什么是最简单的算法来确定一个点是否在一个二维三角形内部?
最简单的方法,适用于所有类型的三角形,就是确定P点A、B、C点的角度。如果任何一个角度大于180.0度,则它在外面,如果是180.0度,则在圆周上,如果acos欺骗了你并且小于180.0度,则在内部。请查看以下链接以便理解http://math-physics-psychology.blogspot.hu/2015/01/earlish-determination-that-point-is.html
下面是我在JavaScript中改编的高性能代码(见下文):
function pointInTriangle (p, p0, p1, p2) {
return (((p1.y - p0.y) * (p.x - p0.x) - (p1.x - p0.x) * (p.y - p0.y)) | ((p2.y - p1.y) * (p.x - p1.x) - (p2.x - p1.x) * (p.y - p1.y)) | ((p0.y - p2.y) * (p.x - p2.x) - (p0.x - p2.x) * (p.y - p2.y))) >= 0;
}
pointInTriangle(p, p0, p1, p2)
- 逆时针三角形pointInTriangle(p, p0, p1, p2)
- 顺时针三角形请查看 jsFiddle(包含性能测试),另外还有一个单独的函数用于检查绕序。或者在下面点击“运行代码片段”。
var ctx = $("canvas")[0].getContext("2d");
var W = 500;
var H = 500;
var point = { x: W / 2, y: H / 2 };
var triangle = randomTriangle();
$("canvas").click(function(evt) {
point.x = evt.pageX - $(this).offset().left;
point.y = evt.pageY - $(this).offset().top;
test();
});
$("canvas").dblclick(function(evt) {
triangle = randomTriangle();
test();
});
document.querySelector('#performance').addEventListener('click', _testPerformance);
test();
function test() {
var result = checkClockwise(triangle.a, triangle.b, triangle.c) ? pointInTriangle(point, triangle.a, triangle.c, triangle.b) : pointInTriangle(point, triangle.a, triangle.b, triangle.c);
var info = "point = (" + point.x + "," + point.y + ")\n";
info += "triangle.a = (" + triangle.a.x + "," + triangle.a.y + ")\n";
info += "triangle.b = (" + triangle.b.x + "," + triangle.b.y + ")\n";
info += "triangle.c = (" + triangle.c.x + "," + triangle.c.y + ")\n";
info += "result = " + (result ? "true" : "false");
$("#result").text(info);
render();
}
function _testPerformance () {
var px = [], py = [], p0x = [], p0y = [], p1x = [], p1y = [], p2x = [], p2y = [], p = [], p0 = [], p1 = [], p2 = [];
for(var i = 0; i < 1000000; i++) {
p[i] = {x: Math.random() * 100, y: Math.random() * 100};
p0[i] = {x: Math.random() * 100, y: Math.random() * 100};
p1[i] = {x: Math.random() * 100, y: Math.random() * 100};
p2[i] = {x: Math.random() * 100, y: Math.random() * 100};
}
console.time('optimal: pointInTriangle');
for(var i = 0; i < 1000000; i++) {
pointInTriangle(p[i], p0[i], p1[i], p2[i]);
}
console.timeEnd('optimal: pointInTriangle');
console.time('original: ptInTriangle');
for(var i = 0; i < 1000000; i++) {
ptInTriangle(p[i], p0[i], p1[i], p2[i]);
}
console.timeEnd('original: ptInTriangle');
}
function pointInTriangle (p, p0, p1, p2) {
return (((p1.y - p0.y) * (p.x - p0.x) - (p1.x - p0.x) * (p.y - p0.y)) | ((p2.y - p1.y) * (p.x - p1.x) - (p2.x - p1.x) * (p.y - p1.y)) | ((p0.y - p2.y) * (p.x - p2.x) - (p0.x - p2.x) * (p.y - p2.y))) >= 0;
}
function ptInTriangle(p, p0, p1, p2) {
var s = (p0.y * p2.x - p0.x * p2.y + (p2.y - p0.y) * p.x + (p0.x - p2.x) * p.y);
var t = (p0.x * p1.y - p0.y * p1.x + (p0.y - p1.y) * p.x + (p1.x - p0.x) * p.y);
if (s <= 0 || t <= 0) return false;
var A = (-p1.y * p2.x + p0.y * (-p1.x + p2.x) + p0.x * (p1.y - p2.y) + p1.x * p2.y);
return (s + t) < A;
}
function render() {
ctx.fillStyle = "#CCC";
ctx.fillRect(0, 0, 500, 500);
drawTriangle(triangle.a, triangle.b, triangle.c);
drawPoint(point);
}
function checkClockwise(p0, p1, p2) {
var A = (-p1.y * p2.x + p0.y * (-p1.x + p2.x) + p0.x * (p1.y - p2.y) + p1.x * p2.y);
return A > 0;
}
function drawTriangle(p0, p1, p2) {
ctx.fillStyle = "#999";
ctx.beginPath();
ctx.moveTo(p0.x, p0.y);
ctx.lineTo(p1.x, p1.y);
ctx.lineTo(p2.x, p2.y);
ctx.closePath();
ctx.fill();
ctx.fillStyle = "#000";
ctx.font = "12px monospace";
ctx.fillText("1", p0.x, p0.y);
ctx.fillText("2", p1.x, p1.y);
ctx.fillText("3", p2.x, p2.y);
}
function drawPoint(p) {
ctx.fillStyle = "#F00";
ctx.beginPath();
ctx.arc(p.x, p.y, 5, 0, 2 * Math.PI);
ctx.fill();
}
function rand(min, max) {
return Math.floor(Math.random() * (max - min + 1)) + min;
}
function randomTriangle() {
return {
a: { x: rand(0, W), y: rand(0, H) },
b: { x: rand(0, W), y: rand(0, H) },
c: { x: rand(0, W), y: rand(0, H) }
};
}
<script src="https://cdnjs.cloudflare.com/ajax/libs/jquery/1.9.1/jquery.min.js"></script>
<button id="performance">Run performance test (open console)</button>
<pre>Click: place the point.
Double click: random triangle.</pre>
<pre id="result"></pre>
<canvas width="500" height="500"></canvas>
function isInTriangle(t,p){
function isInBorder(a,b,c,p){
var m = (a.y - b.y) / (a.x - b.x); // calculate the slope
return Math.sign(p.y - m*p.x + m*a.x - a.y) === Math.sign(c.y - m*c.x + m*a.x - a.y);
}
function findAngle(a,b,c){ // calculate the C angle from 3 points.
var ca = Math.hypot(c.x-a.x, c.y-a.y), // ca edge length
cb = Math.hypot(c.x-b.x, c.y-b.y), // cb edge length
ab = Math.hypot(a.x-b.x, a.y-b.y); // ab edge length
return Math.acos((ca*ca + cb*cb - ab*ab) / (2*ca*cb)); // return the C angle
}
var pas = t.slice(1)
.map(tp => findAngle(p,tp,t[0])), // find the angle between (p,t[0]) with (t[1],t[0]) & (t[2],t[0])
ta = findAngle(t[1],t[2],t[0]);
return pas[0] < ta && pas[1] < ta && isInBorder(t[1],t[2],t[0],p);
}
var triangle = [{x:3, y:4},{x:10, y:8},{x:6, y:10}],
point1 = {x:3, y:9},
point2 = {x:7, y:9};
console.log(isInTriangle(triangle,point1));
console.log(isInTriangle(triangle,point2));
bool point2Dtriangle(double e,double f, double a,double b,double c, double g,double h,double i, double v, double w){
/* inputs: e=point.x, f=point.y
a=triangle.Ax, b=triangle.Bx, c=triangle.Cx
g=triangle.Ay, h=triangle.By, i=triangle.Cy */
v = 1 - (f * (b - c) + h * (c - e) + i * (e - b)) / (g * (b - c) + h * (c - a) + i * (a - b));
w = (f * (a - b) + g * (b - e) + h * (e - a)) / (g * (b - c) + h * (c - a) + i * (a - b));
if (*v > -0.0 && *v < 1.0000001 && *w > -0.0 && *w < *v) return true;//is inside
else return false;//is outside
return 0;
}
A---B
|..\\.o|
|....\\.|
D---C
点O在ABC三角形内,如果要测试第二个三角形,请调用此函数CDA方向,并且在四边形中*v=1-*v;
和*w=1-*w;
后结果应该是正确的。
检查由三角形顶点(x1,y1),(x2,y2),(x3,y3)形成的区域是否为正数的最简单方法之一。
可以通过以下公式计算面积:
1/2 [x1(y2–y3) + x2(y3–y1) + x3(y1–y2)]
或者可以使用Python代码实现:
def triangleornot(p1,p2,p3):
return (1/ 2) [p1[0](p2[1]–p3[1]) + p2[0] (p3[1]–p1[1]) + p3[0] (p1[0]–p2[0])]