1gr2-00 gr2 advanced computer graphics agr lecture 17 radiosity - conclusion non-photorealistic...
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GR2Advanced Computer
GraphicsAGR
GR2Advanced Computer
GraphicsAGR
Lecture 17Radiosity - Conclusion
Non-PhotoRealistic Rendering
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Radiosity - SubstructuringRadiosity - 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
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Substructuring - Managing the Complexity
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 patchpatch to subpatchsubpatch
What is the resulting complexity?
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Algorithm : Progressive Refinement with Substructuring
Algorithm : Progressive Refinement with Substructuring
Initial set-up stageFor 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)
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Algorithm : Progressive Refinement with Substructuring
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
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Algorithm : Progressive Refinement with Substructuring
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
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Radiosity - SoftwareRadiosity - 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
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Combining Radiosity and Ray Tracing
Combining Radiosity and Ray Tracing
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Non Photorealistic RenderingNon Photorealistic Rendering
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Why Photorealistic?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...
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Why Non-Photorealistic?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
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Pen and Ink IllustrationsPen and Ink Illustrations
As an example of this approach, we shall look at computer-generated ‘pen-and-ink’ illustration
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Pen and Ink IllustrationPen 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
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Strokes, Tones, Textures, Indication
Strokes, Tones, Textures, Indication
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Pen and Ink IllustrationPen 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
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Computer-generated Pen and Ink Illustration
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
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StrokesStrokes
Generated by moving nib along path
Character added by:– waviness function– pressure function
Note clipping needs to be adapted to this style of drawing
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Stroke TexturesStroke 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
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Stroke TexturesStroke Textures
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IndicationIndication
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
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Indicator LinesIndicator Lines
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IndicationIndication
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IndicationIndication
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OutlineOutline
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
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Another Example - Digital Facial Engraving
Another Example - Digital Facial Engraving
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Further ReadingFurther 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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