错误的面积计算objective-c？

Question

我有一个包含 5 个 CGPoints（v0、v1、v2、v3、v4）的数组，它们是这样实现的形状的顶点：

_arrayVertex = [[NSMutableArray alloc] initWithObjects:[NSValue valueWithCGPoint:v0], [NSValue valueWithCGPoint:v1], [NSValue valueWithCGPoint:v2], [NSValue valueWithCGPoint:v3], [NSValue valueWithCGPoint:v4], nil];

然后，我用这些点计算面积：

- (float)calculateArea {

    float area = 0;
    int N = (int)_arrayVertex.count;

    for (int i = 0; i < N-1; i++) {

        float term = ([_arrayVertex[i] CGPointValue].x * [_arrayVertex[i+1 % N] CGPointValue].y -
                  [_arrayVertex[i+1 % N] CGPointValue].x * [_arrayVertex[i] CGPointValue].y)/2.0;

        area += term;
    }

    return fabsf(area);

}

面积计算正确吗？我错过了什么吗？提前谢谢你。

Answer 1

sbim：

- (float)calculateArea {

    float area = 0;
    int N = (int)_arrayVertex.count;

    for (int i = 0; i < N; i++) {

        float term = ([_arrayVertex[i] CGPointValue].x * [_arrayVertex[(i+1) % N] CGPointValue].y -
              [_arrayVertex[(i+1) % N] CGPointValue].x * [_arrayVertex[i] CGPointValue].y)/2.0;

        area += term;
    }

    return fabsf(area);

}