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From: Nathan Hurst <njh@nj...>  20110830 22:21:10

On Tue, Aug 30, 2011 at 09:00:46PM +0200, Diederik van Lierop wrote: > 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 P0P1 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 sbasisgeometric.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 No more than a matrix multiply. You need to convert it into a form with a suitable algebra. It could be done with bernstein basis (bezier curves) but we currently lack code to do that. >  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? lib2geom is full of solutions looking for problems :) >  How do I implement perpendicular snapping? There's no method yet > called find_normals() Ok, there are a bunch of operations that are interesting: * given a direction D, find all points on the curve A that point in that direction: roots(dot(derivative(A), rot90(D))) * given a point P, find all points on the curve A whose tangent passes through P: find_tangents(P, A) * given a point P, find all points on the curve A whose normal passes through P, same as find tangents, but use the dot product: SBasis crs (dot(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 (t1)^2 followed by a divide by s^2 (shift) return roots(crs); * given a pair of curves, find the lines which are normal to both curves: collinear_normals * same, but for tangents (fillets): There is no code for this currently, but I'm sure it can be built from collinear_normals. * snap arcs to matching curvature: filetminion (I particularly like the name of this one :) There are more snappy things, but I can't think of them right now. I'm going away for a few days (no internet!!!), but don't hestitate to ask for more help. njh 
From: Diederik van Lierop <mail@di...>  20110830 19:01:01

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 P0P1 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 sbasisgeometric.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 (t1)^2 followed by a divide by s^2 (shift) return roots(crs); } 
From: Johan Engelen <jbc.engelen@sw...>  20110821 10:32:30

On 2082011 15:25, Alvin Penner wrote: > attached is a modified version of file src/2geom/hvlinesegment.h. It > contains two minor modifications, in the routine 'pointAt(Coord t)' and > the routine 'pointAndDerivatives(Coord t, unsigned n)'. These changes > were made/proposed in order to interchange x and y coordinates for > vertical lines to fix LP Bug 813829. > > https://bugs.launchpad.net/inkscape/+bug/813829 > http://article.gmane.org/gmane.comp.graphics.inkscape.devel/37125 > > Does this seem like a reasonable way to fix this bug, or is there a > better way? Thanks for pointing to the fix Alvin. I committed a fix. (modified your fix, because I didn't like that it depended on the exact numerical definition of 'axis') Ciao, Johan 
From: Alvin Penner <penner@va...>  20110820 13:42:42

attached is a modified version of file src/2geom/hvlinesegment.h. It contains two minor modifications, in the routine 'pointAt(Coord t)' and the routine 'pointAndDerivatives(Coord t, unsigned n)'. These changes were made/proposed in order to interchange x and y coordinates for vertical lines to fix LP Bug 813829. https://bugs.launchpad.net/inkscape/+bug/813829 http://article.gmane.org/gmane.comp.graphics.inkscape.devel/37125 Does this seem like a reasonable way to fix this bug, or is there a better way? tia, Alvin Penner 