Better flip-check criterion for almost degenerate face removal #8743
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The reasoning is that in this function, we are maybe not so interested in having nicely shaped elements as much as we are trying to remove the worst elements. Unfortunately, the current criterion (see CGAL#7125) can prevent some edge flips that would resolve a cap because sometimes this Delaunay-like criterion can prefer a cap if it is balanced by a very small angle on the other side. Then you have alpha + beta < 180 and you're stuck with the cap, but maybe you would have liked to flip still because the largest angles created by a flip would be e.g. 120 and 120, which is not that bad. So this PR tries to improve the worst cap angle instead by checking only (and greedily) if we do not create a worse cap angle when we flip. This produces better results for the cases that were problematic with alpha + beta < 180
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WIP, so what is the todo? |
sloriot
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Note that it makes |
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This shouldn't be tested as it is: the API changes in the internal function (I added the capness threshold) breaks isotropic remeshing and related functions. I didn't want to fix it until we discuss this criterion change (and with @janetournois). |
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Summary of Changes
While trying to remove caps,
PMP::remove_almost_degenerate_faces()does a Delaunay-like check before allowing the edge flip. Unfortunately, the current criterion (see #7125) can prevent some edge flips that would resolve a cap because sometimes this Delaunay-like criterion can prefer a cap if it is balanced by a very small angle on the other side. Then you have alpha + beta < 180 and you're stuck with the cap, but maybe you would have liked to flip still because the largest angles created by a flip would be e.g. 120 and 120, which is not that bad.However in this function, we are maybe not so interested in having nicely shaped elements as much as we are trying to remove the worst elements. So this PR tries to improve the worst cap angle instead, by checking only (and greedily) if we do not create a worse cap angle when we flip.
This produces better results for the cases that were problematic with alpha + beta < 180
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PMP