Non-Photorealistic Rendering: Toon Shading

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Non-Photorealistic RenderingToon Shading

• David Luebke
• University of Virginia

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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 dont
  strive for realism, but style, expressiveness,
  abstraction, uncertainty, etc
• A terrible, terrible term that were probably
  stuck with
• Better terms stylized rendering, artistic
  rendering, abstract rendering

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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

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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

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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)

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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

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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!

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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)

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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)

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Image Processing Silhouetting

• Idea analyze the image after its rendered, and
  extract silhouettes (like edge detection)
• Can help by rendering e.g. depth image, object-ID
  image, normal image
• Perfect for fragment program!

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Silhouette Edge Detection

• Idea find silhouette edges geometrically and
  render them explicitly
• Randomized Appels algorithm
• Lots of interesting geometric approaches
• Most work, but gives the most flexibility in how
  silhouettes are drawn

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Results

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