Contour Algorithms

Left: DVR of the reference model, completely opaque with constant color; the scene is illuminated from the top-left according to the Blinn-Phong light model. Right: zooming in we can see some noise on the data, which will be inevitably amplified when computing the derivatives.

• Basic Contours

  
  
  
  Basic contours computed according to [1]
  

  contour_intensity = window(gradient) * (1.0 - dot(normal, view))^thickness

  The model is not shaded. In the right image the contour intensity is thresholded.
  

  ---

• Curvature-Weighted Contours

  
  
  Curvature weighted contours: a point is considered part of the contour iff

  dot(normal, view) < T * K * (2 - T * C)

  The curvature is computed as finite differences along the view ray (see [2]), the result is very similar to the simple contours. On the right, same idea but using the mean curvature (computed according to [3]) instead: thinner contours but emphasized noise and not view dependent.
  
  
  

  ---

• Suggestive Contours

  
  
  As explained in [4], suggestive contours are defined as points satisfying:

  1) dot(view, normal) > threshold

  2) curvature == 0.0

  3) D_w(curvature) > 0.0

  Left: curvature computed as finite difference along the ray; right: mean curvature is used; below: same images with simple contours added.
  
  
  
  
  

  ---

• Ridges and Valleys

  
  
  Ridges and valleys of the mean curvature; according to the thresholds, more or less noise is amplified.
  
  
  

  ---

• References:

  • [1] Csébfalvi B., Mroz L., Hauser H., Konig A., Groller E.: Fast visualization of object contours by non-photorealistic volume rendering. Computer Graphics Forum 20, 3, C452-C460, 2001.
  • [2] S. Bruckner and M. E. Groller: Style transfer functions for illustrative volume rendering. Computer Graphics Forum, 26 (3): 715724, 2007.
  • [3] Lefohn A. E., Whitaker R. T.: A GPU-Based, Three-Dimensional Level Set Solver with Curvature Flow. Tech. Rep. UUCS-02-017, University of Utah , 2002.
  • [4] Doug DeCarlo , Adam Finkelstein , Szymon Rusinkiewicz , Anthony Santella: Suggestive contours for conveying shape. ACM Transactions on Graphics (TOG), v.22 n.3, July 2003
  
  

  ---