各种计算还挺繁琐的, 关键点在角度的计算, 根据鼠标位置, 利用atan(y/x) 得到反正切值 , 角度 (tan输入的是r和x围成的那个角，输出的是y/x。反tan就是输入y/x输出角。)
<!DOCTYPE html>
<html>
<body>
<canvas id="canvas" width="800" height="950" style="border:1px solid #d3d3d3;margin-top:10px">
Your browser does not support the HTML5 canvas tag.
</canvas>
<script>
const canvas = document.getElementById('canvas');
const ctx = canvas.getContext('2d');
const PI = Math.PI;
const number = 4;//画4个外圆；
const reduceRadio = 15;//递减半径；
const maxRadius = 80;//最大半径
const minRadio = maxRadius-number*reduceRadio;
const translateX = 100;//canvas移动位置
const translateY = 100;
//canvas移动位置
ctx.translate(translateX,translateY);
//画所有圆
function drawAllOuterCircle(number,reduceRadio,maxRadius){
//画虚线
ctx.setLineDash([2,2]);
for(let i=0;i<number;i++){
ctx.beginPath();
//半径递减
ctx.arc(0,0, maxRadius-i*reduceRadio, 0, 2*PI);
if(i+n == 4){
ctx.strokeStyle = 'red';
}else{
ctx.strokeStyle = 'black';
}
ctx.stroke();
ctx.closePath();
}
//重置
ctx.restore();
//最里面小圆
ctx.beginPath();
ctx.arc(0,0,maxRadius-number*reduceRadio,0,2*PI);
ctx.fillStyle="red";
ctx.fill();
ctx.closePath();
ctx.restore();
}
drawAllOuterCircle(number,reduceRadio,maxRadius);
//画分割直线
const lineNum = 8;//分割线条数
const minAngel = 2*PI/lineNum;//最小角度
function drawSplitLine(lineNum,maxRadius){
for(let i=0;i<lineNum;i++){
ctx.setLineDash([]);
ctx.beginPath();
ctx.moveTo(0,0);
ctx.lineTo(0,maxRadius);
ctx.strokeStyle = 'black';
ctx.stroke();
//旋转
ctx.rotate(2*PI/lineNum);
ctx.closePath();
}
}
drawSplitLine(lineNum,maxRadius);
//画阴影部分
function drawShadowPanel(ceilMouseRadio,floorMouseRadio,ceilMouseAngel,floorMouseAngel){
//清屏
ctx.beginPath();
ctx.moveTo(0,0);
ctx.arc(0,0,ceilMouseRadio+reduceRadio,0,2*PI);
ctx.fillStyle = "white";
ctx.fill();
ctx.closePath();
//调试: 先画一个大的绿色扇形
ctx.beginPath();
ctx.moveTo(0,0);
ctx.arc(0,0,ceilMouseRadio,floorMouseAngel,ceilMouseAngel);
ctx.fillStyle = "green";
ctx.fill();
ctx.closePath();
//再画一个小的白色扇形 覆盖 大的扇形, 视觉上形成一个弧形
ctx.beginPath();
ctx.moveTo(0,0);
ctx.arc(0,0,floorMouseRadio,floorMouseAngel,ceilMouseAngel);
ctx.fillStyle = "white";
ctx.fill();
ctx.closePath();
}
const maxN = (maxRadius-minRadio)/reduceRadio;
var n;
//获取ceilMouseRadio,floorMouseAngel,ceilMouseAngel
//获取canvas 的 位移
const canvasX = canvas.getBoundingClientRect().left + document.documentElement.scrollLeft +translateX;
const canvasY = canvas.getBoundingClientRect().top + document.documentElement.scrollTop +translateY;
function getMouseDetail(mouseX,mouseY){
//利用勾股定理 算出鼠标 离 canvas原点的距离
let mouseRadio = Math.sqrt( Math.pow(mouseX-canvasX, 2) + Math.pow(mouseY-canvasY, 2));
//因为 cavas原点已经移动到 100 100, 判断鼠标是否在圆圈内
if(mouseRadio>maxRadius||mouseRadio<=minRadio){
return {ceilMouseRadio:0,floorMouseRadio:0,floorMouseAngel:0,ceilMouseAngel:0};
}
//计算鼠标半径和最小半径的比例,画阴影块的大小半径 reduceRadio:15
n = Math.ceil((mouseRadio-minRadio)/reduceRadio);
let ceilMouseRadio = minRadio+n*reduceRadio;
let floorMouseRadio = minRadio+(n-1)*reduceRadio;
//角度
let mouseAngel;
if(mouseX-canvasX<0){
//计算当前鼠标 相对原点的的弧度值 , x轴 顺时针
// 比如 Math.atan(1)/Math.PI*180 是 45度
mouseAngel = Math.atan((mouseY-canvasY)/(mouseX-canvasX)) + PI;
}else{
mouseAngel = Math.atan((mouseY-canvasY)/(mouseX-canvasX));
}
// 计算当前鼠标 有多少个象限
let m = Math.ceil(mouseAngel/minAngel);
let floorMouseAngel = (m-1)*minAngel;
let ceilMouseAngel = m*minAngel;
return {ceilMouseRadio,floorMouseRadio,floorMouseAngel,ceilMouseAngel};
}
//监听事件画图
canvas.addEventListener('mousemove', (e)=>{
let mouseX = e.pageX;
let mouseY = e.pageY;
getMouseDetail(mouseX,mouseY);
let mouseDetail = getMouseDetail(mouseX,mouseY);
let {ceilMouseRadio,floorMouseRadio,floorMouseAngel,ceilMouseAngel} = mouseDetail;
if(ceilMouseRadio){
//先画阴影,
drawShadowPanel(ceilMouseRadio,floorMouseRadio,ceilMouseAngel,floorMouseAngel);
//再画圆圈
drawAllOuterCircle(number,reduceRadio,maxRadius);
drawSplitLine(lineNum,maxRadius);
}
});
</script>
</body>
</html>