How Swiffy antialias canvas?

Question

I don't understand how swiffy succefully antialias clipping mask when it exports a flash animation that contains mask.

Here is my exemple :

• Full canvas, with mask. Super dirty in Chrome : http://goo.gl/n8yB5h

• And a swiffy export where there is a picture that move inside a mask. Super clean on Chrome : http://www.creaktif.com/lab/test4.html

I tried a lot of things, including draw a 200% canvas then scale it down, adding more points when I draw my mask, but no way to get a clean mask in my canvas.

How does swiffy ? It just turn me crazy.

Thanks. :)

Answer 1

Instead of using clip() you can draw the clipping path into an off-screen canvas and use that as a "matte", or rather, an alpha mask.

Now you can use the mask with various composite modes to get anti-aliased edges. Draw in the background you want to clip, set a composite mode (operator) and draw in mask. Depending on operator you can cut out center or the surroundings (destination-in is typical equivalent to using clip() though).

Note: demo below is only useful in Chrome/Opera as Firefox/IE already apply anti-aliasing to the clip mask - here's a snapshot showing the difference:

var ctxC = document.getElementById("clip").getContext("2d");
var ctxM = document.getElementById("mask").getContext("2d");
var w = ctxC.canvas.width, h = ctxC.canvas.height;
var img = new Image();
img.onload = demo;
img.src = "http://i.imgur.com/s9ksOb1.jpg";

function demo() {
  
  // define a clip path
  ctxC.save();
  createPath(ctxC);
  ctxC.clip();
  ctxC.drawImage(this, 0, 0, w, h);
  ctxC.restore();
  
  // create a "matte" / alpha mask
  var matte = document.createElement("canvas"),
      ctx = matte.getContext("2d");
  matte.width = w;
  matte.height = h;

  // fill the path instead:
  createPath(ctx);
  ctx.fill(); // color doesn't matter, alpha does
  
  // now use composition to "clip"
  ctxM.drawImage(this, 0, 0, w, h);  // draw image
  ctxM.globalCompositeOperation = "destination-in"; // will keep bg where mask cover
  ctxM.drawImage(matte, 0, 0);
  ctxM.globalCompositeOperation = "source-over";    // default mode
  
  // zoom some details:
  zoom(ctxC);
  zoom(ctxM);
}

function createPath(ctx) {
  var r = 88;
  ctx.beginPath();
  ctx.moveTo(100 + r, 90);
  ctx.arc(100,90,r, 0, 6.28);
}

function zoom(ctx) {
  ctx.imageSmoothingEnabled =
    ctx.webkitImageSmoothingEnabled =
    ctx.mozImageSmoothingEnabled = false;
  ctx.drawImage(ctx.canvas, 10, 10, 100, 100,  70,0, 400, 400);
}

html, body {margin:4px 0 0 4px;overflow:hidden}
canvas{background:#000;border:1px solid #000;margin:0 1px 0 0}

<canvas id="clip" height=180></canvas>
<canvas id="mask" height=180></canvas>