globalCompositeOperation and concentric, hollow, moving shapes

问题

I'm trying to achieve the following:

A number of concentric circles (or rings) are drawn on a canvas. Each circle has a "hole" in it, so the smaller circles, drawn behind it are partially visible. Each frame (we're using window.requestAnimationFrame to render) the radius of each circle/shape/ring is slightly increased.

A scenario with two rings is depicted in the image here.

The code:

function draw() {
    drawBgr();
    for (var i = 0, len = rings.length; i < len; i++) {
        rings[i].draw();
    }
}

function drawBgr() {
    context.globalCompositeOperation = "source-over";
    context.clearRect(0, 0, WIDTH, HEIGHT);
    context.rect(0, 0, WIDTH, HEIGHT);
    context.fillStyle = '#FFFFFF';
    context.fill();
}

function squareRing(ring) { //called by rings[i].draw();
    context.globalCompositeOperation = "source-over";

    context.fillRect(ring.centerX - ring.radius / 2, ring.centerY - ring.radius / 2, ring.radius, ring.radius);
    context.globalCompositeOperation = "source-out";

    context.beginPath();
    context.arc(CENTER_X, CENTER_Y, ring.radius, 0, 2 * Math.PI, false);
    //context.lineWidth = RING_MAX_LINE_WIDTH * (ring.radius / MAX_SIDE);
    context.fillStyle = '#000000';
    context.fill();
    context.globalCompositeOperation = "source-over";

}

1. What exactly is the problem here? I'm calling clearRect before the circles are drawn. See "What I'm actually getting" image. This is the result of a SINGLE RING being drawn over a number of frames. I shouldn't be getting anything different than a black circle with a hollow square in the middle. (Note that radius is increasing each frame.)

2. I do realize switching globalCompositeOperation might not suffice for the effect I desire. How can I draw a "hole" in an object drawn on the canvas without erasing everything in the "hole" underneath the object I'm trying to modify?

This is the tutorial I used as a reference for the globalCompositeOperation values.

I'm using Firefox 28.0.

回答1:

I would not try to use globalCompositeOperation, since i find it hard to figure out what will happen after several iterations, and even harder if the canvas was not cleared before.

I prefer to use clipping, which gets me to that :

http://jsbin.com/guzubeze/1/edit?js,output

So, to build a 'hole' in a draw, how to use clipping ?
-->> Define a positive clipping sub-path, and within this area, cut off a negative part, using this time a clockwise sub-path :

Clipping must be done with one single path, so rect() cannot be used : it does begin a path each time, and does not allow to choose clockwisity (:-)), so you have to define those two functions which will just create the desired sub-paths :

// clockwise sub-path of a rect
function rectPath(x,y,w,h) {
  ctx.moveTo(x,y);
  ctx.lineTo(x+w,y);
  ctx.lineTo(x+w,y+h);
  ctx.lineTo(x,y+h);
}

// counter-clockwise sub-path of a rect
function revRectPath(x,y,w,h) {
  ctx.moveTo(x,y);
  ctx.lineTo(x,y+h);
  ctx.lineTo(x+w,y+h);
  ctx.lineTo(x+w,y);  
}

then you can write your drawing code :

function drawShape(cx, cy, d, scale, rotation) {
  ctx.save();
  ctx.translate(cx,cy);
  scale = scale || 1;
  if (scale !=1) ctx.scale(scale, scale);
  rotation = rotation || 0;
  if (rotation) ctx.rotate(rotation);
  // clip with rectangular hole
  ctx.beginPath();
  var r=d/2; 
  rectPath(-r,-r, d, d);
  revRectPath(-0.25*r,-0.8*r, 0.5*r, 1.6*r);
  ctx.closePath();
  ctx.clip();
  ctx.beginPath();
  // we're clipped !
  ctx.arc(0,0, r, 0, 2*Math.PI);
  ctx.closePath();
  ctx.fill();
  ctx.restore();
}

Edit :

For the record, there is a simpler way to draw the asked scheme : just draw a circle, then draw counter clockwise a rect within. What you fill will be the part inside the circle that is outside the rect, which is what you want :

function drawTheThing(x,y,r) {
   ctx.beginPath();
   ctx.arc(x ,y, r, 0, 2*Math.PI);
   revRectPath(x-0.25*r, y-0.8*r, 0.5*r, 1.6*r);
   ctx.fill();
   ctx.closePath();
}

(i do not post image : it is the same).

Depending on your need if you change the draw or if you want to introduce some kind of genericity, use first or second one. If you do not change the scheme later, the second solution is simpler => better.

来源：https://stackoverflow.com/questions/22972222/globalcompositeoperation-and-concentric-hollow-moving-shapes