You have to close and re-open the JPEG (quality 12) otherwise you won't see the effect of the JPEG compression as PS maintains the pre-save data.
If you do try to save/close/re-open JPEG12 and do a difference with the original PSD/BMP/TIF you will find there is a difference. Individual primary colour channels can easily be affected by 10% in darker areas. I tried several images and then the coloured squares section in the middle of a Q60 and found that the Green channel was always least affected by a significant amount and the Blue channel was always affected the most by a significant amount. No idea if this is a deliberate design feature. Steve ----- Original Message ----- From: "Bill Fernandez" <[EMAIL PROTECTED]> To: <[EMAIL PROTECTED]> Cc: <[EMAIL PROTECTED]> Sent: Sunday, October 21, 2001 5:08 PM Subject: Re: filmscanners: Lossless JPEG's? was Hello > G'day Mark-- > > Phil Lippincot recently posted about the existence of lossless JPEG > and is the one who first mentioned that quality level 12 in Photoshop > 6 invokes JPEG's lossless mode. He produces drum scanner software > and says his software supports lossless JPEG as a way of making those > huge scans a bit more manageable with no image degradation. Phil > suggests that you can prove it to yourself my saving an image in both > uncompressed TIFF and JPEG quality 12 (in Photoshop), then re-opening > both files, then merging them into one file with two layers, then > setting the blend mode to difference. > > JPEG LS is part of the next-generation JPEG standard and it's not > clear whether it's been finalized or not. > > --Bill > > > > At 10:55 PM +0930 21-10-01, Mark T. wrote: > >At 04:06 AM 21/10/01 -0600, Bill wrote: > >>... > >>o The JPEG standard includes a lossless setting. Photoshop 6 > >>supports it: set the quality level to 12. it will compress to, say, > >>1/3 of the original size. JPEG only supports 24-bit images. > > > >G'day Bill. > > > >I had never heard of a lossless JPG, so I checked the JPEG FAQ, > >which basically says that there *was* an early version of a lossless > >JPEG, but it never took off. They also referred to a new standard > >called JPEG-LS - is this what you meant? I couldn't see anything > >about it in the PS Help file, but I only took a quick look. I would > >be most interested if PS6 really does supprt a lossless JPG.. As > >far as I knew, the main players were/are: > > > >TIFF > >- 48-bit, lossless, large files > > > >TIFF with LZ compression > >- As above but files can be much smaller (esp if image is not grainy > >or detailed), eg typically 1/2 to 1/5 original size > > > >JPEG > >- 24-bit, lossy but adjustable. File sizes often less than 1/5 of > >the uncompressed TIFF (depending on quality setting and image > >content) > > > >PNG > >- 24-bit, lossless. File sizes usually a bit smaller than > >compressed TIFF, but not as small as JPEG. > >(PNG's are also readable by most browsers, which makes them useful > >for 'critical' web-display.) > > > >FWIW, I always use TIFF without compression if in any doubt (I have > >had quite a few problems with lack of portability of LZ'd TIFs), and > >I am now moving over to PNG's for my own file storage in order to > >save CD space. The lack of 48-bit quality hasn't yet been an issue > >for me.. > > > >mt > > -- > > ====================================================================== > Bill Fernandez * User Interface Architect * Bill Fernandez Design > > (505) 346-3080 * [EMAIL PROTECTED] * http://billfernandez.com > ====================================================================== >
