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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.