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What determines thumbnail sizes?

freephile

Two wikis which are ostensibly setup the same way have different results when it comes to thumbnail quality and the sizes available by default.

Compare:
A vs. B

Neither site had $wgSVGConverters set so I tried setting that to ImageMagick on B with no change in results.

But there are two problems (which are probably related): quality; and the default set of thumbnails created. Whereas A offers

Size of this PNG preview of this SVG file: 512 × 512 pixels. Other resolutions: 240 × 240 pixels | 480 × 480 pixels | 600 × 600 pixels | 768 × 768 pixels | 1,024 × 1,024 pixels.

B only offers a couple resolutions.

Tgr

$wgImageLimits is the list of sizes offered for download, and $wgThumbLimits determines the actual thumbnail size.