Non-Photorealistic Rendering: Toon Shading David Luebke University of Virginia

Non-Photorealistic Rendering “Using a term like ‘nonlinear science’ is like referring to the bulk of zoology as ‘the study of nonlinear animals’” Photorealism: age-old goal of graphics Non-photorealistic rendering –Broadly, techniques for rendering that don’t strive for realism, but style, expressiveness, abstraction, uncertainty, etc –A terrible, terrible term that we’re probably stuck with Better terms: stylized rendering, artistic rendering, abstract rendering

Stylized Rendering NPR is most commonly used to refer to graphics techniques that emulate a particular artistic style or medium –Impressionistic painting –Pen-and-ink (cross hatching, outlining, etc) –Colored-pencil, copperplate engraving, you name it –Cartoon drawing

Toon shading Toon shading is the term that we use to refer to cartoon-like rendering effects Tendencies in cartoon/comic style: –Simple, flat shading (cel shading) Two-tone (light/shadow) or three-tone (light/shadow/highlight) –Edge highlighting Boundary (border edge) Crease (hard edge) Material edge Silhouette edge

Toon Shading Cel shading –Threshold the lighting equation Ex: the Cg program we looked at Diffuse below 0.5 ? dark color : light color Specular above 0.5 ? hilight color : diffuse color If no pixel shaders, can use N ● L to look up a 1-D texture containing the shaded colors –In general, can think of these procedures as remapping the traditional lighting equation elements (tone mapping)

Silhouette Rendering Boundary (border edge) –Not shared by two polygons –E.g. the edge of a sheet of paper –Solid models usually have no boundaries Crease (hard edge) –Shared by two polygons at a dihedral angle greater than some threshold (often 60°) –Or a vertex with two normals/two colocated vertices –Ridge or valley edges

Silhouette Rendering Material edge –Triangles sharing edge have different materials/texture maps/etc –Or just an edge that the artist wants to emphasize Silhouette edge –Triangles sharing edge face different directions (towards/away from viewer) –Lots of techniques to find at runtime!

Finding/Rendering Silhouette Edges Surface angle silhouetting –Again, like the Cg program we looked at –Calculate N●V, if below threshold  draw black –Problem: depends on surface curvature (see Fig 7.5)

Procedural Geometry Silhouetting Idea: render the geometry in such a way that the silhouettes “fall out” Ex: –Draw frontfacing polygons –Draw backfacing polygons But draw them in thick (2-pixel) wireframe Or draw them in wireframe, z-biased forward a bit Or “fatten” them Or displace them along their normals (“halo” effect)

Image Processing Silhouetting Idea: analyze the image after it’s rendered, and extract silhouettes (like edge detection) Can help by rendering e.g. depth image, object-ID image, normal image Perfect for fragment program!

Silhouette Edge Detection Idea: find silhouette edges geometrically and render them explicitly –Randomized Appel’s algorithm –Lots of interesting geometric approaches Most work, but gives the most flexibility in how silhouettes are drawn

Results WireframeTranslationFattening 40 fps 50 fps 11.5 fps Original Venus model : 5672 triangles, 66 fps