Re: A bug in perfly ?

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Greg Edwards, SGI UK. (gedwards++at++cordoba.reading.sgi.com)
Tue, 14 Nov 1995 09:43:13 +0000

Well explained Jim. This (temporal aliasing) is a common problem. I've had
to explain this to several customers who were convinced the graphics
was broken. I think we all get used to it and ignore it, but if you look
at high contrast edges in any 30Hx or 20Hz scene (on 60Hz minitir) you can see
two or three ghosts. It's like the coarse Gouraud shading on many objects,
we just accept it as normal.

On Nov 13, 10:39am, Jim Helman wrote:
> Subject: Re: A bug in perfly ?
> This is simply an artifact of rendering at a rate lower than the video
> refresh rate, e.g. 30Hz frame rate on a 60Hz monitor. It's most
> evident during rapid changes of heading, but you can see it with
> linear motion as well, e.g. the telephone poles on the side of the
> road.
>
> The explanation is quite simple. If your eye is tracking an object as
> it moves across the screen, e.g. a telephone pole. Your eye is moving
> at a continuously, tracking the average motion of the object. But
> with a 30Hz frame rate, the object itself is being drawn in the same
> location on the screen at two instants 1/60th of a second apart.
> During this 1/60th of a second, your eye's direction has moved
> slightly causing the telephone pole to be imaged at a slightly
> different position on the retina than the previous one. This results
> in two images on the retina (ghosting or doubling) when rendering at
> 30Hz, three images at 20Hz, and so on.
>
> Some solutions are to render at the video rate, to reduce the rates of
> motion such that the distance separating the images is very small, or
> to reduce the contrast of edges in the scene.
>
> In field sequential color displays (e.g. 180Hz R+G+B), the same
> artifact appears. With a 60Hz frame rate on 180Hz field sequential,
> you see three ghosts each with a different color, which gives the
> illusion of little rainbows along high contrast edges.
>
> rgds,
>
> -jim helman
>
> jimh++at++surreal.asd.sgi.com
> 415/933-1151
>
>
>-- End of excerpt from Jim Helman

-- 
__________________________________________________________________________
Greg Edwards, Graphics Support/Consulting Group, Silicon Graphics UK Ltd.        
Forum 1, Theale, Reading, UK, RG7 4RA. 
switch +44 1734 257500, direct +44 1734 257740, fax +44 1734 257553
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