D26648: Improved quality of JPEG thumbnails

[Stefan Brüns]

bruns added a comment.


  Having had a look at https://code.woboq.org/qt5/qtbase/src/plugins/imageformats/jpeg/qjpeghandler.cpp.html#_M/HIGH_QUALITY_THRESHOLD, the jpeg handler actually know two different quality settings, `< 50` and `> 50`, the default being 75, i.e. high.
  
  High quality means floating point DCT and bilinear upsampling (Qt::SmoothTransformation), while low means IDCT and nearest neighbor upsampling.
  
  https://code.woboq.org/qt5/qtbase/src/plugins/imageformats/jpeg/qjpeghandler.cpp.html#337
  https://code.woboq.org/qt5/qtbase/src/plugins/imageformats/jpeg/qjpeghandler.cpp.html#432
  
  In either case, it uses libjpeg's capability to reconstruct smaller versions of the image, only decoding it partially, up to DC components only (i.e. 1/8 scale). So for a 24MPixel (6000x4000), libjpeg returns a 750x500 Pixel image, which is then downscaled by Qimage to 256x171 Pixel. Using SmoothTransformation for this size should be not to heavy.
  
  https://code.woboq.org/qt5/qtbase/src/plugins/imageformats/jpeg/qjpeghandler.cpp.html#297
  
  I think best is to set the quality explicitly and mention in a comment what intended effect the setting has (so if the jpeghandler behavior changes, the code can be adjusted to have the wanted effect).

REVISION DETAIL
  https://phabricator.kde.org/D26648

To: chroniceel, broulik, #frameworks, #vdg
Cc: meven, volkov, cfeck, bruns, ngraham, kde-frameworks-devel, kfm-devel, pberestov, iasensio, fprice, LeGast00n, MrPepe, fbampaloukas, alexde, GB_2, Codezela, feverfew, michaelh, spoorun, navarromorales, firef, andrebarros, emmanuelp, mikesomov
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