On 6 Dec 2005 at 7:52, Shel Belinkoff wrote: > Over on the Adobe Photoshop User to User forum the question of image > degradation with JPEGs came up. One of the Photoshop wizards posted a test > that > actually showed how many pixels changed or were lost each time a JPEG was > resaved. There were changes and loss of information with ~every~ save.
It's quite easy to do this yourself and to see how insignificant the differences really are. Open a jpeg file in PS, save it with a new file name at "quality 12", open both images and paste one over the other as a layer then set the top layer to "difference". Now you can scan around the image with the "dropper tool" and read actual difference levels in the info tab. You'll find a maximum difference of about 3/256 (~1.2%) for any one colour channel but generally less in the other channels at that point so the overall colour change may be very insignificant. You can also flatten the image and adjust the levels so as to exaggerate the difference (as many seem to do when arguing about jpeg quality) but this doesn't mean it will be visible. Cumulative changes due to multiple saves may be visually apparent but one or two generations wouldn't likely be visible particularly in print. Cheers, Rob Studdert HURSTVILLE AUSTRALIA Tel +61-2-9554-4110 UTC(GMT) +10 Hours [EMAIL PROTECTED] http://members.ozemail.com.au/~distudio/publications/ Pentax user since 1986, PDMLer since 1998
