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25 May 2011, 11:42

Google and Mozilla tussle over WebP image format

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WebP logo While Google is incorporating its WebM-derived still picture format, WebP, into Chrome and various of its web services, Mozilla has said no to integrating the format into Firefox as it currently stands. Google introduced WebP in October 2010 as an alternative to JPEG and it has been a controversial format since its introduction.

Now, Mozilla has rejected WebP. In a bugzilla entry asking for the incorporation of WebP, Mozilla developer Joe Drew said in a comment "As the WebP image format exists currently, I won't accept a patch for it". Readers were directed to a blog posting by Jeff Muizelaar for the rationale for the rejection. He explained that Mozilla is unhappy with the quality versus size claims made by Google for WebP and questioned the methodology used to test: this converted existing JPEG images into WebP images and then compared quality using PSNR (Peak Signal-to-Noise Ratio) and file size.

A month after Muizelaar made these comments, Google released a new study which used PNG images as source and compared quality using SSIM (Structural Similarity). This new study showed WebP images to be on average 25%-34% smaller than similar quality JPEGs, but the quality value is calculated and there is some discussion over whether WebP imagery is perceived by the untrained eye to be of the same quality.

Muizelaar also pointed out that WebP only supports 4:2:0 chroma sub sampling (JPEG also supports 4:2:2 and 4:4:4) and has no support for EXIF data, ICC colour profiles or alpha channels. "Every image format that becomes 'part of the web platform' exacts a cost for all time", he says, and the lack of a compelling reason to adopt WebP makes that cost quite high. Another Mozilla developer, Justin Lebar, pointed out in bugzilla comments that Mozilla's mission is to improve the web in the long term "even if doing so is unpopular in the short term" and compared the rejection to the position taken by the company with H.264. Lebar suggests that Mozilla is holding out for improvements to be made to WebP, both in perceived quality and features.

Google is, in the meantime, pushing the format forward. In a recent blog posting, it highlighted an improved compression algorithm and the addition of a "fancy upsampler" to the WebP decoder. WebP is currently supported by Chrome and Opera browsers and Google is supporting the format on Gmail and Picasa Web Albums and will be bringing WebP support to the AppEngine platform soon. Google says it is working on transparency (alpha channel) support and will add it to the "next stable version of the codec".


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