有了一个点列表,我如何确定它们是否是顺时针顺序的?
例如:
point[0] = (5,0)
point[1] = (6,4)
point[2] = (4,5)
point[3] = (1,5)
point[4] = (1,0)
会说它是逆时针的(对某些人来说是逆时针的)
有了一个点列表,我如何确定它们是否是顺时针顺序的?
例如:
point[0] = (5,0)
point[1] = (6,4)
point[2] = (4,5)
point[3] = (1,5)
point[4] = (1,0)
会说它是逆时针的(对某些人来说是逆时针的)
当前回答
这是我使用其他答案中的解释的解决方案:
def segments(poly):
"""A sequence of (x,y) numeric coordinates pairs """
return zip(poly, poly[1:] + [poly[0]])
def check_clockwise(poly):
clockwise = False
if (sum(x0*y1 - x1*y0 for ((x0, y0), (x1, y1)) in segments(poly))) < 0:
clockwise = not clockwise
return clockwise
poly = [(2,2),(6,2),(6,6),(2,6)]
check_clockwise(poly)
False
poly = [(2, 6), (6, 6), (6, 2), (2, 2)]
check_clockwise(poly)
True
其他回答
c#代码实现lhf的答案:
// https://en.wikipedia.org/wiki/Curve_orientation#Orientation_of_a_simple_polygon
public static WindingOrder DetermineWindingOrder(IList<Vector2> vertices)
{
int nVerts = vertices.Count;
// If vertices duplicates first as last to represent closed polygon,
// skip last.
Vector2 lastV = vertices[nVerts - 1];
if (lastV.Equals(vertices[0]))
nVerts -= 1;
int iMinVertex = FindCornerVertex(vertices);
// Orientation matrix:
// [ 1 xa ya ]
// O = | 1 xb yb |
// [ 1 xc yc ]
Vector2 a = vertices[WrapAt(iMinVertex - 1, nVerts)];
Vector2 b = vertices[iMinVertex];
Vector2 c = vertices[WrapAt(iMinVertex + 1, nVerts)];
// determinant(O) = (xb*yc + xa*yb + ya*xc) - (ya*xb + yb*xc + xa*yc)
double detOrient = (b.X * c.Y + a.X * b.Y + a.Y * c.X) - (a.Y * b.X + b.Y * c.X + a.X * c.Y);
// TBD: check for "==0", in which case is not defined?
// Can that happen? Do we need to check other vertices / eliminate duplicate vertices?
WindingOrder result = detOrient > 0
? WindingOrder.Clockwise
: WindingOrder.CounterClockwise;
return result;
}
public enum WindingOrder
{
Clockwise,
CounterClockwise
}
// Find vertex along one edge of bounding box.
// In this case, we find smallest y; in case of tie also smallest x.
private static int FindCornerVertex(IList<Vector2> vertices)
{
int iMinVertex = -1;
float minY = float.MaxValue;
float minXAtMinY = float.MaxValue;
for (int i = 0; i < vertices.Count; i++)
{
Vector2 vert = vertices[i];
float y = vert.Y;
if (y > minY)
continue;
if (y == minY)
if (vert.X >= minXAtMinY)
continue;
// Minimum so far.
iMinVertex = i;
minY = y;
minXAtMinY = vert.X;
}
return iMinVertex;
}
// Return value in (0..n-1).
// Works for i in (-n..+infinity).
// If need to allow more negative values, need more complex formula.
private static int WrapAt(int i, int n)
{
// "+n": Moves (-n..) up to (0..).
return (i + n) % n;
}
从其中一个顶点开始,计算每条边对应的角度。
第一个和最后一个将是零(所以跳过它们);对于其余部分,角度的正弦值将由归一化与(点[n]-点[0])和(点[n-1]-点[0])的单位长度的叉乘给出。
如果这些值的和是正的,那么你的多边形是逆时针方向绘制的。
下面是基于这个答案的一个简单的Python 3实现(反过来,它是基于已接受答案中提出的解决方案)
def is_clockwise(points):
# points is your list (or array) of 2d points.
assert len(points) > 0
s = 0.0
for p1, p2 in zip(points, points[1:] + [points[0]]):
s += (p2[0] - p1[0]) * (p2[1] + p1[1])
return s > 0.0
我认为为了使某些点顺时针方向,所有的边都必须是正的而不仅仅是边的和。如果一条边是负的,则逆时针方向给出至少3个点。
求出这些点的质心。
假设有直线从这个点到你们的点。
求line0 line1的两条直线夹角
而不是直线1和直线2
...
...
如果这个角是单调递增的,而不是逆时针递增的,
如果是单调递减,则是顺时针递减
Else(它不是单调的)
你不能决定,所以这是不明智的