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GR2 Advanced Computer Graphics AGR

GR2 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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GR2 Advanced Computer Graphics AGR

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  1. GR2Advanced 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

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

  8. Combining Radiosity and Ray Tracing

  9. Non Photorealistic Rendering

  10. 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...

  11. 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

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

  13. 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

  14. Strokes, Tones, Textures, Indication

  15. Pen and Ink Illustration • Outlines: • realistic scenes have no outlines • non-photorealism depends heavily on exterior and interior outlines (eg leaves) • thick outlines can indicate shadow • indication again important

  16. 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

  17. Strokes • Generated by moving nib along path • Character added by: • waviness function • pressure function • Note clipping needs to be adapted to this style of drawing

  18. 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

  19. Stroke Textures

  20. Indication • 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

  21. Indicator Lines

  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. 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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