Contour Tracing

Gradient Vs Sobel Operator | | Iso Surface Examples

Main disadvantage of iso surface extraction: For each iso value all tetrahedra need to be visited, although the iso surface does only intersect a small fraction of those. Visiting all cells does not allow interactive change of the iso value.

Based on the assumption that iso surfaces are continuous (water-tight) the contour tracing method accelerates the standard iso surface extraction, by only touching those cell that are actually cut by the iso surface:

Find a cell which intersects the iso surface

  • Extract the surface element
  • Examine the neighbour cells
    • Since the iso surface is closed (C1-continuous), at least one adjacent cell needs to be cut by the iso surface, also.
    • If an adjacent cell is cut, repeat the procedure unless the cell has already been visited.

Further acceleration of the intersection test by MIN/MAX-Test:

  • Octrees [Wilhelms ‘92]
  • Interval-trees [Cignoni ‘97]

Gradient Vs Sobel Operator | | Iso Surface Examples