Re: percieving fast motion (was: Re: gettimeofday() and clock)

[Steve Baker <sjbaker1@airmail.net>]

Adam D. Moss wrote:

> An accumulation buffer initially sounds like the 'right' way
> to perform the above quoted effect, but then thinking about it
> I realized that this probably only adds two useful options:
> 
> 1) Accumulate (say) 3 time-interpolated frames' worth and display
> the result.  Unless you're after a particular 'effect' then
> this is probably useless on PCs with high-refresh monitors (that
> is to say, just get your new frames to the front-buffer ASAP
> instead of accumulating them), but lovely if your display device
> is low-fixed-refresh (say 50Hz) but your engine and hardware can
> push out (say) 150fps and you have fast-moving scenery.
> 
> 2) Accumulate the majority of the new frame with a fair pinch of
> the previous frame.  In theory this does nothing except simulate
> pretty lousy relatively-long-retention phosphor, but I've seen
> it used in a few games (Magic Carpet, GTA3) and it's not an
> *un*pleasant effect, though I wonder why they bother.

3Dfx were pushing that heavily in their last death-throes with the
"T-buffer".  Games designers seemed less than 100% impressed though.

I've used the trick of rendering the scene, sucking it into texture
and then splatting it on top of the previous frame at (say) 50%
transparency.  With that technique, you get half of the brightness
of the current frame plus a quarter of the brightness of the one
before, and eighth of the one before that - and so on down to nothing.

Tweaking the alpha of that re-rendering from 50% to some smaller
value increases the duration of the blur effect.

The result does indeed look like long-persistance phosphor - which is
*close* to looking like motion blur providing your frame rate is fast
enough that the distance between two consecutive object positions is
small...otherwise you just see multiple images.

Fortunately, I was doing this to simulate a long-persistance phosphor
(in an F16 cockpit) and the multiple images were *precisely* the artifact
I wanted to simulate!

Of course if your frame rate is high enough, you probably don't need
motion blur - so this isn't helping the 30Hz frame rate guys.

> Is your skeletal animator still alive, Steve?  It looked very
> interesting.

Yes - it's just been released (two days ago) with the PLIB demo bundle.

     http://plib.sf.net/examples.html

It's changed a *bit* from the last release - but nothing earth-shattering.

----------------------------- Steve Baker -------------------------------
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