Function get_scale_factor

third-part/SkMatrix/src/SkMatrix.cpp:1395–1472

};

template <MinMaxOrBoth MIN_MAX_OR_BOTH> bool get_scale_factor(SkMatrix::TypeMask typeMask,
                                                              const SkScalar m[9],
                                                              SkScalar results[/*1 or 2*/]) {
    if (typeMask & SkMatrix::kPerspective_Mask) {
        return false;
    }
    if (SkMatrix::kIdentity_Mask == typeMask) {
        results[0] = SK_Scalar1;
        if (kBoth_MinMaxOrBoth == MIN_MAX_OR_BOTH) {
            results[1] = SK_Scalar1;
        }
        return true;
    }
    if (!(typeMask & SkMatrix::kAffine_Mask)) {
        if (kMin_MinMaxOrBoth == MIN_MAX_OR_BOTH) {
             results[0] = SkMinScalar(SkScalarAbs(m[SkMatrix::kMScaleX]),
                                      SkScalarAbs(m[SkMatrix::kMScaleY]));
        } else if (kMax_MinMaxOrBoth == MIN_MAX_OR_BOTH) {
             results[0] = SkMaxScalar(SkScalarAbs(m[SkMatrix::kMScaleX]),
                                      SkScalarAbs(m[SkMatrix::kMScaleY]));
        } else {
            results[0] = SkScalarAbs(m[SkMatrix::kMScaleX]);
            results[1] = SkScalarAbs(m[SkMatrix::kMScaleY]);
             if (results[0] > results[1]) {
                 SkTSwap(results[0], results[1]);
             }
        }
        return true;
    }
    // ignore the translation part of the matrix, just look at 2x2 portion.
    // compute singular values, take largest or smallest abs value.
    // [a b; b c] = A^T*A
    SkScalar a = sdot(m[SkMatrix::kMScaleX], m[SkMatrix::kMScaleX],
                      m[SkMatrix::kMSkewY],  m[SkMatrix::kMSkewY]);
    SkScalar b = sdot(m[SkMatrix::kMScaleX], m[SkMatrix::kMSkewX],
                      m[SkMatrix::kMScaleY], m[SkMatrix::kMSkewY]);
    SkScalar c = sdot(m[SkMatrix::kMSkewX],  m[SkMatrix::kMSkewX],
                      m[SkMatrix::kMScaleY], m[SkMatrix::kMScaleY]);
    // eigenvalues of A^T*A are the squared singular values of A.
    // characteristic equation is det((A^T*A) - l*I) = 0
    // l^2 - (a + c)l + (ac-b^2)
    // solve using quadratic equation (divisor is non-zero since l^2 has 1 coeff
    // and roots are guaranteed to be pos and real).
    SkScalar bSqd = b * b;
    // if upper left 2x2 is orthogonal save some math
    if (bSqd <= SK_ScalarNearlyZero*SK_ScalarNearlyZero) {
        if (kMin_MinMaxOrBoth == MIN_MAX_OR_BOTH) {
            results[0] = SkMinScalar(a, c);
        } else if (kMax_MinMaxOrBoth == MIN_MAX_OR_BOTH) {
            results[0] = SkMaxScalar(a, c);
        } else {
            results[0] = a;
            results[1] = c;
            if (results[0] > results[1]) {
                SkTSwap(results[0], results[1]);
            }
        }
    } else {