Howzieky Howzieky - 25 days ago 15
Javascript Question

HTML5 Canvas ctx.clip() method used on a circle leaves a line underneath the circle

I've created a program to generate planet sprites. I'm doing that by creating a circular path, running

ctx.clip()
to keep all the following layers inside of the circle, then drawing a black and transparent texture layer, then a randomly colored rectangle over the full canvas, then a shadow and glow on top of it all. The issue is that a colored line also appears under the circle after clipping, and I'm not sure why. I need this removed.

Here is a fiddle. The last line sets the code to loop every half second: https://jsfiddle.net/tzkwmzqu/4/

Answer

I am not sure I do understand your problem, but I will assume that you are talking about the anti-aliasing problem.

Currently, you are drawing a lot over your clipped area.
At each draw, new anti-aliasing artifacts will come to smooth the latest drawing. At the end, what should have been semi-transparent pixels are now fully opaque ones.
In the other hand, with globalCompositeOperation like 'destination-in', you need only one drawing to make the compositing (~clipping). So you don't accumulate artifacts. But even if you did, gCO is global and since it takes transparency into account, the accumulation would be less important.

var ctx1 = clip.getContext('2d');
var ctx2 = gCO.getContext('2d');

ctx1.beginPath();
ctx1.arc(150, 150, 150, 0, Math.PI*2)
ctx1.clip();
// drawing multiple times on this clipped area will increase artifacts
ctx1.fillRect(0,0,300, 150);
ctx1.fillRect(0,0,300, 150);
ctx1.fillRect(0,0,300, 150);
ctx1.fillRect(0,0,300, 150);

ctx2.beginPath();
ctx2.arc(150, 150, 150, 0, Math.PI*2)
ctx2.fillRect(0,0,300, 150);
ctx2.globalCompositeOperation = 'destination-in';
//With gCO you only draw once, but even if you did draw multiple times, there would still be less artifacts
ctx2.fill();
ctx2.fill();
ctx2.fill();
ctx2.fill();
ctx2.globalCompositeOperation = 'source-over';

ctx1.fillStyle = ctx2.fillStyle = "white";
ctx1.fillText('clipping', 120, 100);
ctx2.fillText('compositing', 120, 100);
canvas{
  border: 1px solid;
  }
<canvas id="clip"></canvas><canvas id="gCO"></canvas>

A few unrelated notes about your code :

closePath does not mark the end of your path declaration, only a new beginPath() call does. ctx.fillStyle = 'transparent'; ctx.fill() won't do anything. Only putImageData, clearRect methods and globalCompositeOperation + drawing method can produce transparent pixels.

So here is all the above in one snippet :

/* Load images */
var texture = new Image();
texture.src = "http://i.imgur.com/0qMwa8p.png";
var shadow = new Image();
shadow.src = "http://i.imgur.com/pX3HVFY.png";

/* Create the canvas and context references */
var canvas = document.getElementById("game");
canvas.style.width = (canvas.width = 512) + "px";
canvas.style.height = (canvas.height = 512) + "px";
var ctx = canvas.getContext("2d");

/* render */
function render() {
  /* Size of planets */
  var scale = Math.random() + 1

  
  // We don't need to save/restore the canvas state now,
  // simply remember to set the gCO back to 'source-over'
  // here it done at the end of the function
  
  /* Clear canvas for redraw */
  ctx.clearRect(0, 0, canvas.width, canvas.height);
  /* Place texture onto planet */
  ctx.globalAlpha = Math.random() * .5 + .5;
  ctx.drawImage(texture, (Math.round(Math.random() * 256) - 128 * scale), (Math.round(Math.random() * 256) - 128 * scale), texture.naturalWidth * scale, texture.naturalHeight * scale)

  /* Color Planet */
  ctx.globalAlpha = 1;
  ctx.globalCompositeOperation = "multiply";
  var color = "hsl(" + Math.random() * 256 + ", 100%, 50%)"
  ctx.fillStyle = color;
  ctx.fillRect(0, 0, canvas.width, canvas.height)

  /* Give planet its shine and shadow */
  ctx.globalCompositeOperation = "source-over";
  ctx.drawImage(shadow, Math.round(Math.random() * 200 - 128 * scale), Math.round(Math.random() * 200 - 128 * scale), shadow.naturalWidth * scale, shadow.naturalHeight * scale)

  // instead of clipping, use gCO
  ctx.globalCompositeOperation = 'destination-in';
  ctx.beginPath();
  ctx.arc(256, 256, 128 * scale, 0, 2 * Math.PI);
  ctx.fill();
  // reset gCO
  ctx.globalCompositeOperation = 'source-over';
}
render()
window.interval = setInterval(render, 500)
#game {
  border: 1px solid black;
  background-color: black;
}
<canvas id="game"></canvas>

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