[cairo] Concerns about using filters for downscaling

[Owen Taylor]

On Mon, 2014-03-24 at 17:04 -0400, Owen Taylor wrote:
> > Owen Taylor <otaylor at redhat.com> writes:
> I took a stab at implementing a pretty straightforward port of the
> pixman-fast-path.c code to SSE2 - attached. I was able to get roughly
> a 2x speedup:
> 
> >    scale    NEAREST BILINEAR   GOOD    BEST
> >    ----     ------  --------  -----   ------
> >      1.1      656     2860    15163   149147 
> >      1.5      350     1574     7918   108820 
> >      1.9      220      965     7176    89324 
> >      2.0       51      873     6397    95665 
> >      2.5       34      114     4331    71975 
> >      3.0       25      392     4074    67211 
> >      3.5       66      290     3101    60168 
> >      4.0       15      223     3135    59207 
> >      4.5       12       37     2524    58586 
> >      5.0       34      143     2643    56798
> 
>     scale    NEAREST BILINEAR   GOOD    BEST     MIPMAP
>     -----    ------- --------   ----    ----     ------
>       1.1      478     3073    10267    74341     3022 
>       1.5      264     1662     5549    53639     1680 
>       1.9      171     1053     3467    50495     1058 
>       2.0       60      943     3080    46139      151 
>       2.5       40      115     2709    40281      729 
>       3.0       29      431     1907    37104      556 
>       3.5       55      324     1878    35542      439 
>       4.0       19      244     1515    34569      173 
>       4.5       16       39     1451    33327      364 
>       5.0       30      157     1219    33239      330 
> 
> I've added another column which is an implementation (in my test
> program) of the technique of creating a temporary power-of-two scaled
> down image and bilinear-filtering from that. As you can see, even
> tossing away the mipmaps, the performance is much better than with
> convolution - even though either way we have to touch all the pixels in
> the source image, it's much easier to write fast, cache friendly
> average-of-four-pixels code, then convolution code.

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

Warning, large PNGs! (using JPEG would defeat the point of trying to look
for subtle differences in the fine details.)

The images are mostly chosen to be somewhat to very difficult to downscale.

I've also included trilinear scaling since it's come up a couple of times
in the discussion. 

- Owen