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Thumbnailer giving random changes in contrast/white point for greyscale PNG
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Description

Have a look at https://commons.wikimedia.org/wiki/File:William_Grant_Still_by_Carl_Van_Vechten.png - and the upload history. The last two should look identical to https://commons.wikimedia.org/wiki/File:William_Grant_Still_by_Carl_Van_Vechten.jpg - but seem to readjust everything randomly. Firefox displays the image correctly, so it's something to do with the thumbnail.

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AdamCuerden raised the priority of this task from to Unbreak Now!.
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Restricted Application added a subscriber: Aklapper. · View Herald TranscriptJan 12 2016, 5:46 PM

Last three images now, sorry: Wanted to fix some bright spots.

Aklapper renamed this task from PNG thumbnailer giving random changes in contrast/white point to Thumbnailer giving random changes in contrast/white point for greyscale PNG.Jan 20 2016, 4:42 PM
Aklapper lowered the priority of this task from Unbreak Now! to Normal.
Aklapper set Security to None.
Restricted Application added a project: Commons. · View Herald TranscriptJan 20 2016, 4:42 PM
Restricted Application added a subscriber: Steinsplitter. · View Herald Transcript
Steinsplitter moved this task from Incoming to Backlog on the Commons board.Jan 22 2016, 5:59 PM
Restricted Application added a project: Multimedia. · View Herald TranscriptJan 26 2016, 4:18 PM

Is this the same issue as T135862?

Restricted Application added subscribers: Poyekhali, Matanya. · View Herald TranscriptDec 5 2016, 10:03 PM
MarkTraceur lowered the priority of this task from Normal to Low.Dec 5 2016, 10:04 PM
MarkTraceur moved this task from Untriaged to Triaged on the Multimedia board.
AlexisJazz added a subscriber: AlexisJazz.EditedApr 11 2018, 4:27 PM

https://commons.wikimedia.org/wiki/File:Delegation_of_Turks_which_was_sent_to_Lausanne.png same issue?

..confirm, I converted the image from grayscale to color (not actually adding any color), file size increased some 2.5MB and it now thumbnails correctly.

I discovered something really strange! And maybe this makes the bug easier to solve.

For example: https://upload.wikimedia.org/wikipedia/commons/thumb/archive/1/16/20160112173819%21William_Grant_Still_by_Carl_Van_Vechten.png/395px-William_Grant_Still_by_Carl_Van_Vechten.png looks too bright.

Now I download this thumbnail to my computer and open it with the image viewer on my computer. It looks normal! Now I drag this saved thumbnail to Firefox and it looks too bright again!

Different software applications use different PNG rendering software stacks.

Different software applications use different PNG rendering software stacks.

I know, but what it means is that the PNG thumbnails are not fully broken or unrecoverable. It's probably some flag or something that isn't set correctly.

@Aklapper I think I know a solution! Technically workaround but okay.

https://upload.wikimedia.org/wikipedia/commons/thumb/archive/1/16/20160112174139%21William_Grant_Still_by_Carl_Van_Vechten.png/79px-William_Grant_Still_by_Carl_Van_Vechten.png looks too bright.
https://upload.wikimedia.org/wikipedia/commons/thumb/archive/1/16/20160112174139%21William_Grant_Still_by_Carl_Van_Vechten.png/79px-William_Grant_Still_by_Carl_Van_Vechten.jpg looks okay.

This means the input is okay. So if you always force color PNG output, the problem will be solved. Only downside is that color PNG files are slightly larger, but given that greyscale PNG are basically unusable as it is and people are just overwriting files with color PNGs when they encounter this, it's no real loss. Once this is done a feature request could be opened to support greyscale PNG thumbnails.