mcxsvmc isosurface normal direction is flipped in octave compared to matlab

[fangq]

This was initially reported by Josef Probst, and lately by Vincent in the below mailing list thread

https://groups.google.com/g/mcx-users/c/OYRhYRAyFg8

Apparently, the isosurface function produces opposite clock-wise node orders in the output isosurface mesh. this makes the octave surface normal direction flipped compared to the expected direction as those computed in matlab.

here is an example

[xi,yi,zi]=meshgrid(0:60,0:60,0:60);
vol=((xi-30).*(xi-30)+(yi-30).*(yi-30)+(zi-30).*(zi-30))<400;
vol((xi-30).*(xi-30)+(yi-30).*(yi-30)+(zi-30).*(zi-30)<100)=0;
iso=isosurface(double(vol));
nn=nodesurfnorm(iso.vertices, iso.faces);
plotmesh(iso.vertices, iso.faces, 'z<40', 'facealpha', 0.4)
hold on
plotmesh(iso.vertices+nn*0.3, 'z<40', 'r.')

the below plot shows the discrepancy - in matlab, normal direction points from low-to-high across the isosurface threshold, but octave points from high-to-low