Non-Photorealistic Rendering FORMS

Mesh (~170) Parametric (~16) Implicit/CSG (~9) Volumetric (~28) 3D Object Representation in NPR Point Cloud (~4)

Fundamental Goals Traditional Illustration Create convincing impressions of 3D forms on paper Create convincing impressions of 3D forms on paper Depict solid masses in space Depict solid masses in space Show their various structural conditions Show their various structural conditions Suggest their different surface characteristics Suggest their different surface characteristics

Communication Pipeline in Science Illustration SCIENTISTILLUSTRATOR Provides material Requests information description Records information specimen Studies specimen Makes rough drawing Makes detailed preliminary drawing Checks detailed preliminary drawing Corrects preliminary drawing Checks corrections Produces rendering Checks rendering Labels drawing Checks labeling Return specimen

Composition Principles Finished Rendering Initial Sketch Rendering Techniques Line (Ink) Scratch Board Pencil Coquille Board Wash, … Subject Anatomy Botany Zoology Paleontology Archeology Study Selection Forms Lighting Steps

Study the Subject! Illustration terminology Communicate subtleties and eliminate ambiguities Communicate subtleties and eliminate ambiguities Move the subject around, do not just concentrate on one part Move the subject around, do not just concentrate on one part Observe the general appearance first Observe the general appearance first Identify the most significant features Identify the most significant features Only include detail which is necessary in your drawing Only include detail which is necessary in your drawing

Study the Subject! Illustration terminology Extract major features (folding regions, surfaces areas, volumes, curvatures) Highlights to visually eliminate extraneous details and reduce the subject to simple lights and darks Textures, angles where different forms join, perspective, foreshortening

Analyze the Object! NPR terminology Exactly same goals as in traditional illustration!

Model dependent Threshold dependent View dependent Outline form of the object Interior form of the object

Model dependent Threshold dependent View dependent Outline form of the object Interior form of the object Boundary

Outline : Model  Boundary Only in non-closed models Polygonal model : at least one polygon has at least one edge that is not shared with any other polygon

The vertex is adjacent to a boundary edge Outline : Model  Boundary  Cusps (Vertex)

Model dependent Threshold dependent View dependent Outline form of the object Interior form of the object Boundary Silhouette

Outline : View  Silhouette Silhouette point : point on the surface where V (u, v). N (u, v) = 0 or the angle between V(u, v) and N(u, v) is 90 0

Outline : View  Silhouette N. V < 0 N. V > 0 N. V = 0 Polygon is perpendicular to the view direction An edge between two polygons is a silhouette edge if the edge is shared by a front-facing and a back-facing polygon

The vertex is adjacent exactly 2 silhouette edges, one front-facing and the other back-facing Outline : View  Silhouette  Cusps (Vertex)

The vertex is adjacent to more than 2 silhouette edges Outline : View  Silhouette  Cusps (Vertex)

The vertex is adjacent exactly 2 silhouette edges, one front- facing and the other back-facing The vertex is adjacent to more than two silhouette edges Outline : Cusps (Vertex) The vertex is adjacent to a boundary edge

Terminology on Silhouettes Traditional Illustration NPR Profile (Computer Vision, Modeling) Silhouette Boundary

Model dependent Threshold dependent View dependent Outline form of the object Interior form of the object Boundary Silhouette X

Model dependent Threshold dependent View dependent Outline form of the object Interior form of the object Boundary Crease (+ other discontinuities) Silhouette X

Polygonal model: an edge between two front-facing polygons whose dihedral angle theta is above some threshold NURBS surface: any region where the surface normal changes abruptly, i.e., C 1 discontinuities Interior : Model/Threshold  Crease

Model dependent Threshold dependent View dependent Outline form of the object Interior form of the object Boundary Crease (+ other discontinuities) Crease (+ other discontinuities) Silhouette X

Interior : Model/Threshold  Other Discontinuities Sousa et al., Precise Ink Drawing of 3D Models, Eurographics 2003

Pulla, Curvature Based Segmentation of 3D Meshes, Prism/3DK Pulla, Curvature Based Segmentation of 3D Meshes, Prism/3DK ASU, 2000 Interior : Model/Threshold  Other Discontinuities

Alliez et al., Anisotropic Polygonal Remeshing, SIGGRAPH 2003 Interior : Model/Threshold  Other Discontinuities

Sousa and Prusinkiewicz, A Few Good Lines: Suggestive Drawing of 3D Models, Eurographics 2003

Sousa et al., Depicting Shape Features with Directional Strokes and Spotlighting, Computer Graphics International 2004 Interior : Model/Threshold  Other Discontinuities

Model dependent Threshold dependent View dependent Outline form of the object Interior form of the object Boundary Crease (+ other discontinuities) Crease (+ other discontinuities) Silhouette X X

1979 Appel et al The haloed line effect for hidden line elimination SIGGRAPH 1982Roth 1982 Roth Ray Casting for Modeling Solids Computer Graphics and Image Processing 1987Wyvill et al 1987 Wyvill et al Sketches by Ray Tracing Computer Graphics International 1987Sasada 1987 Sasada Drawing Natural Scenery by Computer Graphics Computer-Aided Design 1987Kamada, T. and Kawai, S. An Enhanced Treatment of Hidden Lines 1987 Kamada, T. and Kawai, S. An Enhanced Treatment of Hidden Lines ACM Transactions on Graphics 1988Veenstra and Ahuja 1988 Veenstra and Ahuja Line drawings of octree-represented objects ACM Transactions on Graphics Line Drawing of 3D Models

1979 Appel et al The haloed line effect for hidden line elimination Kamada, T. and Kawai, S. An Enhanced Treatment of Hidden Lines 1987 Kamada, T. and Kawai, S. An Enhanced Treatment of Hidden Lines Veenstra and Ahuja 1988 Veenstra and Ahuja Line drawings of octree-represented objects Line Drawing of 3D Models