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; }
