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Your 'funny' cat pics are weighing down the web, so here's a better JPEG encoder from Mozilla

Claims to reduce file sizes by 10 per cent

Internet Security Threat Report 2014

Mozilla has announced a new open-source JPEG encoding library, which it says could significantly reduce the amount of network traffic used by the web.

Dubbed "mozjpeg," the library incorporates image-shrinking algorithms from the public domain JPEG-optimizing utility jpgcrush into the encoder itself, resulting in images that are smaller than those produced by most other JPEG encoders at the same quality levels.

Because images are typically much larger than HTML, CSS, or JavaScript files, it follows that images take up most of the bandwidth consumed with each web page load. And on today's web, "images" generally means JPEG files.

"Nearly every photograph on the Web is served up as a JPEG," Mozilla technology strategist Josh Aas said in a blog post.

Reducing the bandwidth burden of the web by shrinking the size of images has been a perennial topic of discussion among web developers. But most attempts at solving the problem have involved replacing JPEG with some new, more efficient image format.

Google's WebP format, for example, claims to be able to shrink JPEG, GIF, and PNG images by an average of 39 per cent, without any perceptible loss of image quality.

The problem with this approach, however, is that before major content sites adopt a new format for their images, they have to be sure it will be supported by the majority of web browsers.

In the specific case of WebP, while Chrome and Opera can already decode the format natively, Firefox, Internet Explorer, and Safari can't – at least, not without workarounds.

But all browsers support JPEG images, so by producing a more efficient JPEG encoder, Mozilla is hoping to give web developers some of the benefits of alternatives like WebP while avoiding the backward-compatibility problem.

"Given this situation, we wondered if JPEG encoders have really reached their full compression potential after 20+ years," Aas wrote. "We talked to a number of engineers, and concluded that the answer is 'no,' even within the constraints of strong compatibility requirements."

For mozjpeg, Mozilla has forked the hardware-accelerated JPEG encoding library libjpeg-turbo and baked in algorithms from jpgcrush, which shrinks images by comparing various progressive encoding configurations and picking the one that produces the smallest output.

Mozilla says the new library can reduce the size of JPEGs by an average of 10 per cent, based on a sample of 1,500 images downloaded from Wikimedia – hardly as much as WebP, but still a meaningful savings.

For the next version of the library, Aas said Mozilla plans to implement trellis quantization, an algorithm that should reduce image sizes even further.

Mozilla has made the source code to mozjpeg available on Github under a BSD-style open source license. Developers who are interested in contributing to the project are invited to join the newly created mailing list. ®

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