var canvas=document.getElementById("canvas");
var ctx=canvas.getContext("2d");
var cw=canvas.width;
var ch=canvas.height;
function reOffset(){
var BB=canvas.getBoundingClientRect();
offsetX=BB.left;
offsetY=BB.top;
}
var offsetX,offsetY;
reOffset();
window.onscroll=function(e){ reOffset(); }
window.onresize=function(e){ reOffset(); }
var PI=Math.PI;
var PI2=PI*2;
var cx=cw/2;
var cy=ch/2;
var a=120;
var b=80;
var points=getPointsOnEllipse(cx,cy,a,b);
$("#canvas").mousemove(function(e){handleMouseMove(e);});
drawEllipse(points);
function dot(x,y,color){
ctx.beginPath();
ctx.arc(x,y,3,0,PI2);
ctx.closePath();
ctx.fillStyle=color;
ctx.fill();
}
function isInEllipse(x,y){
var dx=x-cx;
var dy=y-cy;
return ((dx*dx)/(a*a)+(dy*dy)/(b*b)<=1);
}
function drawEllipse(points){
ctx.beginPath();
ctx.moveTo(points[0].x,points[0].y);
for(var j=0;j<points.length;j++){
ctx.lineTo(points[j].x,points[j].y);
}
ctx.closePath();
ctx.lineWidth=1;
ctx.strokeStyle='forestgreen';
ctx.stroke();
}
function getPointsOnEllipse(cx,cy,a,b){
var startAngle=-PI/2;
var lastX=cx-(a*Math.cos(startAngle));
var lastY=cy+(b*Math.sin(startAngle));
var points=[];
for(var i=0;i<1000;i++){
var angle=startAngle+PI2/1000*i;
var x=cx-(a*Math.cos(angle));
var y=cy+(b*Math.sin(angle));
var dx=x-lastX;
var dy=y-lastY;
var length=parseInt(Math.sqrt(dx*dx+dy*dy));
var eAngle=(Math.atan2(dy,dx)+PI2)%PI2;
if(length>0){
points.push({x:x,y:y,angle:eAngle});
lastX=x;
lastY=y;
}
}
return(points);
}
function handleMouseMove(e){
e.preventDefault();
e.stopPropagation();
mouseX=parseInt(e.clientX-offsetX);
mouseY=parseInt(e.clientY-offsetY);
var hit=isInEllipse(mouseX,mouseY);
var color=(hit)?'red':'green';
dot(mouseX,mouseY,color);
}
body{ background-color: ivory; }
#canvas{border:1px solid red; }
<script src="https://ajax.googleapis.com/ajax/libs/jquery/1.9.1/jquery.min.js"></script>
<h4>Move mouse in & out of ellipse.<br>Red mouse dots are inside ellipse, Green outside.</h4>
<canvas id="canvas" width=300 height=300></canvas>
x = a cos t, y = b sin t
或等效方式,还是通过使用一对context.bezierCurveTo()
函数来模拟椭圆? - Brian Peacockcontext.scale(1,0.5)
然后context.arc
- Justin Taddei