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Plotting the Implementation Space: Experiments on Images

  For the image proxy, the key points in the implementation space occur when the user detects a reduction in quality due to the adaptation taking place in the proxy. For the scale method, this is too dependent upon context to determine - the authors of the HTML may have fixed ona particular width of images and any deviation will reduce the quality. However, for the reduce method operating on JPEGs, we can generally reduce the quality factor without impact upon the user. We could find no hard numbers within the literature, and from observation, most JPEGs were encoded with too high a quality factor, so we designed an experiment to determine the limits of the quality factor on user perceptions. We could then relate this to the corresponding files sizes.

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