Google's new VP8-based image format could replace JPEG


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In an effort to speed up page loading, Google has introduced an experimental new image format called WebP. The format is intended to reduce the file size of lossy images without compromising the quality. Google's tests, which involved converting a million images, show that the format delivers an average reduction in file size of 39 percent.

To achieve this reduction, the WebP format relies on the advanced, still-image compression methods that the VP8 video codec uses to compress individual frames. Google has coupled these advanced compression techniques with a very slim container format. Due to its efficacy for compressing lossy images, WebP might someday replace JPEG as the standard format for photos and similar content on the Web.

"We applied the techniques from VP8 video intra frame coding to push the envelope in still image coding," wrote Google product manager Richard Rabbat in a post on the official Chromium blog. "We also adapted a very lightweight container based on RIFF. While this container format contributes a minimal overhead of only 20 bytes per image, it is extensible to allow authors to save meta-data they would like to store."

Google obtained the VP8 video codec last year when it acquired media technology company On2. Google later opened the source code of the standard VP8 implementation and introduced WebM, an open multimedia format for the Web that relies on VP8 and Ogg Vorbis. It was a significant milestone for open video on the Web, because it finally supplied a competitive, royalty-free format that is suitable for use in standards-based video implementations.

Google's experimental WebP project reflects the possibility of putting On2's technology to work in unexpected ways. Although the results of Google's tests are very promising, practical adoption of the format will have to wait until it is supported in browsers. Google has already produced a patch for WebKit and intends to support the format natively in future versions of its own Chrome Web browser.

Google has published a gallery of samples that compare WebP-encoded images next to equivalent JPEG images. Subjectively, they look good to my eye, but not everybody is impressed. Jason Garret-Glaser, a well-known x264 developer who is participating in an effort to produce an independent VP8 implementation for FFmpeg, contends that WebP falls flat compared to JPEG due to weaknesses in the VP8 encoder. He reiterated his previous complaint that the VP8 reference implementation emphasizes optimizations for PSNR instead of psychovisual factors, causing it to deliver lower quality than it could if it was designed better.

"Is Google nuts? I could understand the push for 'WebP' if it was better than JPEG," he wrote in a blog entry. "And sure, technically as a file format it is, and an encoder could be made for it that's better than JPEG. But note the word 'could'. Why announce it now when libvpx is still such an awful encoder? You'd have to be nuts to try to replace JPEG with this blurry mess as-is."

As Garret-Glaser also points out, previous attempts to displace the antiquated JPEG format with superior alternatives haven't gained much traction. It may take a long time for WebP to gain real-world support in the browser ecosystem. The potential advantages of more effective compression are compelling, however, when you consider the performance gains that it could bring for mobile Web browsing where bandwidth is still constrained.