How to Transition an Image from B&W to Color with Canvas

Tuesday, 21 June 2011 06:30

Recently, in the CodeCanyon forums, a question was brought up: “How do I transition an image from black and white, to color — using only one image?” Unfortunately, at this point in time, it’s not possible with CSS. However, if we’re creative with JavaScript and canvas, we can create a solution relatively easily. I’ll show you how in today’s video tutorial!

Choose 720p for maximum clarity.
Subscribe to our YouTube and Blip.tv channels to watch more screencasts.

Final Source

<!DOCTYPE html> 

<html lang="en">
<head>
   <meta charset="utf-8">
   <title>untitled</title>
	<style>
		/* Setup...not important. */
		.img-wrap {
			width: 500px;
			margin: 100px auto;
			position: relative;
			cursor: pointer;
		}

		/* Handles animation of b*w to color */
		canvas {
			position: absolute;
			left: 0;
			top: 0;
			opacity: 1;
			-webkit-transition: all 1s;
			-moz-transition: all 1s;
			-o-transition: all 1s;
			-ms-transition: all 1s;
			transition: all 1s;
		}

		canvas:hover {
			opacity: 0;
		}

		/* If you MUST have IE support */
		#cvs-src {
		   filter: progid:DXImageTransform.Microsoft.BasicImage(grayscale=1);
		}

		#cvs-src:hover {
			filter: none;
		}
	</style>
</head>
<body> 

<div class="img-wrap">
	<img id="cvs-src" src="/your-image.jpg" />
	<canvas width=500 height=500></canvas>
</div> 

<script>
	(function() {
		var supportsCanvas = !!document.createElement('canvas').getContext;
		supportsCanvas && (window.onload = greyImages);

		function greyImages() {
			var ctx = document.getElementsByTagName("canvas")[0].getContext('2d'),
				img = document.getElementById("cvs-src"),
				imageData, px, length, i = 0,
				grey;

			ctx.drawImage(img, 0, 0);

			// Set 500,500 to the width and height of your image.
			imageData = ctx.getImageData(0, 0, 500, 500);
			px = imageData.data;
			length = px.length;

			for ( ; i < length; i+= 4 ) {
				grey = px[i] * .3 + px[i+1] * .59 + px[i+2] * .11;
				px[i] = px[i+1] = px[i+2] = grey;
			}

			ctx.putImageData(imageData, 0, 0);
		}
	})();
</script> 

</body>
</html>

Conclusion

So what do you think? Would you use this technique in your own projects? Can you think of a better way that doesn’t involve using a server-side language or sprites? Let me know in the comments!