SkMatrix22.cpp File Reference

#include "src/utils/SkMatrix22.h"
#include "include/core/SkMatrix.h"
#include "include/core/SkPoint.h"
#include "include/core/SkScalar.h"

Go to the source code of this file.

Functions
void	SkComputeGivensRotation (const SkVector &h, SkMatrix *G)

Function Documentation

SkComputeGivensRotation()

void SkComputeGivensRotation	(	const SkVector &	h,
		SkMatrix *	G
	)

Find the Givens matrix G, which is the rotational matrix that rotates the vector h to the positive hoizontal axis. G * h = [hypot(h), 0]

This is equivalent to

SkScalar r = h.length(); SkScalar r_inv = r ? SkScalarInvert(r) : 0; h.scale(r_inv); G->setSinCos(-h.fY, h.fX);

but has better numerical stability by using (partial) hypot, and saves a multiply by not computing r.

Definition at line 14 of file SkMatrix22.cpp.

                                                             {
    const SkScalar& a = h.fX;
    const SkScalar& b = h.fY;
    SkScalar c, s;
    if (0 == b) {
        c = SkScalarCopySign(SK_Scalar1, a);
        s = 0;
        //r = SkScalarAbs(a);
    } else if (0 == a) {
        c = 0;
        s = -SkScalarCopySign(SK_Scalar1, b);
        //r = SkScalarAbs(b);
    } else if (SkScalarAbs(b) > SkScalarAbs(a)) {
        SkScalar t = a / b;
        SkScalar u = SkScalarCopySign(SkScalarSqrt(SK_Scalar1 + t*t), b);
        s = -SK_Scalar1 / u;
        c = -s * t;
        //r = b * u;
    } else {
        SkScalar t = b / a;
        SkScalar u = SkScalarCopySign(SkScalarSqrt(SK_Scalar1 + t*t), a);
        c = SK_Scalar1 / u;
        s = -c * t;
        //r = a * u;
    }
 
    G->setSinCos(s, c);
}