Android线段与矩形碰撞检测函数

原理不明白。。网上找的代码。测试通过，记录一下。

    /**
    * <p> 判断线段是否在矩形内
    * <p> 
    * 先看线段所在直线是否与矩形相交，
    * 如果不相交则返回false，
    * 如果相交，
    * 则看线段的两个点是否在矩形的同一边（即两点的x(y)坐标都比矩形的小x(y)坐标小，或者大）,
    * 若在同一边则返回false，
    * 否则就是相交的情况。
    * </p>
    * 
    * @param linePointX1 线段起始点x坐标
    * @param linePointY1 线段起始点y坐标
    * @param linePointX2 线段结束点x坐标
    * @param linePointY2 线段结束点y坐标
    * @param rectangleLeftTopX 矩形左上点x坐标
    * @param rectangleLeftTopY 矩形左上点y坐标
    * @param rectangleRightBottomX 矩形右下点x坐标
    * @param rectangleRightBottomY 矩形右下点y坐标
    * @return 是否相交
    */
    private boolean isLineIntersectRectangle(int linePointX1,int linePointY1,int linePointX2,int linePointY2,int rectangleLeftTopX,int rectangleLeftTopY,int rectangleRightBottomX,int rectangleRightBottomY) {

    int lineHeight = linePointY1 - linePointY2;
    int lineWidth = linePointX2 - linePointX1;
    // 计算叉乘
    int c = linePointX1 * linePointY2 - linePointX2 * linePointY1;

    if ((lineHeight * rectangleLeftTopX + lineWidth * rectangleLeftTopY + c >= 0 && lineHeight * rectangleRightBottomX + lineWidth * rectangleRightBottomY + c <= 0)
    || (lineHeight * rectangleLeftTopX + lineWidth * rectangleLeftTopY + c <= 0 && lineHeight * rectangleRightBottomX + lineWidth * rectangleRightBottomY + c >= 0)
    || (lineHeight * rectangleLeftTopX + lineWidth * rectangleRightBottomY + c >= 0 && lineHeight * rectangleRightBottomX + lineWidth * rectangleLeftTopY + c <= 0)
    || (lineHeight * rectangleLeftTopX + lineWidth * rectangleRightBottomY + c <= 0 && lineHeight * rectangleRightBottomX + lineWidth * rectangleLeftTopY + c >= 0)) {
            if (rectangleLeftTopX > rectangleRightBottomX) {
                int temp = rectangleLeftTopX;
                rectangleLeftTopX = rectangleRightBottomX;
                rectangleRightBottomX = temp;
            }
            if (rectangleLeftTopY < rectangleRightBottomY) {
                int temp = rectangleLeftTopY;
                rectangleLeftTopY = rectangleRightBottomY;
                rectangleRightBottomY = temp;
            }
            if ((linePointX1 < rectangleLeftTopX && linePointX2 < rectangleLeftTopX) 
            || (linePointX1 > rectangleRightBottomX && linePointX2 > rectangleRightBottomX) 
            || (linePointY1 > rectangleLeftTopY && linePointY2 > rectangleLeftTopY) 
            || (linePointY1 < rectangleRightBottomY && linePointY2 < rectangleRightBottomY)) {
                return false;
            } else {
                return true;
            }
        } else {
            return false;
        }
    }