NPR Shading Algorithms

DVR of the reference model with semi-transparent skin and opaque bones; the scene is illuminated from the top-left and the colors are computed according to a simplified version of the Blinn-Phong model:

color = k_diffuse * base_color + k_specular * (1, 1, 1)

where base_color is computed from the transfer function.

• Toon Shading

  
  
  
  
  
  Toon Shading: the final color is still computed according to the previous shading model, but both k_diffuse and k_specular are quantized according to the specified number of levels.
  Above: 2 levels (1 threshold at 0.5, values mapped to either 0.0 or 1.0); below: 4 levels (3 thresholds at 0.17, 0.5, 0.83, values mapped to 0.0, 0.33, 0.66, 1.0).
  
  
  

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• Cool to Warm Shading

  
  
  
  
  
  Cool to Warm Shading computed according to [1]; the base_color of the object is discarded, but the alpha value is still taken into account.
  Above: blue to yellow with alpha = beta = 0.2; below: green to red with alpha = 0.1, beta = 0.5.
  
  
  

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• Metal Shading

  
  
  Metal Shading computed according to [1]: the results are not so great when applied to semi-transparent models, but they are better with solid surfaces (see video below).
  

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• Combined

  - Play Movie -
  
  Metal Shading, Metal+Cool2Warm Shading, Metal+Cool2Warm+Toon Shading.
  

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• References:

  • [1] Amy Gooch , Bruce Gooch , Peter Shirley , Elaine Cohen: A non-photorealistic lighting model for automatic technical illustration. Proceedings of the 25th annual conference on Computer graphics and interactive techniques, p.447-452, July 1998
  
  

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