CGAL users discussion list

[Ben Supnik <>]

Hi Y'all,

I think I may have found either a bug or an unstated condition for 
merge_edge...

Merge_edge picks one of the two half-edge-pairs to become the new 
consolidated edge when an edge-pair is merged.

From what I can tell, Merge_edge never recalculates the "direction()" 
cached value on the halfedges.

But...in some cases, merging the edges (which are not required to be 
colinear - imagine merging two lines when the underlying curve type is 
"poly-line") might cause the direction of the "recycled" halfedge to 
change it's direction.  This causes the planar map to be invalid.

I'm not sure my fix is so great...I'm using Arr_accessor's declaration 
of the dcel halfedge type to cast my way into the dcel, where I can flip 
the direction manually.

Bug?  Design limitation?  Is there a better work-around?

cheers
Ben
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