Reversal of boundary segments

[krober10nd]

• fixing bug from Error in specifying riverine boundary segments! #166
• Second to last and last point in drawn boundary segments were reversed.

[WPringle]

I also encountered this strange ordering before but I remember depending on the clockwise/anticlockwise choice would produce different results. Would be good to find out the source of the error instead of the ad-hoc adjustment at the end.

[krober10nd]

It isn't ad hoc but it does suggest other changes. The edges are ordered in a way that creates this problem

Perhaps we should get rid of this CW and CCW option and instead just make people switch the ordering of the inputs. It's not clear what CW and CCW always mean for complex polygons and most people (including me) simply guess.

[krober10nd]

Perhaps the default should be 1 and leave it like that.

[WPringle]

Right so tell people they have to enter it in Clockwise ordering? Which is 1 again?
I still would like to see where the wrong ordering is coming from and I'm worried that this doesn't always apply. It must be right most of the time because doesn't often produce this error.

[krober10nd]

Right so tell people they have to enter it in Clockwise ordering? Which is 1 again?

Yes something like. Just get rid of this option in the API.

In the file utilities/extract_boundary.m I describe the algorithm for the "traversal". We can have a closer look at it, it's been years since I wrote it.

[krober10nd]

Running the JBAY example and picked an arbitrary segment just switching the order of the input indices keeping the order set to 1.

[krober10nd]

I think I've found the origin of the problem. Line 66-67 of utilities/extract_boundary adds the entire selected edge to the polygon list ignoring the v_start and v_next while all other segments consider this.

[krober10nd]

Okay put in a fix. If v_next doesn't appear as the last entry of the first edge selected, the edge is flipped so that is the case.

[WPringle]

Ok cool so it definitely works for normal open boundary selection as well as small river like segments?

[krober10nd]

I've tested it with a one edge segment with order=0 and order=1 on two different meshes and they all worked. I've also tested segments for length 3 vertices and on larger domains as well and the original problem did not crop up.