FIT 14.6.4 & Web Accelerator with Improved Image Quality Assessment

With this release we are proud to take our Automatic Image Quality Assessment out of beta. We have made three significant improvements to this Web Accelerator module. In order to find the highest acceptable compression ratio, AIQA ensures that the visual quality is not degraded. Therefore it is essential that the assessment score closely correlates with human perception. The SSIM algorithm provides good results in many cases. However, especially commercial product images often have a very uneven distribution of image details. Consider a cut-out, in-focus product framed by a passe-partout. Conventional algorithms tend to dilute the errors introduced in complex image areas with the “perfect reproduction” of large background areas.

We have created a Saliency Weighted variant of the SSIM algorithm that is almost immune to this kind of overrating. For example, enlarging the monochrome backdrop around a product should not change the calculated image distance introduced by distortion. Images in natural settings benefit, too, by correcting the weight of errors in background areas. This produces very accurate distance values that help us minimize visible compression artifacts.

To ease the use of this technique, we have compiled a set of Compression Presets that makes configuration as simple as deciding between performance and quality – or using the balanced default values.

Image Assessment is a very CPU intensive task, because finding the optimal compression settings usually involves multiple compress-and-assess cycles. We have analyzed the compression characteristics of thousands of images in order to train a prediction model. It estimates near optimal settings after a single cycle with a very low reject rate.

For developers we have created more possibilities to control HTTP requests: Super fine-grained timeouts and origin response header injection. The latter can be used to correct erroneous or provide missing Cache-Control headers.

There are a lot of smaller improvements, bug fixes and third party updates, too. The changelog provides a thorough list.

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