experimental/SkSetPoly3To3.cpp

#include "SkMatrix.h"

static void computeOuterProduct(SkScalar op[4],
                                const SkPoint pts0[3], const SkPoint& ave0,
                                const SkPoint pts1[3], const SkPoint& ave1) {
    sk_bzero(op, 4 * sizeof(op[0]));
    for (int i = 0; i < 3; i++) {
        SkScalar x0 = pts0[i].fX - ave0.fX;
        SkScalar y0 = pts0[i].fY - ave0.fY;
        SkScalar x1 = pts1[i].fX - ave1.fX;
        SkScalar y1 = pts1[i].fY - ave1.fY;
        op[0] += SkScalarMul(x0, x1);
        op[1] += SkScalarMul(x0, y1);
        op[2] += SkScalarMul(y0, x1);
        op[3] += SkScalarMul(y0, y1);
    }
}

static SkScalar dot(SkScalar ax, SkScalar ay, SkScalar bx, SkScalar by) {
    return SkScalarMul(ax, bx) + SkScalarMul(ay, by);
}

bool SkSetPoly3To3(SkMatrix* matrix, const SkPoint src[3], const SkPoint dst[3]) {
    const SkPoint& srcAve = src[0];
    const SkPoint& dstAve = dst[0];
    
    SkScalar srcOP[4], dstOP[4];
    
    computeOuterProduct(srcOP, src, srcAve, src, srcAve);
    computeOuterProduct(dstOP, src, srcAve, dst, dstAve);

    SkScalar det = SkScalarMul(srcOP[0], srcOP[3]) - SkScalarMul(srcOP[1], srcOP[2]);

    // need SkScalarNearlyZeroSquared for this (to match Chrome's fix)
    if (SkScalarNearlyZero(det)) {
        return false;
    }
    
    SkScalar invDet = SkScalarInvert(det);
    
    // now compute invDet * [srcOP]T * [dstOP]
    
    // scale and transpose
    const SkScalar srcOP0 = SkScalarMul( srcOP[3], invDet);
    const SkScalar srcOP1 = SkScalarMul(-srcOP[1], invDet);
    const SkScalar srcOP2 = SkScalarMul(-srcOP[2], invDet);
    const SkScalar srcOP3 = SkScalarMul( srcOP[0], invDet);

    matrix->reset();
    matrix->setScaleX(dot(srcOP0, srcOP1, dstOP[0], dstOP[2]));
    matrix->setSkewX( dot(srcOP2, srcOP3, dstOP[0], dstOP[2]));
    matrix->setSkewY (dot(srcOP0, srcOP1, dstOP[1], dstOP[3]));
    matrix->setScaleY(dot(srcOP2, srcOP3, dstOP[1], dstOP[3]));
    matrix->setTranslateX(dstAve.fX - dot(srcAve.fX, srcAve.fY,
                                    matrix->getScaleX(), matrix->getSkewX()));
    matrix->setTranslateY(dstAve.fY - dot(srcAve.fX, srcAve.fY,
                                    matrix->getSkewY(), matrix->getScaleY()));
    return true;
}