[FFmpeg-devel] [RFC]] swscale modernization proposal

[Hendrik Leppkes]

On Sat, Jul 6, 2024 at 6:42 PM Michael Niedermayer
<michael at niedermayer.cc> wrote:
>
> On Sat, Jul 06, 2024 at 02:11:30AM +0200, Hendrik Leppkes wrote:
> > On Fri, Jul 5, 2024 at 11:34 PM Michael Niedermayer
> > <michael at niedermayer.cc> wrote:
> > > > /**
> > > >  * The exact interpretation of these quality presets depends on the backend
> > > >  * used, but the backend-invariant common settings are derived as follows:
> > > >  */
> > > > enum AVScaleQuality {
> > > >     AV_SCALE_ULTRAFAST = 1,  /* no dither,      nearest+nearest     */
> > > >     AV_SCALE_SUPERFAST = 2,  /* no dither,      bilinear+nearest    */
> > > >     AV_SCALE_VERYFAST  = 3,  /* no dither,      bilinear+bilinear   */
> > > >     AV_SCALE_FASTER    = 4,  /* bayer dither,   bilinear+bilinear   */
> > > >     AV_SCALE_FAST      = 5,  /* bayer dither,   bicubic+bilinear    */
> > > >     AV_SCALE_MEDIUM    = 6,  /* bayer dither,   bicubic+bicubic     */
> > > >     AV_SCALE_SLOW      = 7,  /* bayer dither,   lanczos+bicubic     */
> > > >     AV_SCALE_SLOWER    = 8,  /* full dither,    lanczos+bicubic     */
> > > >     AV_SCALE_VERYSLOW  = 9,  /* full dither,    lanczos+lanczos     */
> > > >     AV_SCALE_PLACEBO   = 10, /* full dither,    lanczos+lanczos     */
> > >
> > > I dont think its a good idea to hardcode dither and the "FIR" filter to the quality level in the API
> > >
> >
> > The entire point of presets is to have them provide a predefined set
> > of parameters, easy for users to pick one value, rather than a bunch.
> > And what a preset actually means should be documented.
> > How do you define "presets" if they don't hardcode a list of choices
> > for all the relevant options?
> >
> > Advanced settings exist for a user to select any particular detail, if
> > they so desire.
>
> The problem is if new features are added and you have a hardcoded list in
> the API what each quality corresponds to change it you have to bump major
>
> also, do we really have or want to have optimized nearest neighbor scaler
> code ?
> If not the AV_SCALE_ULTRAFAST could be slower than AV_SCALE_VERYFAST
> simply because it now "has to" do something we actually have not optimized
>

So.. you object to the comments that explain what it does?
Someone that uses presets will never have a guarantee to the selected
algorithms and options, if thats your desire - set what you want. That
doesn't mean you can't inform users what they are at this time. Its a
comment indicating a behavior, we don't make any stability guarantees
about that, and no API linkage occurs.

- Hendrik