# [RFC v2] Wayland presentation extension (video protocol)

Bill Spitzak spitzak at gmail.com
Tue Feb 11 12:03:41 PST 2014

```I think in absolute time you are right, the "P" points do not move.
Instead everything else would move left by .5, resulting in the same
image I described with the vertical arrows always tilting to the right.

The green decision lines move left by .5 to align with the "P" points
since the decision is strictly whether a "T" is between two "P".

The "T" points move left by .5 because the client will have to timestamp
everything .5 earlier.

I still feel it would be less confusing to avoid negative numbers and to
match what I am pretty certain is mainstream usage of integer timestamps
on frames.

Pekka Paalanen wrote:
> 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
```