Studying Lossy Image Compression Efficiency

Josh Aas

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JPEG has been the only widely supported lossy compressed image format on the Web for many years. It was introduced in 1992, and since then a number of proposals have aimed to improve on it. A primary goal for many proposals is to reduce file sizes at equivalent qualities.

We’d like to share a study that compares three frequently-discussed alternatives, HEVC-MSP, WebP, and JPEG XR, to JPEG, in terms of compression efficiency.

The data shows HEVC-MSP performing significantly better than JPEG and the other formats we tested. WebP and JPEG XR perform better than JPEG according to some quality scoring algorithms, but similarly or worse according to others.

We consider this study to be inconclusive when it comes to the question of whether WebP and/or JPEG XR outperform JPEG by any significant margin. We are not rejecting the possibility of including support for any format in this study on the basis of the study’s results. We will continue to evaluate the formats by other means and will take any feedback we receive from these results into account.

In addition to compression ratios, we are considering run-time performance (e.g. decoding time), feature set (e.g. alpha, EXIF), time to market, and licensing. However, we’re primarily interested in the impact that smaller file sizes would have on page load times, which means we need to be confident about significant improvement by that metric, first and foremost.

We’d like to hear any constructive feedback you might have. In particular, please lets us know if you have questions or comments about our code, our methodology, or further testing we might conduct.

Also, the four image quality scoring algorithms used in this study (Y-SSIM, RGB-SSIM, IW-SSIM, and PSNR-HVS-M) should probably not be given equal weight as each has a number of pros and cons. For example: some have received more thorough peer review than others, while only one takes color into account. If you have input on which to give more weight please let us know.

We’ve set up a thread on Google Groups in order to discuss.

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