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POCanvas - Fill a rectangle in all areas that are fully transparent
 

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    <p>I'm writing a simple 2D game engine using the HTML5 canvas. I've come to adding a lighting engine. Each light source has a radius value and an intensity value (0-1, eg 1 would be very bright). There's also an ambient light value that is used to light everything else in the world that isn't near a light source (0-1, eg 0.1 would be moonlight). The process of lighting is done on a separate canvas above the main canvas:</p> <ol> <li>For each light source, a radial gradient is drawn at that position with the same radius as the light source. The gradient is given two stops: the center is black with an alpha of 1-intensity of the light and the end/edge is black with alpha of 1-ambient light value. That all works fine.</li> <li>This is where it goes wrong :/ I need to fill the whole canvas with black with and alpha of 1-ambient light value and at the moment I do this by setting the context.globalCompositeOperation to source-out then fillRecting the whole canvas. </li> </ol> <p>My code for this stuff is:</p> <pre><code>var amb = 'rgba(0,0,0,' + (1-f.ambientLight) + ')'; for(i in f.entities) { var e = f.entities[i], p = f.toScreenPoint(e.position.x, e.position.y), radius = e.light.radius; if(radius &gt; 0) { var g = cxLighting.createRadialGradient(p.x, p.y, 0, p.x, p.y, radius); g.addColorStop(0, 'rgba(0,0,0,' + (1-e.light.intensity) + ')'); g.addColorStop(1, amb); cxLighting.fillStyle = g; cxLighting.beginPath(); cxLighting.arc(p.x, p.y, radius, 0, Math.PI*2, true); cxLighting.closePath(); cxLighting.fill(); } } //Ambient light cxLighting.globalCompositeOperation = 'source-out'; cxLighting.fillStyle = amb; cxLighting.fillRect(0, 0, f.width, f.height); cxLighting.globalCompositeOperation = 'source-over'; </code></pre> <p>However instead of getting what I wan't out of the engine (left) I get a kind of reversed gradient (right). I think this is because when I draw the rectangle with the <code>source-out</code> composite operation it affects the colours of the gradient itself because they are semi-transparent.<br /><br /> <img src="https://i.stack.imgur.com/T6fsx.png" alt="Desired lighting effect"> <img src="https://i.stack.imgur.com/u6itZ.png" alt="I get this"></p> <p>Is there a way to do this differently or better? Using clipping maybe, or drawing the rect over everything first?</p> <p>Also, I modified the Mozila Dev Centre's <a href="https://developer.mozilla.org/en/Canvas_tutorial/Compositing" rel="noreferrer">example</a> on composting to replicate what I need to do and none of the composite modes seemed to work, <a href="http://chopapp.com/#1b0f6c3e" rel="noreferrer">check that out</a> if it would help.</p> <p>Thanks very much, any answer would be great :)</p>
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