Mega Code Archive
Get the perpendicular point on segment from external point
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Title: get the perpendicular point on segment from external point?
{---------------------------------------------------------------------
Two routines are made available, the first takes into account a segment
and will produce point on the given segment that is of least distance
to an external point.
The second routine is similar to the first other than the fact that it
will extended the given segment into a ray and hence also produce a point
on the ray which will produce a segment of least distance between that
point and an external point, the segment produced between the point on
the ray and the external point is guaranteed to be perpendicular to the
ray in all cases except for the instance where the external point is
collinear to ray.
Both routines come from the computational geometry library
FastGEO (url: http://fastgeo.partow.net)
---------------------------------------------------------------------}
procedure PerpendicularPntToSegment(const x1, y1, x2, y2, Px, Py: Double;
out Nx, Ny: Double);
var
Ratio: Double;
Dx: Double;
Dy: Double;
begin
Dx := x2 - x1;
Dy := y2 - y1;
Ratio := ((Px - x1) * Dx + (Py - y1) * Dy) / (Dx * Dx + Dy * Dy);
if Ratio then
begin
Nx := x1;
Ny := y1;
end
else if Ratio 1 then
begin
Nx := x2;
Ny := y2;
end
else
begin
Nx := x1 + (Ratio * Dx);
Ny := y1 + (Ratio * Dy);
end;
end;
(* End PerpendicularPntSegment *)
procedure PerpendicularPntToLine(const Rx1, Ry1, Rx2, Ry2, Px, Py: Double;
out Nx, Ny: Double);
var
Ratio: Double;
Gr1, Gr2: Double;
Gr3, Gr4: Double;
begin
(* The ray is defined by the coordinate pairs (Rx1,Ry1) and (Rx2,Ry2) *)
if NotEqual(Rx1, Rx2) then
Gr1 := (Ry2 - Ry1) / (Rx2 - Rx1)
else
Gr1 := 1e300;
Gr3 := Ry1 - Gr1 * Rx1;
if NotEqual(Gr1, 0) then
begin
Gr2 := -1 / Gr1;
Gr4 := Py - (Gr2 * Px);
Ratio := (Gr4 - Gr3) / (Gr1 - Gr2);
Nx := Ratio;
Ny := (Gr2 * Ratio) + Gr4;
end
else
begin
Nx := Px;
Ny := Ry2;
end;
end;
(* End PerpendicularPntToLine *)
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