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" <bill_sub@billfernandez.com>
To: <filmscanners@halftone.co.uk>
Cc: <filmscanners@halftone.co.uk>
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 * bill@billfernandez.com *
> ======================================================================
>
