[Digikam-devel] [digikam] [Bug 358848] New: Chroma subsampling incorrectly described

[DrSlony via KDE Bugzilla]

https://bugs.kde.org/show_bug.cgi?id=358848

            Bug ID: 358848
           Summary: Chroma subsampling incorrectly described
           Product: digikam
           Version: unspecified
          Platform: Other
                OS: Linux
            Status: UNCONFIRMED
          Severity: normal
          Priority: NOR
         Component: Export
          Assignee: digikam-devel at kde.org
          Reporter: bugs at londonlight.org

Continuation of https://plus.google.com/u/0/+digikam/posts/SExHL5iCt8f

https://quickgit.kde.org/?p=digikam.git&a=blob&h=f33c15ca92309ff6e8ec2e7a296bd88df0682c63&hb=b5be6c56a43af98a639e2668ff99d8a47fd6504a&f=libs%2Fdimg%2Floaders%2Fjpegloader.cpp

Case 1: 2x1 1x1 1x1 = 4:2:2
Chroma halved horizontally.
You call it "low".

Case 2: 2x2 1x1 1x1 = 4:2:0
Chroma quartered in 2x2 blocks.
You call it "medium".

Case 3: 4x1 1x1 1x1 = 4:1:1
Chroma quartered in 4x1 blocks.
You call it "high".

Tooltip:
https://quickgit.kde.org/?p=digikam.git&a=blob&h=afa4beb83a6173345f516261ec442c199557738e&hb=b5be6c56a43af98a639e2668ff99d8a47fd6504a&f=libs%2Fdimg%2Floaders%2Fjpegsettings.cpp

4:2:0 and 4:1:1 both quarter the chroma resolution, but in different ways.
4:1:1 is uncommon, but if you want to have it then I think it is misleading to
call 4:1:1 "high" and 4:2:0 "medium", as they both destroy 3/4 of the color but
in different ways.

Just saying "little to no visual difference" is not good, because the
difference can be the opposite of "little" depending on the image, and this
explanation does nothing to prepare the user for what sort of difference to
expect. I would not call this "little to no" http://i.imgur.com/zfvnrML.png

Saying that only 4:1:1 "tends to alter colors" leads the user to think the
other options don't when in fact they all butcher colors.

Lastly, 4:2:0 and 4:1:1 both quarter the resolution, so I wouldn't call one
"high" and the other "medium".

Here is my proposal:

    d->subSamplingCB->setWhatsThis(i18n("<p>Chroma subsampling reduces file
size by taking advantage of the eye's lesser sensitivity to color resolution.
How perceptible the difference is depends on the image - real-life full-sized
photos will generally show no difference, while sharp, down-scaled photos and
pixel-fine colorful watermarks and text may lose fine color detail.</p>"
"<p><b>None</b><br>J:a:b 4:4:4, h/v 1/1<br>No chroma subsampling, highest
quality but lowest compression.</p>"
"<p><b>Medium</b><br>J:a:b 4:2:2, h/v 2/1<br>Chroma halved horizontally,
average compression, average quality.</p>"
"<p><b>High 2x1</b><br>J:a:b 4:2:0, h/v 2/2<br>Chroma quartered in 2x2 blocks,
high compression but low quality.</p>"
"<p><b>High 4x1</b><br>J:a:b 4:1:1, h/v 4/1<br>Chroma quartered in 4x1 blocks,
high compression but low quality.</p>"));

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