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PNG thumbnail get smudgy and streaky (near garbled)
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Description

The png [[File:Dpm2.png]] has a weird graphical artifact; size 640 (× 272) pixels https://upload.wikimedia.org/wikipedia/commons/thumb/archive/d/dd/20140805183822%21Dpm2.png/640px-Dpm2.png (the actual upload is also not fully fixed with transparency workaround), the C2–C3 bond of 2-methoxy-1-propanol (the vertical blue line in the top-left structure) has a ghost to its right. At larger renderings (2-3 px are sufficient), it disappears, so it could just be a wikimedia renderer goof or a hidden problem in the image itself. (by -- user: DMacks)


Version: 1.24rc
Severity: minor

Details

Reference
bz69155

Event Timeline

bzimport raised the priority of this task from to Normal.Nov 22 2014, 3:37 AM
bzimport set Reference to bz69155.
bzimport added a subscriber: Unknown Object (MLST).
Perhelion created this task.Aug 5 2014, 8:58 PM
brion added a comment.Aug 6 2014, 9:09 AM

Problem with VPSscaler?

This case seems be very rare!? I found until yet no other picture (with exception of small thumbs with raster pattern, which get very big). But I've also not very intensively searched. So this bug may be negligible?

Another example, there are some slightly visible vertical stripes. The image is indexed with only 6 colors. [[File:Wappen_Samtgemeinde_Bevensen-Ebstorf.png]]

(In reply to Brion Vibber from comment #2)

Problem with VPSscaler?

Yes I think so. It looks similar to artifacts observed with tiffs when using vips shrink (instead of im_shrink) with non-integer shrink factors. Notice the lack of artifacts when using integral scale factors like https://upload.wikimedia.org/wikipedia/commons/thumb/archive/d/dd/20140805183822!Dpm2.png/2357px-Dpm2.png

That said, previously these types of artifacts were only observed on tiff files. Based on comment 4, maybe only observable on indexed pngs?

(In reply to Bawolff (Brian Wolff) from comment #5)

(In reply to Brion Vibber from comment #2)
... Based on comment 4, maybe only observable on indexed pngs?

Yes I think so. I tested the "integral scale factors" and yes it's depending on the original resolution (unusually / odd pixel dimension seems a part of this bug).

For example 2309 is Prime number (so this should be the last good render resoltion): http://upload.wikimedia.org/wikipedia/commons/thumb/d/d3/Wappen_Samtgemeinde_Bevensen-Ebstorf.png/2309px-Wappen_Samtgemeinde_Bevensen-Ebstorf.png

Renderings lower than 400px seems ok.

Restricted Application added a subscriber: Steinsplitter. · View Herald TranscriptAug 9 2015, 1:13 PM

Archive of one test case:

CFCF added a subscriber: CFCF.Aug 16 2015, 10:12 PM
Jdforrester-WMF moved this task from Untriaged to Backlog on the Multimedia board.Sep 4 2015, 6:35 PM
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