This article is part of a series which is continued here.

This article presents a proof of concept to greatly reduce/compress the filesize of alpha transparent PNG images via PHP without loosing the alpha information and reassembling the separated channels with the help of jQuery and HTML5 canvas.

Current evolution of HTML5, CSS3, widespread availability of broadband internet connections and (finally!) Internet Explorer 6 becoming more and more obsolete we see ourselves, at times, faced by new kinds of challenges.

As we frequently develop promotional microsites/raffles for our clients (and these kind of sites do actually have very special requirements in terms of product/brand identity and representation) we, as developers, are often forced to use huge amounts of PNG-images with full alpha-channel. Their sheer amount plus the fact that PNG is an almost uncrompessed format can become such an aforementioned challenge as you will see in the following article.

The Problem: huge filesize of PNG-images

In one of my latest projects I ended up having a page with 4.5MB of data in total which was mainly caused by one image-slider making extensive use of PNG files with alpha-channel information. The images of this slider, in fact, summed up to a total of 3.8MB alone which would lead to a loading time of approx. 20-30 seconds for a 2MBit connection! Otherwise the code was quite clean and making extensive use of CSS3 and sprites to reduce loading time and the number of requests. To put it in a nutshell: an unacceptable condition in terms of a developer's honor.

I discussed my concerns with Christoph when he finally joked about trying to use canvas to somehow circumvent the problem of huge filesizes when being forced to use PNGs. We both had an extended laugh, at first.

The idea: separate color- and alpha-channel and reassemble them in a canvas element

I then thought about it for a moment and had an idea: Canvas could indeed be part of a possible solution if I would somehow manage to separate the image's color-information from its alpha-channel and then save them as separate files. Due to the fact that the most problematic PNGs are those that would normally best be saved as JPG (if they did not need an alpha-channel) I was almost sure that it should be possible to save the separated color-information as JPG thus greatly reducing total filesize.

At first I did some research (always remember: Google is your friend) and found some interesting articles. None of them covered all aspects of what I wanted to achieve but they were a great source of inspiration and knowledge:

The abstract: four challenges on the way to PNG size reduction

So I ended up facing four questions needed for a simple proof of concept:

  1. Would it be easily possible to separate color- and alpha-information via PHP?
  2. Is it possible to re-combine these two channels via Javascript and the canvas-element?
  3. Is there any chance to write the combined channels back into an image?
  4. Will there be a way to circumvent the doubled amount of requests due to the separation?

1. Separating color- and alpha-information via PHP

I actually started with some far more simple code for the proof of concept but this is what I came up with in the end. In this example, I am leaving out some parts, including mainly the saving of the final images to keep it short and easy to understand. (See the end of this articles for the helper functions used in the PHP-part)

			<?php
			$sourceFile = 'Path to your source-image';

			// temporary storage for alpha-colors
			$colorCache = array(127 => rgb2color(0, 0, 0, 0));

			// read width and height from source-image
			$size   = @getimagesize($sourceFile);
			$width  = $size[0];
			$height = $size[1];

			// create image from source-image
			$imageSource = @imagecreatefrompng($sourceFile);

			// create image for color-information
			$imageJpg    = @imagecreatetruecolor($width, $height);

			// create image for alpha-information
			$imagePng    = @imagecreatetruecolor($width, $height);

			// enable alpha-handling for source- and alpha image
			@imagealphablending($imageSource, false);
			@imagesavealpha($imageSource, true);
			@imagealphablending($imagePng, false);
			@imagesavealpha($imagePng, true);

			// fill color- and alpha-image
			@imagefill($imageJpg, 0, 0, rgb2color(127, 127, 127));
			@imagefill($imagePng, 0, 0, $colorCache[127]);

			// iterate over source-image
			for($x = 0; $x < $width; $x++) {
				for($y = 0; $y < $height; $y++) {
					// convert GDLib-color to RGBa-values
					$color = color2rgb(imagecolorat($imageSource, $x, $y));

					// check if color alpha is less than 127 (which means transparent)
					if($color['a'] < 127) {
						// write color-information to color-image
						imagesetpixel($imageJpg, $x, $y, rgb2color($color['r'], $color['g'], $color['b']));

						// check if alpha-color is in temporary storage
						if(!isset($colorCache[$color['a']])) {
							// put alpha-color in temporary storage
							$colorCache[$color['a']] = rgb2color(0, 0, 0, 127 - $color['a']);
						}

						// write alpha-information to alpha-image
						imagesetpixel($imagePng, $x, $y, $colorCache[$color['a']]);
					}
				}
			}

			// convert GDLib-resources to color- (as JPEG) and alpha-image (as PNG)
			$imageJpg = getImage($imageJpg, 'jpeg', true, 80);
			$imagePng = getImage($imagePng, 'png', false, 9, PNG_ALL_FILTERS);
			?>
			

Just to give you a quick & dirty example the following source-image (leftmost: approx. 129 KB) is split into separate color- and alpha-images (approx. 32 KB in total using 80% compression for the JPEG)

Note: The background pattern assigned is not part of the images but only used to visualize transparency. All images are resized via CSS to fit the layout.

2. Re-combine color- and alpha-image via Javascript and canvas

This is the main part of the javascript I ended up with. It combines the two separate channels from their img-elements via a canvas element. Remember to process this on window load. Otherwise the result will be an empty canvas.

			<script type="text/javascript">
			var context,
				width   = 256, // width of your source-image
				height  = 256, // height of your source-image
				jpg     = '', // img-element holding color-image
				png     = '', // img-element holding alpha-image
				canvas  = document.createElement('canvas');

			canvas.style.display = 'none';
			canvas.width         = width;
			canvas.height        = height;

			context = canvas.getContext('2d');
			context.clearRect(0, 0, width, height);
			context.drawImage(jpg, 0, 0, width, height);

			context.globalCompositeOperation = 'xor';
			context.drawImage(png, 0, 0, width, height);
			</script>
			

3. Write combined channels back into image

This is by far the simplest part as the HTML5 canvas element provides a native method to directly fetch a valid dataURL. As an example I use jQuery to create an image, assign the dataURL and append it to the document body.

			<script type="text/javascript">
			var dataURL = canvas.toDataURL('image/png');

			jQuery('<img />', { src: dataURL, alt: '' }).appendTo('body');
			</script>
			

Look at the final outcome on the right and carefully compare it to the source PNG on the left (remember, this is 32 KB instead of 129 KB, so only 25% of the original filesize):

Note: The background pattern assigned is not part of the images but only used to visualize transparency.

Savings in filesize will vary roughly between 60% to 80% reduction of the original filesize (using 80% compression for the JPEG).

4. Circumvent doubled amount of HTTP-requests due to image separation

Well, up to now everything was quite simple due to the scripts being more or less a proof of concept. As stated before I left out some parts which, to be honest, mainly dealt with exactly this problem. The solution is in fact not that complicated but would simply be too much for this part of the article. Just to give you a hint: .htaccess and modRewrite come to the rescue in the second part of this article which can now be found here.

PHP helper functions
			<?php
			function rgb2color($r, $g, $b, $a = 0) {
				return ($r << 16) + ($g << 8) + $b + ($a << 24);
			}

			function color2rgb($color) {
				$return = null;

				if(preg_match('/^\d+$/', $color)) {
					$return = array(
						'r' => ($color >> 16) & 0xFF,
						'g' => ($color >> 8) & 0xFF,
						'b' => $color & 0xFF,
						'a' => ($color & 0x7F000000) >> 24,
					);
				}

				return $return;
			}

			function getImage($resource, $type = 'png', $interlace = false, $quality = NULL, $filter = 248) {
				if($interlace === true) {
					@imageinterlace($resource, 1);
				}

				ob_start();

				switch($type) {
					case 'png':
						$quality = ($quality === NULL) ? 9 : max(0, min(9, (int) $quality));

						@imagepng($resource, NULL, $quality, $filter);
						break;
					case 'jpeg':
						$quality = ($quality === NULL) ? 100 : max(0, min(100, (int) $quality));

						@imagejpeg($resource, NULL, $quality);
						break;
				}

				return trim(ob_get_clean());
			}
			?>
			
This article is part of a series which is continued here.
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Dirk Lüth Dirk Lüth, 38
MINISTRY
Code poetry in Javascript and PHP