Thumbnails not color-managed + severely distorted shadow tonality for thumbs, previews

Elle Stone ellestone at
Wed Nov 15 20:08:59 GMT 2017

On 11/15/2017 02:08 PM, Maik Qualmann wrote:
> The thumbnails are color managed. But the color depth of the thumbnails is
> only 8 bit, that's probably the cause.

Hmm, no, on my monitor, if color management isn't being used, the image 
has a slight magenta color cast. All the digiKam thumbnails have a 
slight magenta color cast that for example changes the color of blue 
skies to make them slightly more purple, and also gives grayscale images 
a slight magenta color cast. This is completely separate from and 
independent of the crushed/posterized shadows in the thumbnails. The 
previews don't have the magenta color cast, but the thumbnails do.

But yes, the fact that the thumbnails are 8-bits accounts for the 
crushed/posterized shadows for the thumbnails, but hopefully not also 
for the previews. Are the previews 8-bits also? Because the tonality in 
the previews doesn't have the type of posterized shadows I'd expect from 
a linear gamma 8-bit image. Rather the shadows are crushed with fairly 
abrupt transitions but not completely posterized tonal shifts.

Now that you've explained why the shadows are posterized for the 
thumbnails, I should have realized that was the issue - I suspect 
thumbnails are only 8-bits fairly universally. But magenta color cast 
for the thumbnails is another issue altogether.

If you are saying the thumbnails *should* be color-managed (and the 
settings would indicate that yes, they should be), how do I troubleshoot 
why they are showing the typical (for my monitor, no doubt not for all 
monitors) magenta color cast that results when I view images in 
non-color-managed applications?

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