[GRASS-dev] symbol rotation is go

Glynn Clements glynn at gclements.plus.com
Thu Apr 26 12:56:19 EDT 2007

Hamish wrote:

> I have just added a new libgis function: G_rotate_around_pt() in 6.3cvs.
>  (make distclean, yet again)
> It rotates a point around a given center coord by a given angle.

I would suggest an alternate version:

void G_rotate_around_point(double X0, double Y0, double *X1, double *Y1, double angle)
	double dx = *X1 - X0;
	double dy = *X1 - X0;
	double c = cos(D2R(angle));
	double s = sin(D2R(angle));
	double dx1 = dx * c - dy * s;
	double dy1 = dx * s + dy * c;

	*X1 = X0 + dx1;
	*Y1 = Y0 + dy1;


1. Uses double for coordinates instead of int.
2. Doesn't have a singularity at (X0,Y0) (atan2(0,0) may return NaN in
some implementations).
3. "void" return type; functions returning a meaningless "int" is
usually a hold-over from pre-ANSI C.

Glynn Clements <glynn at gclements.plus.com>

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