[gmpi] Re: Reqs 3.9. Time - opening arguments.1

  • From: Chris Grigg <gmpi-public@xxxxxxxxxxxxxx>
  • To: gmpi@xxxxxxxxxxxxx
  • Date: Thu, 5 Feb 2004 12:27:00 -0800

I would assume that when scheduling an event to sync with a video display, the UST would represent the moment when the video frame appears to the audience's eyes, not the realtime time when the video or audio buffer was getting rendered, which is always earlier than display time. If so, then all of this needs to take into account the downstream audio output latency from the plug's place in the chain. Including possible downstream graphs other than GMPI, the audio output buffering, etc. Nontrivial, as currently the GMPI sample clock seems to be relative to the current GMPI graph only, with no adjustment for what happend downstream.

-- Chris G.

On Thu, Feb 05, 2004 at 11:29:14AM -0500, Paul Davis wrote:
 Its still king, but you still need UST to link the two clocks
 together. You need to know that sample S (more precisely, the first
 sample of a given process block) "happened" at U1, and that video
 frame F was drawn at U2, and then you can sync them together.

How does it know what UST time that first sample "happened" at when you the sample has not happened until everyone is done processing it AND you can't predict the future of UST timing?

You either assign it a UST time in the past, or you need to be able to
predict UST times (i.e. UST is linear).

 Remember: you need to be able to get some time (U1, U2) from a
 non-process() context (i.e. when doing video sync). Nothing we have in
 GMPI so far is designed to allow an arbitrary component of a
 multithreaded plugin to call the host to get "the current sample
 time", and i don't actually know of any way you could reliably get
 that anyway.

What is the "current" sample time? the sample number of the start of the current timeslice? An offset into the current timeslice based on % of plugins processed? The current sample being calculated in the current plugin being processed?

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