CGAL users discussion list

[Sylvain Pion <>]

Pedro Machado Manhães de Castro a écrit :
> The answer is no.

No, the answer to the first question is yes.  RTFM:
http://www.cgal.org/Manual/3.3/doc_html/cgal_manual/Kernel_23_ref/Class_Circle_2.html

As for the relative position of a segment wrt a circle, there are more cases 
to
distinguish, and you may need to do something along the lines of what Pedro 
suggests.
See squared_distance(), between the center and the segment and/or the 
endpoints
or the supporting line.
http://www.cgal.org/Manual/3.3/doc_html/cgal_manual/Kernel_23_ref/Function_squared_distance.html

> However:
>
> You could do that by:
>
> Check whether the supporting line of your segment and the circle 
> intersect (with do_intersect). If it does not intersect, then the 
> segment is completely outside or inside the circle (if it does interest 
> you, you can find which of those two cases are applied by checking 
> whether one end point is inside the circle or not). Otherwise, you check 
> whether the squared_distance between the center and the supporting line 
> is equal to the squared_radius of the circle or not. If yes, then you 
> project the center onto the supporting line of the segment and check if 
> the segment contains that point (with Has_on_2) (I am assuming that a 
> 'tangent segment' is the one such that its supporting line is tangent to 
> the circle + the segment intersects the circle). And now, if the 
> distance is not equal to the radius, check whether both end points of 
> the segment are outside/on the circle or not (so you are able to decide 
> whether it divides or not the circle).
>
> Search the manual index for Has_on_2, Do_intersect_2, 
> Has_on_bounded_side_2, Intersect_2 and squared_distance.
>
> Cheers,
> Pedro Machado
>
> On Wed, Nov 12, 2008 at 8:22 PM, Emerson Carride < 
> <mailto:>> wrote:
>
>     In CGAL has a function that returns if a given point is inside or
>     outside or in boundary of a circle.
>
>     I want to know if CGAL has a function that returns if a given
>     segment is tangent to a circle or divided the circle into two parts?
>
>
>
>
>     -- 
>     Emerson Amade Carride
>     Matemática Aplicada e Computação Científica
>     Instituto de Ciências Matemáticas e Computação
>     USP - São Carlos
>     tel: (16) 3411 0877; (11) 4538 3315 ; cel: (11) 71289888 


--
Sylvain Pion
INRIA Sophia-Antipolis
Geometrica Project-Team
CGAL, http://cgal.org/