Compute Geodesic Path and Distance in RvtkStatismo

27 Oct 2016

The development branch of RvtkStatismo now contains an implementation of vtkDijkstraGraphGeodesicPath. The R-functions vtkGeodesicPath and vtkGeodesicPathForPointPair allow to compute the (pseudo-) geodesic path by tracking the shortest route along connected vertices - this means the higher the mesh resolution, the smoother the resulting path will be. vtkGeodesicPath needs a mesh and two vertex indices (defining start and end positions of the path) as input. vtkGeodesicPathForPointPair requires a mesh and two vectors containing points on the surface. For the latter the search will start and and at the vertices closest to those coordinates.

Example 1: `vtkGeodesicPath` (Figure 1)

 require(Rvcg)
 data(humface)
 gp <- vtkGeodesicPath(humface,1,1000)
 gp$distance
 ## The distance is 98.36738 mm
 
 ## render it
 require(rgl);require(Morpho)
 points3d(vert2points(humface)[gp$index,])
 lines3d(vert2points(humface)[gp$index,],col="blue",lwd=4)
 spheres3d(vert2points(humface)[c(1,1000),],col=2)
 shade3d(humface,col="white")

Example 2: `vtkGeodesicPathForPointPair` (Figure 2)

 require(Rvcg)
 data(humface)
 gp <- vtkGeodesicPathForPointPair(humface,humface.lm[1,],humface.lm[2,])
 gp$distance
 ## The distance is 30.98827 mm
 
 ## render it
 require(rgl);require(Morpho)
 points3d(vert2points(humface)[gp$index,])
 lines3d(vert2points(humface)[gp$index,],col="blue",lwd=4)
 spheres3d(humface.lm[1:2,],col=2)
 shade3d(humface,col="white")

The paths are quite crooked because of the mesh’s low resolution. To obtain a smoother path, we can e.g. apply a triangle subdivision first (Figure 3):

Example 3: `vtkGeodesicPathForPointPair` on a high resolution mesh (Figure 3)

 humface <- vcgSubdivide(humface,threshold = 0.5,looptype = "cont",iterations=5)
 gp <- vtkGeodesicPathForPointPair(humface,humface.lm[1,],humface.lm[2,])
  
 ## render it
 require(rgl);require(Morpho)
 points3d(vert2points(humface)[gp$index,])
 lines3d(vert2points(humface)[gp$index,],col="blue",lwd=4)
 spheres3d(humface.lm[1:2,],col=2)
 shade3d(humface,col="white")