[cairo] Concerns about using filters for downscaling

[Krzysztof Kosiński]

2014-03-25 22:18 GMT+01:00 Owen Taylor <otaylor at redhat.com>:
> On Tue, 2014-03-25 at 16:39 -0400, Owen Taylor wrote:
>
>> The other consideration is things look actually like, of course.
>> I've put up samples of the different downscaling methods on different
>> images at:
>>
>>  http://fishsoup.net/misc/downscaling/samples.html
>
> Turned out that some box filtering crept into the Mipmap/Trilinear
> samples - I've fixed that now, but if you looked at the images
> immediately after I posted the link and drew any conclusions, you should
> reload the page and look again.

Two comments:

1. For interactve use, I think that any method except NEAREST has
acceptable quality, while for non-interactive use it is clear that
Lanczos3 gives excellent quality even for pathological images. There
should be a way to use both one of the 'not perfect but reasonably
fast' methods and full Lanczos3.

2. For "Window Screen", it looks like the 1 / 1.1 scaling example for
Lanczos3 shows the same Moire pattern as NEAREST and box filtering.
This may be due to the subsampling always being set to 1x. Can you
regenerate this image with a larger value e.g. 4x? (Those are the last
two parameters to the Pixman call that creates the convolution.)

Regards, Krzysztof