使用canvas画多个多边形，并在质心填充文字
<!DOCTYPE html>
<html lang="en">
<head>
<meta charset="UTF-8">
<title>canvas绘制多边形</title>
<script src="jquery-1.12.4.min.js"></script>
</head>
<body>
<style>
canvas {
border: 1px solid #333;
display: block;
}
input {
width: 100px;
margin-left: 200px;
margin-top: 650px;
}
#canvas {
position: absolute;
left: 0;
top: 0;
z-index: 1;
cursor: crosshair;
}
#canvasSave {
position: absolute;
left: 0;
top: 0;
}
</style>
<!--用来和鼠标进行交互操作的canvas-->
<canvas id="canvas" width="1000px" height="600px"></canvas>
<!--存储已生成的点线，避免被清空-->
<canvas id="canvasSave" width="1000px" height="600px"></canvas>
<input id="deleteCanvas" type="button" value="清空选区">
<script>
var can = document.getElementById("canvas");
var ctx = can.getContext('2d');
var canSave = document.getElementById("canvasSave");
var ctxSave = canSave.getContext('2d');
var allPoint = [];
var pointX, pointY;
var pointArr = [];//存放坐标的数组
ctx.strokeStyle = 'rgba(102,168,255,1)';//线条颜色
ctx.lineWidth = 4;//线条粗细
ctxSave.strokeStyle = 'rgba(102,168,255,1)';//线条颜色
ctxSave.lineWidth = 4;//线条粗细
var oIndex = -1;//判断鼠标是否移动到起始点处，-1为否，1为是
/*点击画点*/
$(can).click(function (e) {
var pointObj = getPolygonAreaCenter(pointArr)
if (e.offsetX || e.layerX) {
pointX = e.offsetX == undefined ? e.layerX : e.offsetX;
pointY = e.offsetY == undefined ? e.layerY : e.offsetY;
var piX, piY;
if (oIndex > 0 && pointArr.length > 0) {
piX = pointArr[0].x;
piY = pointArr[0].y;
console.log(piX, piY, "if")
//画点
makearc(ctx, piX, piY, GetRandomNum(2, 2), 0, 180, 'rgba(102,168,255,1)');
canvasSave(pointArr);//保存点线同步到另一个canvas
// saveCanvas();//生成画布
saveCanvas()
ctxSave.fillStyle = 'red';
ctxSave.font = '20px sans-serif';
ctxSave.fillText('hello world!', pointObj.x, pointObj.y);
pointArr = []
} else {
piX = pointX;
piY = pointY;
makearc(ctx, piX, piY, GetRandomNum(2, 2), 0, 180, 'rgba(102,168,255,1)');
console.log(piX, piY, "else")
pointArr.push({ x: piX, y: piY });
canvasSave(pointArr);//保存点线同步到另一个canvas
}
}
});
/* */
$(can).mousemove(function (e) {
if (e.offsetX || e.layerX) {
pointX = e.offsetX == undefined ? e.layerX : e.offsetX;
pointY = e.offsetY == undefined ? e.layerY : e.offsetY;
var piX, piY;
/*清空画布*/
ctx.clearRect(0, 0, can.width, can.height);
/*鼠标下跟随的圆点*/
makearc(ctx, pointX, pointY, GetRandomNum(4, 4), 0, 180, 'rgba(102,168,255,1)');
if (pointArr.length > 0) {
if ((pointX > pointArr[0].x - 15 && pointX < pointArr[0].x + 15) && (pointY > pointArr[0].y - 15 && pointY < pointArr[0].y + 15)) {
if (pointArr.length > 1) {
piX = pointArr[0].x;
piY = pointArr[0].y;
ctx.clearRect(0, 0, can.width, can.height);
makearc(ctx, piX, piY, GetRandomNum(4, 4), 0, 180, 'rgba(102,168,255,1)');
oIndex = 1;
}
} else {
piX = pointX;
piY = pointY;
oIndex = -1;
}
/*开始绘制*/
ctx.beginPath();
ctx.moveTo(pointArr[0].x, pointArr[0].y);
if (pointArr.length > 1) {
for (var i = 1; i < pointArr.length; i++) {
ctx.lineTo(pointArr[i].x, pointArr[i].y);
}
}
ctx.lineTo(piX, piY);
ctx.fillStyle = 'rgba(161,195,255,1)';//填充颜色
ctx.fill();//填充
ctx.stroke();//绘制
}
}
});
// 存储已生成的点线
function canvasSave(pointArr) {
ctxSave.clearRect(0, 0, ctxSave.width, ctxSave.height);
ctxSave.beginPath();
if (pointArr.length > 1) {
ctxSave.moveTo(pointArr[0].x, pointArr[0].y);
for (var i = 1; i < pointArr.length; i++) {
ctxSave.lineTo(pointArr[i].x, pointArr[i].y);
ctxSave.fillStyle = 'rgba(161,195,255,1)';//填充颜色
ctxSave.strokeStyle = "green" //设置border的颜色
ctxSave.fill();
ctxSave.stroke();//绘制
}
ctxSave.closePath();
}
}
/*生成画布 结束绘画*/
function saveCanvas() {
ctx.clearRect(0, 0, can.width, can.height);
ctxSave.closePath();//结束路径状态，结束当前路径，如果是一个未封闭的图形，会自动将首尾相连封闭起来
// ctxSave.fill();//填充
ctxSave.stroke();//绘制
pointArr = [];
}
/*清空选区*/
$('#deleteCanvas').click(function () {
ctx.clearRect(0, 0, can.width, can.height);
ctxSave.clearRect(0, 0, canSave.width, canSave.height);
pointArr = [];
// allPoint = [];
});
/*验证canvas画布是否为空函数*/
function isCanvasBlank(canvas) {
var blank = document.createElement('canvas');//创建一个空canvas对象
blank.width = canvas.width;
blank.height = canvas.height;
return canvas.toDataURL() == blank.toDataURL();//为空 返回true
}
/*canvas生成圆点*/
function GetRandomNum(Min, Max) {
var Range = Max - Min;
var Rand = Math.random();
return (Min + Math.round(Rand * Range));
}
function makearc(ctx, x, y, r, s, e, color) {
ctx.clearRect(0, 0, 199, 202);//清空画布
ctx.beginPath();
ctx.fillStyle = color;
ctx.arc(x, y, r, s, e);
ctx.fill();
}
function Area(p0, p1, p2) {
let area = 0.0;
area =
p0.x * p1.y +
p1.x * p2.y +
p2.x * p0.y -
p1.x * p0.y -
p2.x * p1.y -
p0.x * p2.y;
return area / 2;
}
// 计算多边形质心
function getPolygonAreaCenter(points) {
let sum_x = 0;
let sum_y = 0;
let sum_area = 0;
let p1 = points[1];
for (var i = 2; i < points.length; i++) {
let p2 = points[i];
let area = Area(points[0], p1, p2);
sum_area += area;
sum_x += (points[0].x + p1.x + p2.x) * area;
sum_y += (points[0].y + p1.y + p2.y) * area;
p1 = p2;
}
return {
x: sum_x / sum_area / 3,
y: sum_y / sum_area / 3
};
}
</script>
</body>
</html>