Slide 11 of 42
But opacity functions don't take position into account in how they
assign opacity, so we need to find the boundaries not in the spatial
domain, but in the data value domain. We want to know, what are the
data values that reliably correspond to the middle of a boundary. In
answer to the question, where's the boundary, we want the answere
"here", at v0, because if we make values around v0 opaque in the
opacity function, we should only be making the boundary opaque, and
then in the final rendering we should see the boundary only.
