[Lib2geom-devel] Tangential and perpendicular snapping

[Diederik v. L. <ma...@di...>]

Hi all,

Due to popular demand, I'm thinking of implementing tangential and 
perpendicular snapping in Inkscape. Well, at least for the case where 
the user draws a straight line or manipulates a guide. So I have a curve 
I want to snap to, and some distant point P0. Now I need to find all 
points P1, for which the line P0-P1 is either tangential or 
perpendicular to the curve.

I guess I could first convert the path to sbasis, and then call 
find_tangents() (see the code snippet below, from sbasis-geometric.cpp). 
But a few questions come to mind:
- Can't this be done without having to convert each path to sbasis 
first? Converting will take CPU time
- Am I really the first one to need this functionality? find_tangets() 
is not being used anywhere as far as I can tell. Are there other ways to 
solve this?
- How do I implement perpendicular snapping? There's no method yet 
called find_normals()

It would me much appreciated if someone could shed some light on this.

Diederik


/**
* \brief returns all the parameter values of A whose tangent passes 
through P.
* \relates D2
*/
std::vector<double> find_tangents(Point P, D2<SBasis> const &A) {
     SBasis crs (cross(A - P, derivative(A)));
     crs = shift(crs*Linear(-1, 0)*Linear(-1, 0), -2); // We know that 
there is a double root at t=0 so we divide out t^2
// JFB points out that this is equivalent to (t-1)^2 followed by a 
divide by s^2 (shift)
     return roots(crs);
}