Re: Picture wobbling

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Gerard R. Tyra (gerry++at++sa-tech.com)
Mon, 08 Dec 1997 13:32:46 -0800

Angus Dorbie wrote:
>
> On Dec 1, 12:31pm, Gerard R. Tyra wrote:
> > Subject: Re: Picture wobbling
>
> > A question for ASD:
> >
> > Has gang draw been implemented in newer versions of Irix? If not, why
> > not?
> >
>
> How about it isn't needed, besides I think it's supported now.
>
> The problem with swapready is that it can confuse. It is usefull
> but a software lock is only microseconds slower.
>

Okay, correct me if I am wrong (the finer points of the graphics pipe
are a moving target). I am under the impression that graphics commands
are put on a FIFO while waiting for the graphics hardware to execute
them. While multiple SWAPBUFFERs can be issued within micro-seconds of
each other, they are not executed by the hardware until the FIFO has
cleared. Hence, there can be a significant (measured in micro-seconds)
difference in the execution time of the SWAPBUFFERs on the various
pipes. The greater the difference in pipe loading the greater the
difference in the SWAPBUFFERs.

If this difference occurs across a vertical retrace, you do get a
momentary break between the channels. Even one break every couple of
seconds is extremely noticeable.

Using a software lock on the swap reduces the probability of an async
swap, but only hardware (swap ready) guarantees a lock.

Gerry
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