RE: [Algorithms] FPS Questions

[Tom F. <to...@mu...>]

I like the theory - shame move theaters show each image twice really,
otherwise it'd be a really good one. Though to be fair I see double images
when movies pan (really bad flickery double images too - ugh) - so your
theory still holds.

"it evolved to throw rocks at rabbits running behind trees." How true. How
Quake :-)

Tom Forsyth - Muckyfoot bloke.
Whizzing and pasting and pooting through the day.

> -----Original Message-----
> From: Stephen J Baker [mailto:sj...@li...]
> Sent: 01 August 2000 18:09
> To: gda...@li...
> Subject: RE: [Algorithms] FPS Questions
> 
> 
> On Mon, 31 Jul 2000, Graham S. Rhodes wrote:
> 
> > I like Jason's post. It is fairly consistent with my own 
> observations, which
> > I describe here. He may not agree with my whole discussion, though.
> 
> This stuff is well researched and understood.
> 
> Flight simulator people have been aware of the issue for 20 years.
> 
> > The old "fact" that 25-30 fps is enough for animation that appears
> > continuous to the human eye was discovered when viewing 
> movies of fairly
> > slow moving objects. Slow compared to FPS games during 
> attack sequences, for
> > example. It just happens that 60 fps or 100 fps or 25 or 30 
> fps just works
> > out nicely for most current games.
>  
> There is also a BIG difference between 24Hz movies and (say) 
> 30Hz video.
> 
> In a 24Hz movie, each image is only drawn once - so there are 
> 24 separate
> still frames.
> 
> <apologies to people who've heard me explain this many times before>
> 
> In a 30Hz video animation on a 60Hz CRT, each image is drawn TWICE by
> the CRT - so there are 60 separate still frames - with two consecutive
> identical images being painted onto the phosphor between each 
> swapbuffer
> call.
> 
> That distinction might not seem to matter - so you'd expect 
> 30Hz graphics
> to look better than 24Hz movies - however, they don't - and 
> here is why:
> 
> Our brains evolved for tasks like watching a small furry 
> animal running
> around in a forest - then letting us throw a rock at it and 
> stand a good
> chance of hitting it.
> 
> That means that when the cute bunny runs along and tree 
> trunks, bushes,
> etc get between it and us, we have to mentally interpolate 
> it's position
> in order to fill in the gaps in the imagery coming from our eyes. If
> your brain didn't do that, you'd think that you were seeing a set of
> separate disconnected events - and throwing a rock would be 
> impossible.
> 
> That hardwired interpolation is what screws up our perception of 30Hz
> animation on 60Hz video.
> 
> Look at a graph of position against time for an object moving
> at a constant speed - displayed using 30Hz graphics on a 60Hz video
> screen:
> 
>      |
>      |                         . .
>   ^  |                     . .
>   |  |                 . .
> posn |             . .
>      |         . .
>      |     . .
>      | . .
>      |____________________________
>                 time ->
> 
> Linear motion - but two consecutive images at each position - right?
> 
> Well, when your brain tries to interpolate between those still
> images, it tries to make a straight line through the points,
> but it can't - it's a stair-step function.
> 
> However, one way to view this graph is as TWO parallel straight
> lines. You can easily draw two parallel lines through those
> points - and they fit the data perfectly.
> 
> That's what your brain does.  So you don't see ONE object moving
> jerkily - you see TWO objects moving smoothly but flickering
> at 30Hz.  This means that all fast moving objects double-image
> at 30Hz - and they flicker too.
> 
> When the graphics are only updated at 20Hz, you get triple-imaging,
> as you'd expect.  But there comes a point at poor enough frame rates
> at which your brain accepts that this is jerky motion and not
> multiple moving objects.
> 
> For me, that sometimes happens at 20Hz and sometimes at 15.  It seems
> to depend on the ambient lighting. Somewhere around that speed, I can
> sometimes 'flip' my brain between seeing multiple images and jerkiness
> by concentrating on that - just like you can make some 
> optical illusions
> flip between two states by staring hard at them.
> 
> Different people hit this effect at different speeds. One of 
> my co-workers
> can see quadruple-images at 15Hz - I've never seen that.
> 
> In a movie theater, each image is only painted once - so a perfect
> straight line can be drawn through the points and no double-imaging
> is aparrent - although the flicker is bad and the heavy motion blur
> can get ugly.
> 
> > Does anyone have any reference that states that the human 
> eye *cannot*
> > resolve *much* better than, say, 100 fps?
> 
> No - to the contrary.  There are people who can resolve much 
> better than
> 100Hz.  If you run a CRT at 120Hz but update the graphics at 60, the
> double imaging comes back.  That proves that your eyes are 
> still seeing
> a non-continuous image - even if your higher cognitive centers don't
> notice it.  It would be interesting to do that with 200Hz 
> video and 100Hz
> rendering - but I don't have a CRT that'll go that fast.
> 
> I suspect the limit would be the persistance of the phosphor 
> rather than
> a limitation of the eye/brain.  All those neurons are firing 
> asynchronously
> so there will always be some that see the interruption to the light.
> 
> The idea that the eye/brain somehow doesn't see the black 
> periods between
> the redraws at over 20Hz is a fallacy.  What happens is that the
> interrupted image is reconstructed into smooth interpolated motion for
> your higher level cognitive functions...but when (redraw rate != video
> rate) your mental interpolator gets confused because it evolved to
> throw rocks at rabbits running behind trees.
> 
> Obviously humanity may sometime evolve to be able to interpolate 30Hz
> images - but that presumes that people who play a lot of video games
> successfully will be more likely to pass on their genes to the next
> generation.  The reality is probably the opposite of that!  :-)
> 
> > What happens in 5 years when we all have monitors running 
> at 2000 pixels
> > horizontally? Large immersive displays such as CAVE's 
> already run at 96
> > frames per second at that resolution... What happens when 
> we are all using
> > stereoscopic hardware all the time, so that each eye sees 
> have the total
> > frame rate. 100fps becomes 50fps. We will then need 200fps 
> to match today's
> > 100fps..... Food for thought.
>  
> No - that's not true.  We've run 3500x2200 pixel screens and 
> 60Hz still looks
> just as good as it does on a 1000 pixel screen.
> 
> Steve Baker                      (817)619-2657 (Vox/Vox-Mail)
> L3Com/Link Simulation & Training (817)619-2466 (Fax)
> Work: sj...@li...           http://www.link.com
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