I would like to animate a circle into a line with the radius equaling the width, and I am wondering how I can do this with an Arc? Or perhaps there is a better way?
Here's my arc:
function drawStar(x,y,size,scale,opacity,ctx){
ctx.save();
ctx.beginPath();
ctx.arc(x,y,size+scale,0,size+scale * Math.PI,false);
ctx.globalAlpha = opacity
ctx.closePath();
setFillStyle('rgb(255,237,219)',ctx);
ctx.fill()
ctx.restore();
}
I tried using ctx.scale(n,1), however it does not keep the same radius(width) and it scales the collection of arcs as a whole (zoom in effect).
Use instead a wide line-width value with "round" lineCap
and stroke()
:
var ctx = c.getContext("2d"); ctx.lineWidth = 50; ctx.lineCap = "round"; ctx.moveTo(45 , 25); ctx.lineTo(45.5, 25); // in IE11 this must be slightly offset ctx.moveTo( 45, 100); ctx.lineTo(150, 100); ctx.stroke();
<canvas id=c></canvas>
Remember beginPath()
for animation.
You could draw the left and right halves of a circle using arc
, then do a fillRect
in between to connect them.
Edit: To elaborate on what I said earlier:
function init() {
let canvas = document.getElementById('myCanvas');
canvas.width = 400;
canvas.height = 400;
canvas.style.width = "400px";
canvas.style.height = "400px";
let ctx = canvas.getContext("2d");
function fillArc(ctx, cx, cy, r, startDeg, endDeg) {
ctx.beginPath();
ctx.arc(cx, cy, r, startDeg * Math.PI / 180, endDeg * Math.PI / 180);
ctx.fill();
}
function fillOval(ctx, cx, cy, r, sideLength, skipFirstArc) {
if (!skipFirstArc) {
fillArc(ctx, cx, cy, r, 90, 270);
}
ctx.fillRect(cx, cy - r, sideLength, r * 2);
fillArc(ctx, cx + sideLength, cy, r, 270, 90);
}
let sideLength = 0;
ctx.fillStyle = 'red';
function animateOval() {
if (sideLength === 100) {
ctx.clearRect(0, 0, 400, 400);
}
else {
fillOval(ctx, 30, 30, 25, sideLength, sideLength > 0);
}
++sideLength;
if (sideLength > 100) {
sideLength = 0;
}
}
setInterval(animateOval, 16);
}
Here's a Plunker with the above code running: http://plnkr.co/edit/vNqoUjPKg2lqC7JtYuEb?p=preview
You can use Bezier Curves to 'transform' your arc.
There's some math involved in calculating the perfect ends of your stretched circle but I guessed and tweaked my numbers.
var c = document.getElementById("myCanvas"); var ctx = c.getContext("2d"); ctx.beginPath(); ctx.moveTo(40, 20); ctx.lineTo(100, 20); ctx.bezierCurveTo(130, 20, 130, 60, 100, 60); ctx.lineTo(40, 60); ctx.bezierCurveTo(10, 60, 10, 20, 40, 20); ctx.stroke(); ctx.fill(); ctx.closePath(); ctx.beginPath(); ctx.moveTo(40, 80); ctx.bezierCurveTo(68, 80, 68, 120, 40, 120); ctx.bezierCurveTo(12, 120, 12, 80, 40, 80); ctx.fill(); ctx.stroke();
<canvas id="myCanvas"></canvas>
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