David Luebke
University of Virginia

“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

 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 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

 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 )

 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

 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!

 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)

 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)


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!

 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

Wireframe Translation Fattening
40 fps 50 fps 11.5 fps
Original Venus model : 5672 triangles, 66 fps