Comment by leni536

My acid test for image scaling software:

http://i.imgur.com/X9vKa2s.png

https://i.scaley.io/png-256x128/i.imgur.com/X9vKa2s.png

This means that you don't use a linear color space for blending colors.

Sorry about the language, the image is not made by me [1].

[1] https://web.archive.org/web/20120904060524/http://www.4p8.co...

edit:

I see that your backend relies on libvips to do the resizing. I have no experience with it, but I could make libvips-tools' vipsthumbnail program work correctly:

  $ vipsthumbnail --linear -f tn_%s.png -s 256x128 gamma-1.0-or-2.2.png

Note the "--linear" option, without it produces the same wrong image.