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’”
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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
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Better terms: stylized rendering, artistic rendering,
abstract rendering
Stylized Rendering
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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
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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)
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Two-tone (light/shadow) or three-tone
(light/shadow/highlight)
– Edge highlighting
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Boundary (border edge)
Crease (hard edge)
Material edge
Silhouette edge
Toon Shading
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Cel shading
– Threshold the lighting equation
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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
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Boundary (border edge)
– Not shared by two polygons
– E.g. the edge of a sheet of paper
– Solid models usually have no boundaries
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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
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Material edge
– Triangles sharing edge have different
materials/texture maps/etc
– Or just an edge that the artist wants to
emphasize
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Silhouette edge
– Triangles sharing edge face different
directions (towards/away from viewer)
– Lots of techniques to find at runtime!
Finding/Rendering
Silhouette Edges
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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
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Idea: render the geometry in such a way
that the silhouettes “fall out”
Ex:
– Draw frontfacing polygons
– Draw backfacing polygons
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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
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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
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Idea: find silhouette edges geometrically
and render them explicitly
– Randomized Appel’s algorithm
– Lots of interesting geometric approaches
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Most work, but gives the most flexibility in
how silhouettes are drawn
Results
Wireframe
40 fps
Translation
50 fps
Fattening
11.5 fps
Original Venus model : 5672 triangles, 66 fps
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Non-Photorealistic Rendering: Toon Shading David Luebke University of Virginia