[Digikam-devel] [Bug 149578] libjpeg JPEG subsampling setting is not user-controlable

Gilles Caulier caulier.gilles at gmail.com
Fri Sep 7 14:43:34 CEST 2007

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------- Additional Comments From caulier.gilles gmail com  2007-09-07 14:43 -------
Jean Marc,

The gimp doc is wrong !!!

Look page 70 of http://docs.gimp.org/en.pdf We can read :

// -----------------------------------------------------------

Subsampling Human eye is not sensitive in the same way all over color spectrum. Compression can use this to consider
    as identical slightly different colors. Three methods are available :
    • 2x2, 1x1, 1x1 : important compression; suits images with weak borders but tends to denature colors.
    • 1x1, 1x1, 1x1 : preserves borders and contrasting colors, but compression is less.
    • 1x1, 1x1, 1x1 (4:2:2) : between other both.

// -----------------------------------------------------------

There are both "1x1, 1x1, 1x1" (:=(((

After investiguations using wikipedia page, i will set in digiKam :

"High chroma subsampling" ==> 2x2 1x1 1x1
"Medium chroma subsampling" ==> 2x1 1x1 1x1 (4:2:2)
"Low chroma subsampling" ==> 1x1 1x1 1x1

I will set the "Low chroma subsampling as default value because we always want to preserve the _best_ image quality. If user want a lower quality, it must change it in settings.

Gilles Caulier

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