Re: [boost] GTL - geometric template library - determining interest

From: Suto, Gyuszi (gyuszi.suto_at_[hidden])
Date: 2007-10-08 17:41:07

Fernando Cacciola wrote:
>>> But using basic algebra that becomes:
>>> 
>>> convex += ( cross_product((b-a),(c-a)) > 0 ) ; [...]
>> 
> But we don't want to perpetuate this blasfemy here in boost :)
> So, the correct "orientation test" is to use the determinant of a
> matrix formed with those two vectors (which works in 3D too).
> So I shoud have wrote that as:
> 
> ( determinant((b-a),(c-a)) > 0 )

The following line (admittedly is rectinilear specific)

 concave += (a.towards(b).left() == b.towards(c));

translates into just  a handful of assembly instructions. We did use
cross product 
in the past but it couldn't compete on runtime

Gyuszi