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。 

static inline bool poly_to_point(SkPoint* pt, const SkPoint poly[], int count) { 
    float   x = 1, y = 1; 
    SkPoint pt1, pt2; 

    if (count > 1) { 
        pt1.fX = poly[1].fX - poly[0].fX; 
        pt1.fY = poly[1].fY - poly[0].fY; 
        y = SkPoint::Length(pt1.fX, pt1.fY); 
        if (checkForZero(y)) { 
            return false; 
        } 
        switch (count) { 
            case 2: 
                break; 
            case 3: 
                pt2.fX = poly[0].fY - poly[2].fY; 
                pt2.fY = poly[2].fX - poly[0].fX; 
                goto CALC_X; 
            default: 
                pt2.fX = poly[0].fY - poly[3].fY; 
                pt2.fY = poly[3].fX - poly[0].fX; 
            CALC_X: 
                x = SkScalarDiv(SkScalarMul(pt1.fX, pt2.fX) + 
                                SkScalarMul(pt1.fY, pt2.fY), y); 
                break; 
        } 
    } 
    pt->set(x, y); 
    return true; 
} 


 

  

2.从多边形到多边形。先转换为矩形，再转换为多边形。

 

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;
}