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

From: François Duranleau (duranlef_at_[hidden])
Date: 2007-10-08 11:17:04

On Sun, 7 Oct 2007, Fernando Cacciola wrote:

[...]
>> Let's consider the example below.
>>
>> [really bad code ommitted]
>>
>> Translating this to gtl isotropic code:
>>
>> concave += (a.towards(b).left() == b.towards(c));
>>
> OK. So you are calling isotropic what I know as "coordinate free
> operations".
>
> Notice that I could implement the above as:
>
> concave += ( cross_product((b-a),(c-a)) < 0 ) ;
>
> using basic algebraic stuff (and this would work for non-rectilinear edges
> as well).
[...]
>> convex += (a.towards(b).right() == b.towards(c));
>>
> Right.
> But using basic algebra that becomes:
>
> convex += ( cross_product((b-a),(c-a)) > 0 ) ;
[...]

Just one thing: in linear algebra, cross product returns a vector. Then 
how can you compare that result with a scalar?

-- 
Francois Duranleau