[RFC v2] Wayland presentation extension (video protocol)

Jason Ekstrand jason at jlekstrand.net
Sat Feb 8 14:29:03 PST 2014

OK, that makes more sense. Thank you for stating it in terms of intervals.
I still need to think about it a bit more.
On Feb 8, 2014 4:14 PM, "Axel Davy" <axel.davy at ens.fr> wrote:

>  On 08/02/2014, Axel Davy wrote :
> Hi,
> On 08/02/2014, Jason Ekstrand wrote :
>>       For each surface with queued content updates and matching main
>>       output, the compositor picks the update with the highest
>>       timestamp no later than a half frame period after the predicted
>>       presentation time. The intent is to pick the content update
>>       whose target timestamp as rounded to the output refresh period
>>       granularity matches the same display update as the compositor is
>>       targeting, while not displaying any content update more than a
>  I'm not really following 100% here. It's not your fault, this is just a
> terribly awkward sort of thing to try and put into English.  It sounds to
> me like the following: If P0 is the time of the next present and P1 is the
> time of the one after that, you look for the largest thing less than the
> average of P1 and P2.  Is this correct?  Why go for the average?  The
> client is going to have to adjust anyway.
> If you target t, and P0 and P1 are possible pageflips time,
> if P0<t<(1/2)P0+(1/2)P1 then you take the pageflip at P0
> if (1/2)P0+(1/2)P1<t<P1 then you take the pageflip at P1
> That way the length of the intersection of the interval (t,t+time between
> two pageflips) and 'time interval at which it is displayed' is maximized.
>  Well it isn't really the reason why this is choosen (else one might say
> it would be better to maximize with (t-T/2,t+T/2) with T the time between
> two pageflips.).
> The reason is more that you want to minimize the time at when the pageflip
> happens and t, so the real presentation time and t doesn't differ more than
> T/2.
> Axel Davy
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