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.