[cairo] Concerns about using filters for downscaling

[Krzysztof Kosiński]

2014-03-22 17:40 GMT+01:00 Owen Taylor <otaylor at redhat.com>:
>> >> The downscaling method used in
>> >> 1.12 is simply incorrect and due to severe visual artifacts makes it
>> >> impossible to use Inkscape to edit drawings that contain large
>> >> bitmaps, e.g. photos. It's no use being fast if the result is wrong.
>> >
>> > Hmm, can you use BEST? That seems to be the right constant to use if you
>> > quality is the absolute requirement and performance is secondary.
>>
>> Lanczos3 is far too slow for interactive rendering.
>
> I'm not suggesting that you use Lanczos3 - I'm suggesting that you use
> the constant BEST, and we make that something that is the best we can do
> at a performance level that is "manageably slow" - about what GOOD is
> in the current 1.13.x branch.
>
>> As a compromise, I propose the following mapping:
>>
>> BEST = Lanczos3 / Lanczos3
>> GOOD = linear / box
>> BILINEAR = linear / box
>> DEFAULT = linear / impulse
>> FAST = nearest / impulse
>
> So DEFAULT isn't actually a constant - it's an internal #define. I'm
> pretty sure we want the default state to be a state that is publicly
> selectable patterns to start off in some magic state.
>
> Assuming we don't want to add new constants, a variant of your
> scheme might be:
>
>            BEST = Lanczos3 / Lanczos3
>            GOOD = linear / box
>  [default] BILINEAR = linear / impulse
>            FAST = nearest / impulse
>
> (linear / impulse presumably implemented as currently rather than with
> a convolution.)
>
> There's a number of reasons I think this is non-ideal:
>
>  * The slowdown for BEST with Lanczos3 in 1.13.x is really quite
>    extreme. I think it's probably unfair to people who optimistically
>    specified it in their code despite it doing the same thing as GOOD to
>    slow down their programs by a factor of 50-100x. Yes, it is
>    documented as
>    "performance may not be suitable for interactive use"
>    but from experience, pointing to wording in docs when people complain
>    that you broke their application is not that useful :-)
>
>    If we want to introduce a real non-interactive scaling mode,
>    I think it needs to be a new constant CAIRO_FILTER_EVEN_MORE_BEST or
>    something.
>
>  * Similarly, people may be specifying GOOD instead
>    of leaving filtering at the default state, and slowing that down
>    by 5-10x will also be problematical.
>
>    If we *only* slow down BEST by 5-10x, we still might break some
>    application, but the set of code we break is much smaller than
>    if we slow GOOD by 5-10x and BEST by 50-100x.
>
>  * GOOD is documented to be:
>
>    "A reasonable-performance filter, with quality similar to
>     CAIRO_FILTER_BILINEAR"
>
>    Obviously "reasonable performance" is subjective, but I'd say
>    it's defined by what it is currently and we can't make it many
>    times slower and say that it's still reasonable performance.
>
>  * Creating large inter-backend differences for the performance and
>    quality of GOOD doesn't seem like a good idea to me - if we're
>    going to improve GOOD, we should wait until we can do something
>    consistent across the common backends.
>
> That's why I prefer the approach I suggested of only changing what
> BEST does, and restricting ourselves there to 5-10x slower not 50-100x.
>
> (Søren pointed out on IRC that the pixman implemention of convolution
> is not SIMD optimized, so their would be future opportunity to improve
> what we can do within the constraints of 5-10x slower.)
>
> The final approach approach would be to extend the filter enumeration
> with a specific value for what you want Inkscape - maybe make
> CAIRO_FILTER_BOX be the LINEAR/BOX combination. Main downsides are
> figuring out what it would do on other backends. Trigger fallbacks?
> And having to carry it going forward.

If you want to keep the performance, then CAIRO_FILTER_GOOD and
CAIRO_FILTER_BEST should be deprecated, because they give wrong
results (not matching their documentation), and new values e.g.
CAIRO_FILTER_HIGH_QUALITY, CAIRO_FILTER_MEDIUM_QUALITY should be
introduced. On other backends, they would do what GOOD and BEST do
currently, because this is essentially a workaround for a bug in the
image backend.

Regards, Krzysztof