1GR2-00 GR2 Advanced Computer Graphics AGR Lecture 17 Radiosity - Conclusion Non-PhotoRealistic Rendering

2GR2-00 Radiosity - Substructuring n Dilemma: – accuracy in radiosity demands many, small patches – efficiency in radiosity demands a few, large patches n Substructuring provides a solution – each patch divided into a number of subpatches

3GR2-00 Substructuring - Managing the Complexity n Suppose N patches in all, M subpatches in all n What is complexity if we apply radiosity algorithm at subpatch level? patch subpatch n A compromise is to shoot from patch to subpatch n What is the resulting complexity?

4GR2-00 Algorithm : Progressive Refinement with Substructuring n Initial set-up stage For each patch i – Set initial increments B i to E i – For each subpatch s in patch i » set B s = E i Set initial ambient lighting (proportional to the average radiosity)

5GR2-00 Algorithm : Progressive Refinement with Substructuring Select patch i with greatest unshot radiosity B i A i – build hemicube, calculate form factors F i-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 = R j B i F i-s A i / A s B s = B s + Radiosity B j = B j + Radiosity A s /A j – B i = 0

6GR2-00 Algorithm : Progressive Refinement with Substructuring n Compute vertex radiosities, decrease ambient component (proportional to shot radiosity) n Perform view-dependent projection and Gouraud shading n Repeat until convergence, selecting patch with greatest unshot radiosity

7GR2-00 Radiosity - Software n Radiosity software is commercially available from: – Lightwork Design of Sheffield n Catalogue of radiosity software: –

8GR2-00 Combining Radiosity and Ray Tracing

9GR2-00 Non Photorealistic Rendering

10GR2-00 Why Photorealistic? n Simple graphics rendering techniques produce rather dull, dead images n Hence the research into achieving greater and greater photorealism – textures – bump mapping – environment mapping – ray tracing – radiosity n This research continues...

11GR2-00 Why Non-Photorealistic? n In real-life, photographs are not always the best imagery n Schematic diagrams more useful in many applications n Artist is often able to convey greater expressiveness than a photographer n This has given rise to the field of non- photorealistic rendering

12GR2-00 Pen and Ink Illustrations n As an example of this approach, we shall look at computer-generated pen-and-ink illustration

13GR2-00 Pen and Ink Illustration n 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 n Tones and textures: – combinations of strokes give both tone and texture – indication used to economise on drawing each and every stroke

14GR2-00 Strokes, Tones, Textures, Indication

15GR2-00 Pen and Ink Illustration n 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

16GR2-00 Computer-generated Pen and Ink Illustration n Compared with traditional rendering: – tone and texture combined – 2D projection affects rendering n Pipeline includes: – modelling – texture assignment – reflection model to give tone – outlines added

17GR2-00 Strokes n Generated by moving nib along path n Character added by: – waviness function – pressure function n Note clipping needs to be adapted to this style of drawing

18GR2-00 Stroke Textures n Collection of strokes to give texture and tone n Prioritised so that different tones can be achieved – first only highest priority drawn – to increase tone, lower priorities drawn n For example: – highest priority to outline – next could be horizontal lines – then vertical, and so on

19GR2-00 Stroke Textures

20GR2-00 Indication n This can be handled semi- automatically by marking on drawing a set of indicator lines n Strokes closer to indicator lines have higher probability of being drawn

21GR2-00 Indicator Lines

22GR2-00 Indication

23GR2-00 Indication

24GR2-00 Outline n Boundary and interior outlines n Boundary outline texture associated with each stroke texture n Interior outlines drawn when two faces of similar tone are adjacent n Accented outlines for shadows

25GR2-00 Another Example - Digital Facial Engraving

26GR2-00 Further Reading n Papers by David Salesin, University of Washington n Expressive Rendering: A Review of Nonphotorealistic Techniques, by John Lansdown and Simon Schofield, IEEE Computer Graphics and Applications, 1995.