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Perceptual Limits on JPEGs Quality

  The primary aim of this experiment was to determine the point at which users can no longer perceive the difference between a high quality image and the same imaged reduced in quality through JPEGs compression - this point we shall call the 'just perceivable difference' or JPD. When combined with knowledge about the approximate relationship to file sizes, we can plot the quality threshold for the various implementations of JPEGs images. It is asserted that:

the mean of the JPD (JPEGs index) from a sample of Web users plus twice the standard deviation of the JPD between an image of high quality and the same image compressed, represents the cut-off point to which all JPEG images can be compressed in a default policy: given a normal distribution 98% of users would see no perceivable difference in image quality. Those who notice a difference would only notice a slight variation.

In this study it was hypothesised that a difference would be found between JPDs for different types of images used in the experiment.

Malcolm McIlhagga
Thursday June 11 16:17:19 BST 1998