I'm using C and OpenCV to create a Delaunay triangle mesh from user-specified sample points on an image (which will then be extrapolated across the domain using the FEM for the relevant ODE).

Since the 4 corners of the (rectangular) image are in the list of vertices supplied to Subdiv2D, I expect the outer convex hull of the triangulation to trace the perimeter of the image. However, very frequently, there are missing elements around the outside.

Sometimes I can get the expected result by nudging the coordinates of certain points to avoid high aspect ratio triangles. But this is not a solution as in general the user most be able to specify any valid coordinates.

An example output is like this: