android 学习笔记之图形算法

莫名奇妙地用了叉积分。求完面积之后，再进行平均（除以y）：
奇怪的是
??? pt2.fX = poly[0].fY - poly[3].fY;
??? pt2.fY = poly[3].fX - poly[0].fX;
这两行，竟然与逻辑推理出来的不一致。刚好取了个相反数。?

bool SkMatrix::setPolyToPoly(const SkPoint src[], const SkPoint dst[],                             int count) {    if ((unsigned)count > 4) {        SkDebugf("--- SkMatrix::setPolyToPoly count out of range %d\n", count);        return false;    }    if (0 == count) {        this->reset();        return true;    }    if (1 == count) {        this->setTranslate(dst[0].fX - src[0].fX, dst[0].fY - src[0].fY);        return true;    }    SkPoint scale;    if (!poly_to_point(&scale, src, count) ||            SkScalarNearlyZero(scale.fX) ||            SkScalarNearlyZero(scale.fY)) {        return false;    }    static const PolyMapProc gPolyMapProcs[] = {        SkMatrix::Poly2Proc, SkMatrix::Poly3Proc, SkMatrix::Poly4Proc    };    PolyMapProc proc = gPolyMapProcs[count - 2];    SkMatrix tempMap, result;    tempMap.setTypeMask(kUnknown_Mask);    if (!proc(src, &tempMap, scale)) {        return false;    }    if (!tempMap.invert(&result)) {        return false;    }    if (!proc(dst, &tempMap, scale)) {        return false;    }    if (!result.setConcat(tempMap, result)) {        return false;    }    *this = result;    return true;}

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