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gitea/modules/identicon/polygon.go
2015-08-09 12:06:08 +08:00

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// Copyright 2015 by caixw, All rights reserved.
// Use of this source code is governed by a MIT
// license that can be found in the LICENSE file.
package identicon
var (
// 4个元素分别表示cos(0),cos(90),cos(180),cos(270)
cos = []float64{1, 0, -1, 0}
// 4个元素分别表示sin(0),sin(90),sin(180),sin(270)
sin = []float64{0, 1, 0, -1}
)
// 将points中的所有点以x,y为原点旋转angle个角度。
// angle取值只能是[0,1,2,3],分别表示[090180270]
func rotate(points []float64, x, y float64, angle int) {
if angle > 3 {
panic("rotate:参数angle必须0,1,2,3三值之一")
}
for i := 0; i < len(points); i += 2 {
px := points[i] - x
py := points[i+1] - y
points[i] = px*cos[angle] - py*sin[angle] + x
points[i+1] = px*sin[angle] + py*cos[angle] + y
}
}
// 判断某个点是否在多边形之内,不包含构成多边形的线和点
// x,y 需要判断的点坐标
// points 组成多边形的所顶点,每两个元素表示一点顶点,其中最后一个顶点必须与第一个顶点相同。
func pointInPolygon(x float64, y float64, points []float64) bool {
if len(points) < 8 { // 只有2个以上的点才能组成闭合多边形
return false
}
// 大致算法如下:
// 把整个平面以给定的测试点为原点分两部分:
// - y>0包含(x>0 && y==0)
// - y<0包含(x<0 && y==0)
// 依次扫描每一个点当该点与前一个点处于不同部分时即一个在y>0区一个在y<0区
// 则判断从前一点到当前点是顺时针还是逆时针以给定的测试点为原点如果是顺时针r++否则r--。
// 结果为2==abs(r)。
r := 0
x1, y1 := points[0], points[1]
prev := (y1 > y) || ((x1 > x) && (y1 == y))
for i := 2; i < len(points); i += 2 {
x2, y2 := points[i], points[i+1]
curr := (y2 > y) || ((x2 > x) && (y2 == y))
if curr == prev {
x1, y1 = x2, y2
continue
}
mul := (x1-x)*(y2-y) - (x2-x)*(y1-y)
if mul > 0 {
r++
} else if mul < 0 {
r--
}
x1, y1 = x2, y2
prev = curr
}
return r == 2 || r == -2
}