Hi all,

recent discussion on gimp-user brought up some usuability issues
of the JPEG export dialog [1]. Actually, there's nothing new to say
about it... the big "JPEG quality" thread did cover it all [2].
However, due to the sheer size of that thread, i think a summary of
open issues is useful.

I. Displayed value range for the quality slider

   It's surprisingly delicate to choose an innocuous range for that slider:

   - 0..100 is an invitation for confusion: 100 = 100% quality = 100% 

   - using the same range as another application makes it easy to think those
     settings were comparable between the applications. (Which they usually are 
     for various reasons).
     Photoshop uses 0.. 12  for "save as JPEG"
                    0..100  for "save for web"
     Lightroom:     0..100

   - starting at 0 feels odd: zero quality = nothing at all = no image?!?

   - other don'ts: photoshop allows slider stops between numbers, but doesn't
     display the corresponding values [3]

II. Range of actually useful values for IJG quality value

    For GIMP's target users less than half of all possible settings are useful:
    or, in ASCII:

      .  no-go: blocky garbage
      .  if in dire need
     90  the sweet spot
      .  no-go: just wastes disk space -- ever heard of XCF?

    The IJG library takes an integer for the quality value, so there are
    only about 15-35 distinct useful settings for the quality slider.

    (don't beat me on the exact numbers, i'm willing to provide
     suitable image comparisons when/if these numbers become important)

III. Parameter Triaging

     The "Subsampling" parameter is more important than its current
     position inside the "advanced parameters" section suggests.

     Photoshop is the reference here -- it automatically switches
     from 2x2 to 1x1 subsampling for the higher quality settings [4].


[1] https://lists.xcf.berkeley.edu/lists/gimp-user/2010-January/016436.html

[2] http://marc.info/?l=gimp-developer&m=118377279721823&w=2

[3] https://lists.xcf.berkeley.edu/lists/gimp-user/2010-January/016493.html


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