A test page for the new VIPS image scaler is now available:
https://test2.wikipedia.org/wiki/Special:VipsTest
You can give it names of images from Commons and it will show a
comparison with a moving divider, with the thumbnail from ImageMagick
on the left, and the one from VIPS on the right.
I'll explain what you would expect to see when using this tool:
For JPEG images, using a sharpening radius of 0.8 will make the VIPS
result roughly match the ImageMagick result, as long as the thumbnail
is reduced to less than 85% of the original width. With not enough
sharpening, the resulting image looks blurry. With too much
sharpening, contrast in fine detail will be unrealistically enhanced
and high-contrast borders will develop "halos".
Above about 50% reduction factor, the block average introduces
artifacts in fine detail, so enabling the "bilinear" option will look
better, and will more closely match ImageMagick. But if the bilinear
option is used with a reduction factor much smaller than that, severe
artifacts will be seen in areas of contrasting fine detail.
At small reduction factors, the main difference between ImageMagick
and VIPS is that VIPS uses a simple block average whereas ImageMagick
uses a more complex windowing function. This leads to minor
differences in fine detail.
What we're looking for out of this test is:
* Confirmation that VIPS is not completely failing fpr some class of
images.
* Suggestions for parameter values (sharpening, bilinear) for various
source and destination sizes. VipsScaler allows these parameters to be
configured depending on source size and reduction factor.
-- Tim Starling