Re: Stats Question correction

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Angus Dorbie (dorbie++at++sgi.com)
Mon, 06 Apr 1998 13:42:23 -0700

Marcus Barnes wrote:
>
> On Apr 6, 11:46am, Jay Gischer wrote:
> >The label "frame" appears to be causing a great deal of confusion
> >here. Recall the picture of Performer's pipe
> >
> >APP | Frame 1 | Frame 2 | Frame 3 | Frame 4 | etc
> >
> >CULL | | Frame 1 | Frame 2 | Frame 3 | etc
> >
> >DRAW | | | Frame 1 | Frame 2 | etc
> >
> >If 30.0 Hz is the target frame rate then the target from one pipe
> >stage boundary to the next is 33.3 ms.
> >
> >However, the time reported with the label "frame" is a latency time,
> >and measures the time from when the frame starts the APP stage, to
> >when it completes the draw stage. Ignoring special cases, this time
> >is 3 times as long as the time for a single stage. Three time 33.3 is
> >100, so 96.3 is about right.
>
> I don't believe that this timeline takes the video refresh period into account.
> Recall that the draw is double buffered. Therefore you don't actually see the
> results of the draw process for another "video frame". This adds to your
> latency.
>

This is incorrect. The vertical line is the vertical retrace. When the
draw ends it swaps and the image is switched at the next vertical
retrace,
in the diagram this is the line at the end of draw. Almost immediately
thereafter the video starts getting sent on the wire without any video
buffering and should begin to appear on your screen if you use CRTs.
There are different schools of thought on where latency should be
measured to. Some say the beginning of video, others say the end
but it has little to do with the I.G. You say potato I say potato.

Cheers,Angus.

-- 
"Only the mediocre are always at their best." -- Jean Giraudoux 


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