https://bugzilla.wikimedia.org/show_bug.cgi?id=32721

--- Comment #10 from John Cupitt <[email protected]> 2012-03-19 14:20:17 UTC ---
I've just realised that the -thumbnail option for convert is doing very crude
subsampling, hence the horrible aliasing in the shrunk image. You need to use
-resize to get a proper box filter: 

$ time convert Chicago.png -resize 800 -unsharp 0x.5 tn_Chicago-magick2.jpg
real    0m14.497s
user    0m20.601s
sys    0m1.068s
peak RES: 950M

Which generates this image:

http://www.vips.ecs.soton.ac.uk/development/tn_Chicago-magick2.jpg

the same quality as the vips one, though vips is sharpening more.

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