When expand the mask, houdini don't expand at the terrain boundary.
How to fix this?
Thanks.
It seems that the heightfield_blur node have the same problem.
When blur the mask, the mask don't be a straight at the boundary.
If we have a road go across the terrain boundary, we project the road to terrain
heightfield to get a mask, and blur it, we will found that the mask have
some gap. We want blur the mask and don't have any gap.
