https://bugzilla.wikimedia.org/show_bug.cgi?id=51400
Web browser: ---
Bug ID: 51400
Summary: Severe quality issues when scaling PNGs with VIPS
scaler
Product: MediaWiki extensions
Version: unspecified
Hardware: All
OS: All
Status: UNCONFIRMED
Severity: blocker
Priority: Unprioritized
Component: VipsScaler
Assignee: [email protected]
Reporter: [email protected]
CC: [email protected], [email protected],
[email protected], [email protected],
[email protected]
Classification: Unclassified
Mobile Platform: ---
Created attachment 12852
--> https://bugzilla.wikimedia.org/attachment.cgi?id=12852&action=edit
HTML document containing various real world test-cases side-by-side
While experimenting with the test page [1] I realized that for some PNG images
quality of the new scaler was superior, for others much worse. Also checking
the "bilinear" check-box yielded completely different results depending on
content.
I therefore tried to find some sort of systematics by comparing the results for
various real world examples (e.g. images used on Wikipedia, scaled to the size
the are currently used in the respective articles). I created an HTML page
including those test-cases side-by-side to be able to directly compare them
(see attachment).
My results so far:
1) First of all: Is checking "bilinear" (or the URL parameter bilinear=1) on
the test page actually activating bilinear scaling? Since in all my test-cases
the "bilinear" version yields a very sharp result, with sharp edges and
noticeable steps - something which should actually be avoided by bilinear
scaling and isn't visible in any other image editing software I'm using when
setting a bilinear scaler (e.g. GIMP). So, if the switch is not inverted for a
reason, VIPS incorporates the worst linear scaler I've ever seen.
2) All test-cases have in common, that the old-scaler is always a bit smoother
than VIPS (regardeless of bilinear setting) - maybe a bit too smooth.
3) While the old scaler does not always provide the best quality, it provides
always an acceptable quality without any visual flaws. Nothing one could say
off any of the VIPS scaler settings.
4) Choosing one of VIPS scaler settings is basically impossible. Sometimes
"bilinear" looks better, sometimes worse. Sometimes there are serious visual
flaws with "bilinear" enabled, sometimes there are serious visual flaws with it
disabled.
I also gave some basic notes in the attached HTML document on the issues I see
with every graphic. But porbably it's best if you have a look yourself.
In conclusion - judging from those test-cases - I'd still prefer imagemagick
currently. I don't know how hard this is, but I'd say the scalers used by VIPS
need to be improved before we can deploy them on the WMF Wikis. Otherwise we'd
have to accept much worse quality in some cases which should only be considered
if we really have serious performance issues currently (which I don't think we
have?).
[1] https://test2.wikipedia.org/wiki/Special:VipsTest
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