[RFC v2] Wayland presentation extension (video protocol)

Pekka Paalanen ppaalanen at gmail.com
Tue Feb 11 00:30:25 PST 2014

On Mon, 10 Feb 2014 12:20:00 -0800
Bill Spitzak <spitzak at gmail.com> wrote:

> Pekka Paalanen wrote:
> > This algorithm aims to start showing an update between t-T/2 and t+T/2,
> > which means that presentation may occur a little early or late, with
> > an "average" of zero. Another option would be to show the update between
> > t and t+T, which would mean that presentation would be always late with
> > an "average" of T/2.
> I think there would be a lot less confusion if this was described using 
> the t,t+1 model. I think in your diagram it would move the 'P' line .5 
> to the right so they line up with the green lines, and all the red 
> arrows would tilt to the right. It makes no difference to the result 
> (the same frames are selected) but I think makes it a lot easier to 
> describe.
> Another reason is that media starts at time 0, not time -.5*frame.

Hmm, I'm not sure. The green lines are not frame boundaries, they are
decision boundaries. In the picture, the points P are the exact time
when a framebuffer flip happens, which means that the hardware starts
to scan out a new image. Each image is shown for the interval
P[n]..P[n+1], not the interval between green lines.

So the green lines only divide the queue axis into intervals, that get
assigned to a particular P. Both axes are the same time axis, with
units of nanoseconds which are not marked. The black ticks on both axes
denote when a new frame begins.

Did we have some confusion here?

- pq

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