Complexity • Suppose N patches in all, M subpatches in all • What is complexity if we apply radiosity algorithm at subpatch level? • A compromise is to shoot from patch to subpatch • What is the resulting complexity?

  4. Algorithm : Progressive Refinement with Substructuring • Initial set-up stage For each patch i • Set initial increments Bi to Ei • For each subpatch s in patch i • set Bs = Ei Set initial ambient lighting (proportional to the average radiosity)

  5. Algorithm : Progressive Refinement with Substructuring • Select patch i with greatest unshot radiosity BiAi • build hemicube, calculate form factors Fi-s for all subpatches s in all patches • for each patch j seen by patch i do for each subpatch s in j seen by i Radiosity = RjBi Fi-s Ai / As Bs = Bs + Radiosity Bj = Bj + Radiosity As/Aj • Bi = 0

  6. Algorithm : Progressive Refinement with Substructuring • Compute vertex radiosities, decrease ambient component (proportional to shot radiosity) • Perform view-dependent projection and Gouraud shading • Repeat until convergence, selecting patch with greatest unshot radiosity • Question: how would you do the division into subpatches?

  7. Radiosity - Software • Radiosity software is commercially available from: • Lightwork Design of Sheffield http://www.lightwork.com • Lightscape http://www.lightscape.com • Catalogue of radiosity software: • http://www.ledalite.com/library-/rad.htm

  8. Radiosity – Some Links • Paul Heckbert’s radiosity page • www.cs.cmu.edu/~radiosity • ACM SIGGRAPH Hypergraph • www.education.siggraph.org/materials/HyperGraph/radiosity/radiosity.htm

  9. Combining Radiosity and Ray Tracing

  10. Non-Photorealistic Rendering

  11. Why Photorealistic? • Simple graphics rendering techniques produce rather dull, ‘dead’ images • Hence the research into achieving greater and greater photorealism • textures • bump mapping • environment mapping • ray tracing • radiosity • This research continues...

  12. Medical illustration From IBLS at Univ of Glasgow Why Non-Photorealistic? • In real-life, photographs are not always the best imagery • Schematic diagrams more useful in many applications • Artist is often able to convey greater expressiveness than a photographer • This has given rise to the field of non-photorealistic rendering

  13. Pen and Ink Illustrations • As an example of this approach, we shall look at computer-generated ‘pen-and-ink’ illustration

  14. Pen and Ink Illustration • Strokes: • Tracing out a path with nib of pen, different pressure gives different width • To appear natural, thickness will vary along path, lines will be wavy • Tones and textures: • combinations of strokes give both tone and texture • ‘indication’ used to economise on drawing each and every stroke

  15. Strokes, Tones, Textures, Indication

  16. Notice how non-photorealism depends heavily on outlines Both exterior and interior (eg in drawing leaves) Thick outlines can be used to indicate shadow Pictures from Intel 3D Software Technologies pages Pen and Ink Illustration

  17. Computer-generated Pen and Ink Illustration • Compared with traditional rendering: • tone and texture combined • 2D projection affects rendering • Pipeline includes: • modelling • texture assignment • reflection model to give tone • outlines added

  18. Strokes • Generated by moving nib along path • Character added by: • waviness function • pressure function

  19. Stroke Textures • Collection of strokes to give texture and tone • Prioritised so that different tones can be achieved • first only highest priority drawn • to increase tone, lower priorities drawn • For example: • highest priority to outline • next could be horizontal lines • then vertical, and so on

  20. Stroke Textures

  21. This can be handled semi-automatically by marking on drawing a set of ‘indicator lines’ Strokes closer to indicator lines have higher probability of being drawn Indication

  22. Indication

  23. Indication

  24. Outline • Boundary and interior outlines • Boundary outline texture associated with each stroke texture • Interior outlines drawn when two faces of similar tone are adjacent • Accented outlines for shadows

  25. Another Example - Digital Facial Engraving

  26. … and of course

  27. Further Reading • Papers by David Salesin, University of Washington • ‘Expressive Rendering: A Review of Nonphotorealistic Techniques’, by John Lansdown and Simon Schofield, IEEE Computer Graphics and Applications, 1995.

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