What use do swap interval > 1 and OML_sync_control divisor and remainder have?

Jason Ekstrand jason at jlekstrand.net
Fri Jan 24 10:27:11 PST 2014

I'll just chip in one quick thought.

On Jan 24, 2014 6:32 AM, "Pekka Paalanen" <ppaalanen at gmail.com> wrote:
> Hi,
> I am investigating what kind of Wayland protocol extensions would be
> needed to support proper presentation timing. Looking at existing
> works, I am wondering about two things whether they have any real use.
> Where is swap interval (e.g. eglSwapInterval) greater than one useful?
> Intervals 0 and 1 I understand, and Mesa EGL Wayland already supports
> those. But when would you like to make your framerate a fraction of the
> display's?
> When are the target-MSC related remainder and divisor parameters as
> defined in the GLX_OML_sync_control useful? Why does also X11 Present
> protocol include remainder and divisor?
> GLX_OML_sync_control defines that for interlaced displays MSC is
> incremented for each field. With divisor and remainder you could then
> target only top or bottom fields. Is that useful, and do we care about
> interlaced displays anymore?

I think we do. In particular, we should care about TV set-top boxes. Even
though most TVs are LCD, DLP, or similar, hdmi does support interlacing and
it is still used (particularly in HDTV). I have no idea what implications
this has for a present extension, but I think we could still handle it in a
sane way without going for MSC.

> I am contemplating on not supporting these, because I am going to
> propose using an UST-like clock as the "standard clock language" in
> Wayland present extension. Supporting MSC-based timings would add
> complexity. Therefore I would like to know where and how the above
> mentioned are useful, because I cannot imagine it myself.
> Please, let me know of real actual use cases and existing software,
> where these features offer a visible benefit and what that benefit is
> exactly. I am not interested in what one might do in theory, I am
> interested in real-world examples where they have proved useful. Well,
> maybe also theories if they allow taking advantage of some new cool
> technology.
> Btw. if you think that using UST for presentation timing and feedback
> is nonsense, and MSC is the only right way, let me know and I can start
> another email thread about that detail after preparing my material.
> Thanks,
> pq

I hope that's helpful,
--Jason Ekstrand
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