GR2 Advanced Computer Graphics AGR

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GR2Advanced Computer GraphicsAGR

• Lecture 17
• Radiosity - Conclusion
• Non-PhotoRealistic Rendering

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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

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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?

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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)

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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

• 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 - 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
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Non Photorealistic Rendering
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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?

• 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 Illustrations

• As an example of this approach, we shall look at
  computer-generated pen-and-ink illustration

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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

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Strokes, Tones, Textures, Indication
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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

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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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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

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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

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Stroke Textures
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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

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Indicator Lines
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Indication
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Indication
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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

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Another Example - Digital Facial Engraving
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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.