我试图创建一个快速的2D点内多边形算法,用于命中测试(例如多边形.contains(p:点))。对有效技术的建议将不胜感激。


当前回答

我知道这是旧的,但这里是一个在Cocoa实现的光线投射算法,如果有人感兴趣的话。不确定这是最有效的方法,但它可能会帮助别人。

- (BOOL)shape:(NSBezierPath *)path containsPoint:(NSPoint)point
{
    NSBezierPath *currentPath = [path bezierPathByFlatteningPath];
    BOOL result;
    float aggregateX = 0; //I use these to calculate the centroid of the shape
    float aggregateY = 0;
    NSPoint firstPoint[1];
    [currentPath elementAtIndex:0 associatedPoints:firstPoint];
    float olderX = firstPoint[0].x;
    float olderY = firstPoint[0].y;
    NSPoint interPoint;
    int noOfIntersections = 0;

    for (int n = 0; n < [currentPath elementCount]; n++) {
        NSPoint points[1];
        [currentPath elementAtIndex:n associatedPoints:points];
        aggregateX += points[0].x;
        aggregateY += points[0].y;
    }

    for (int n = 0; n < [currentPath elementCount]; n++) {
        NSPoint points[1];

        [currentPath elementAtIndex:n associatedPoints:points];
        //line equations in Ax + By = C form
        float _A_FOO = (aggregateY/[currentPath elementCount]) - point.y;  
        float _B_FOO = point.x - (aggregateX/[currentPath elementCount]);
        float _C_FOO = (_A_FOO * point.x) + (_B_FOO * point.y);

        float _A_BAR = olderY - points[0].y;
        float _B_BAR = points[0].x - olderX;
        float _C_BAR = (_A_BAR * olderX) + (_B_BAR * olderY);

        float det = (_A_FOO * _B_BAR) - (_A_BAR * _B_FOO);
        if (det != 0) {
            //intersection points with the edges
            float xIntersectionPoint = ((_B_BAR * _C_FOO) - (_B_FOO * _C_BAR)) / det;
            float yIntersectionPoint = ((_A_FOO * _C_BAR) - (_A_BAR * _C_FOO)) / det;
            interPoint = NSMakePoint(xIntersectionPoint, yIntersectionPoint);
            if (olderX <= points[0].x) {
                //doesn't matter in which direction the ray goes, so I send it right-ward.
                if ((interPoint.x >= olderX && interPoint.x <= points[0].x) && (interPoint.x > point.x)) {  
                    noOfIntersections++;
                }
            } else {
                if ((interPoint.x >= points[0].x && interPoint.x <= olderX) && (interPoint.x > point.x)) {
                     noOfIntersections++;
                } 
            }
        }
        olderX = points[0].x;
        olderY = points[0].y;
    }
    if (noOfIntersections % 2 == 0) {
        result = FALSE;
    } else {
        result = TRUE;
    }
    return result;
}

其他回答

简单的解决方案是将多边形划分为三角形,并按这里解释的那样对三角形进行测试

如果你的多边形是凸多边形,可能有更好的方法。把这个多边形看作是无限条线的集合。每一行将空间一分为二。对于每一个点,很容易判断它是在直线的一边还是另一边。如果一个点在所有直线的同一侧,那么它在多边形内。

当我还是Michael Stonebraker手下的一名研究员时,我做了一些关于这方面的工作——你知道,就是那位提出了Ingres、PostgreSQL等的教授。

我们意识到最快的方法是首先做一个边界框,因为它非常快。如果它在边界框之外,它就在外面。否则,你就得做更辛苦的工作……

如果你想要一个伟大的算法,看看开源项目PostgreSQL的源代码的地理工作…

我想指出的是,我们从来没有深入了解过左撇子和右撇子(也可以表达为“内”和“外”的问题……


更新

BKB's link provided a good number of reasonable algorithms. I was working on Earth Science problems and therefore needed a solution that works in latitude/longitude, and it has the peculiar problem of handedness - is the area inside the smaller area or the bigger area? The answer is that the "direction" of the verticies matters - it's either left-handed or right handed and in this way you can indicate either area as "inside" any given polygon. As such, my work used solution three enumerated on that page.

此外,我的工作使用单独的函数进行“在线”测试。

...因为有人问:我们发现当垂直的数量超过某个数字时,边界盒测试是最好的——如果有必要,在做更长的测试之前做一个非常快速的测试……边界框是通过简单地将最大的x,最小的x,最大的y和最小的y放在一起,组成一个框的四个点来创建的……

另一个提示是:我们在网格空间中进行了所有更复杂的“调光”计算,都是在平面上的正点上进行的,然后重新投影到“真实”的经度/纬度上,从而避免了在经度180线交叉时和处理极地时可能出现的环绕错误。工作好了!

令人惊讶的是之前没有人提出这个问题,但是对于需要数据库的实用主义者来说:MongoDB对Geo查询提供了出色的支持,包括这个查询。

你需要的是:

db.neighborhoods。findOne({geometry: {$geoIntersects: {$geometry: { type: "Point",坐标:["经度","纬度"]}}} })

communities是存储一个或多个标准GeoJson格式多边形的集合。如果查询返回null,则表示不相交,否则为。

这里有详细的记录: https://docs.mongodb.com/manual/tutorial/geospatial-tutorial/

在330个不规则多边形网格中,超过6000个点分类的性能不到一分钟,没有任何优化,包括用各自的多边形更新文档的时间。

计算点p与每个多边形顶点之间的有向角和。如果总倾斜角是360度,那么这个点在里面。如果总数为0,则点在外面。

我更喜欢这种方法,因为它更健壮,对数值精度的依赖更小。

计算交集数量的均匀性的方法是有限的,因为你可以在计算交集数量的过程中“击中”一个顶点。

编辑:顺便说一下,这种方法适用于凹凸多边形。

编辑:我最近在维基百科上找到了一篇关于这个话题的完整文章。

VBA版本:

注意:请记住,如果你的多边形是地图中的一个区域,纬度/经度是Y/X值,而不是X/Y(纬度= Y,经度= X),因为从我的理解来看,这是历史含义,因为经度不是一个测量值。

类模块:CPoint

Private pXValue As Double
Private pYValue As Double

'''''X Value Property'''''

Public Property Get X() As Double
    X = pXValue
End Property

Public Property Let X(Value As Double)
    pXValue = Value
End Property

'''''Y Value Property'''''

Public Property Get Y() As Double
    Y = pYValue
End Property

Public Property Let Y(Value As Double)
    pYValue = Value
End Property

模块:

Public Function isPointInPolygon(p As CPoint, polygon() As CPoint) As Boolean

    Dim i As Integer
    Dim j As Integer
    Dim q As Object
    Dim minX As Double
    Dim maxX As Double
    Dim minY As Double
    Dim maxY As Double
    minX = polygon(0).X
    maxX = polygon(0).X
    minY = polygon(0).Y
    maxY = polygon(0).Y

    For i = 1 To UBound(polygon)
        Set q = polygon(i)
        minX = vbMin(q.X, minX)
        maxX = vbMax(q.X, maxX)
        minY = vbMin(q.Y, minY)
        maxY = vbMax(q.Y, maxY)
    Next i

    If p.X < minX Or p.X > maxX Or p.Y < minY Or p.Y > maxY Then
        isPointInPolygon = False
        Exit Function
    End If


    ' SOURCE: http://www.ecse.rpi.edu/Homepages/wrf/Research/Short_Notes/pnpoly.html

    isPointInPolygon = False
    i = 0
    j = UBound(polygon)

    Do While i < UBound(polygon) + 1
        If (polygon(i).Y > p.Y) Then
            If (polygon(j).Y < p.Y) Then
                If p.X < (polygon(j).X - polygon(i).X) * (p.Y - polygon(i).Y) / (polygon(j).Y - polygon(i).Y) + polygon(i).X Then
                    isPointInPolygon = True
                    Exit Function
                End If
            End If
        ElseIf (polygon(i).Y < p.Y) Then
            If (polygon(j).Y > p.Y) Then
                If p.X < (polygon(j).X - polygon(i).X) * (p.Y - polygon(i).Y) / (polygon(j).Y - polygon(i).Y) + polygon(i).X Then
                    isPointInPolygon = True
                    Exit Function
                End If
            End If
        End If
        j = i
        i = i + 1
    Loop   
End Function

Function vbMax(n1, n2) As Double
    vbMax = IIf(n1 > n2, n1, n2)
End Function

Function vbMin(n1, n2) As Double
    vbMin = IIf(n1 > n2, n2, n1)
End Function


Sub TestPointInPolygon()

    Dim i As Integer
    Dim InPolygon As Boolean

'   MARKER Object
    Dim p As CPoint
    Set p = New CPoint
    p.X = <ENTER X VALUE HERE>
    p.Y = <ENTER Y VALUE HERE>

'   POLYGON OBJECT
    Dim polygon() As CPoint
    ReDim polygon(<ENTER VALUE HERE>) 'Amount of vertices in polygon - 1
    For i = 0 To <ENTER VALUE HERE> 'Same value as above
       Set polygon(i) = New CPoint
       polygon(i).X = <ASSIGN X VALUE HERE> 'Source a list of values that can be looped through
       polgyon(i).Y = <ASSIGN Y VALUE HERE> 'Source a list of values that can be looped through
    Next i

    InPolygon = isPointInPolygon(p, polygon)
    MsgBox InPolygon

End Sub