17.1 Si31_2001 SI31 Advanced Computer Graphics AGR Lecture 17 Radiosity - Conclusion Non-PhotoRealistic Rendering

17.2 Si31_2001 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

17.3 Si31_2001 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?

17.4 Si31_2001 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)

17.5 Si31_2001 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

17.6 Si31_2001 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 n Question: how would you do the division into subpatches?

17.7 Si31_2001 Radiosity - Software n Radiosity software is commercially available from: – Lightwork Design of Sheffield – Lightscape n Catalogue of radiosity software: – rary-/rad.htm

17.8 Si31_2001 Radiosity – Some Links n Paul Heckbert’s radiosity page – n ACM SIGGRAPH Hypergraph – yperGraph/radiosity/radiosity.htm

17.9 Si31_2001 Combining Radiosity and Ray Tracing

17.10 Si31_2001 Non-Photorealistic Rendering

17.11 Si31_2001 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...

17.12 Si31_2001 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 Medical illustration From IBLS at Univ of Glasgow

17.13 Si31_2001 Pen and Ink Illustrations n As an example of this approach, we shall look at computer-generated ‘pen-and-ink’ illustration

17.14 Si31_2001 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

17.15 Si31_2001 Strokes, Tones, Textures, Indication

17.16 Si31_2001 Pen and Ink Illustration n Notice how non- photorealism depends heavily on outlines – Both exterior and interior (eg in drawing leaves) n Thick outlines can be used to indicate shadow n Pictures from Intel 3D Software Technologies pages

17.17 Si31_2001 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

17.18 Si31_2001 Strokes n Generated by moving nib along path n Character added by: – waviness function – pressure function

17.19 Si31_2001 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

17.20 Si31_2001 Stroke Textures

17.21 Si31_2001 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

17.22 Si31_2001 Indication

17.23 Si31_2001 Indication

17.24 Si31_2001 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

17.25 Si31_2001 Another Example - Digital Facial Engraving

17.26 Si31_2001 … and of course

17.27 Si31_2001 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.