On Thu, 23 Aug 2007 13:31:27 +0200, Andrew <[EMAIL PROTECTED]> wrote:
> I need to make a JPEG "saved at a high quality setting (i.e. Photoshop 
> level 10 or above)". Since I don't know what Photoshop is ;), could 
> someone please tell me what the equivalent would be using GIMP?

Here is a table that provides an approximate mapping between Photoshop
quality levels and GIMP (actually IJG JPEG library) quality levels:

Adobe Photoshop quality 12  <=  GIMP quality 98, subsampling 1x1
Adobe Photoshop quality 11  <=  GIMP quality 95, subsampling 1x1
Adobe Photoshop quality 10  <=  GIMP quality 93, subsampling 1x1
Adobe Photoshop quality 9   <=  GIMP quality 91, subsampling 1x1
Adobe Photoshop quality 8   <=  GIMP quality 90, subsampling 1x1
Adobe Photoshop quality 7   <=  GIMP quality 89, subsampling 1x1
Adobe Photoshop quality 6   <=  GIMP quality 90, subsampling 2x2
Adobe Photoshop quality 5   <=  GIMP quality 89, subsampling 2x2
Adobe Photoshop quality 4   <=  GIMP quality 88, subsampling 2x2
Adobe Photoshop quality 3   <=  GIMP quality 88, subsampling 2x2
Adobe Photoshop quality 2   <=  GIMP quality 87, subsampling 2x2
Adobe Photoshop quality 1   <=  GIMP quality 86, subsampling 2x2
Adobe Photoshop quality 0   <=  GIMP quality 85, subsampling 2x2

Some remarks:

- I designed this mapping in a way that is a rather pessimistic for
  GIMP.  It tries to be "at least as good", which means that in
  practice you could reduce the GIMP quality by a few points and still
  get about the same quality level.

- Photoshop includes several parameters in a single "quality" setting,
  while GIMP provides separate controls for the "quality level", the
  choice of subsampling and other parameters hidden in the Advanced
  options in the JPEG dialog.  It is likely that future GIMP versions
  (after 2.4) will also offer a simplified interface by default and
  hide the current quality slider inside the advanced options.

- Subsampling 1x1 and 2x2 are short ways to write "1x1,1x1,1x1" and
  "2x2,1x1,1x1" that you find in the GIMP JPEG dialog.

- If you still intend to do more work on the image, you should never
  just save it as JPEG.  Always keep a copy in XCF format (GIMP's
  native file format).

- Because of the change of subsampling, Photshop quality 7 uses a
  "GIMP quality level" that is lower than Photoshop quality 6.  This
  may seem strange, but apparently the Photoshop developers designed
  the quality levels to be related to the expected file size rather
  than the quality of the JPEG quantization.

- The default GIMP quality level is 85.  This matches the worst
  quality level for Photoshop (quality 0).  This is not really true
  because of the different subsampling (GIMP uses 2x1 by default,
  which is better than 2x2) and because the mapping that I described
  here is rather pessimistic for GIMP.  But level 85 is good enough
  in most cases and the default Photoshop quality levels are designed
  for high-quality storage, not for publishing on the web.

- Photoshop includes a totally different quality scale in its "Save
  for web" interface.  It goes from 0 to 100 (like in GIMP) with the
  following mapping (again, rather pessimistic for GIMP):
  Adobe Photoshop Save for web 100 <= GIMP quality 98, subsampling 1x1
  Adobe Photoshop Save for web 75  <= GIMP quality 92, subsampling 1x1
  Adobe Photoshop Save for web 50  <= GIMP quality 86, subsampling 1x1
  Adobe Photoshop Save for web 25  <= GIMP quality 72, subsampling 2x2
  Adobe Photoshop Save for web 0   <= GIMP quality 51, subsampling 2x2

- Setting the GIMP quality level to 95 or higher is a waste of disk
  space and/or bandwidth.

- Setting the GIMP quality level to anything below 50 is a bad idea.

- I just saw that the GIMP user manual has a very confusing
  description for the subsampling parameter and it is not even
  correct.  I will try to fix that soon.

This is probably way more information that you were expecting.  The
short answer is: set your GIMP quality level to 93 or higher (but in
fact, 90 should be OK).  And set the subsampling to 1x1,1x1,1x1.

-Raphael
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