Re: [Algorithms] vrefresh jumpiness

[Madoc E. <tm...@ti...>]

Sounds like you're on to something there. But these wavy lines and double
diagonals don't sound like something you might be able to do in practice,
especially given that the perception doesn't follow constants. You'd have=
 to
get every user to recalibrate with white dots on black backgrounds in dar=
k
rooms every time the framerate or their mood changes.

The only solution I can see is to set a low refresh rate and do your best=
 to
stay above it. We all agree here (at the office) that 60hz graphics with =
the
monitor refresh set higher still looks dreadful.

The other solution is really stiff and jerky control systems, you really
don't notice it much then.

So, crap (excuse my french) graphics that run really fast or crap (pard=F3=
n)
control systems that look jerky anyway. They both seem to be pretty commo=
n
solutions. Next time you see them, think twice before criticising, they
might be the result of extreme cunning.

Cheers,
Madoc

----- Original Message -----
From: "Stephen J Baker" <sj...@li...>
To: <gda...@li...>
Sent: Tuesday, March 30, 2004 7:22 PM
Subject: Re: [Algorithms] vrefresh jumpiness


> Thatcher Ulrich wrote:
> > On Mar 25, 2004 at 05:07 +0100, Nils Pipenbrinck wrote:
> >
> >>If you go into a cinema and watch a movie with 24 or 25 fps you don't
> >>see the ghost images.
> >
> >
> > You should, movies are unbelievably bad in this respect.  Pay
> > attention during any panning shot.  Movies have half-frame-rate-itis
> > all the time, since they always flash the same frame twice.
>
> You are both right.  Almost certainly, one of you sees one thing, the
> other sees something different.
>
> > The best explanation of the double-image artifact I've heard is that
> > your eye tries to smoothly track moving objects.  In the case of 30fp=
s
> > update on a 60Hz screen, with an object moving at a velocity v, your
> > eye tries to track the object at velocity v.
>
> The better way to state this is to imagine the canonical caveman chucki=
ng
> a rock at a rabbit that he fancies for lunch.  As the rabbit runs behin=
d
> a tree, he loses sight of it momentarily - and if his brain didn't deal
> with that, he'd be unable to hit the darned thing.  So after enough
> generations of hungry cavement, we've evolved the ability to interpolat=
e
> the missing data when the object we're tracking vanishes momentarily - =
and
> this is what makes a series of still images in movies and TV appear to =
be
> moving.
>
> I suspect that the lack of interest of some animals in watching TV rela=
tes
> to their failure to have evolved that mechanism.  For them, TV is just =
a
> slide-show.
>
> OK - so what are the implications of this?
>
> If you plot a graph of position against time of an object being updated
> 60 times per second on a 60Hz CRT, the points lie on a nice straight,
> diagonal line - and our brains have no trouble interpolating the
> positions using the million-year-old rabbit-went-behind-a-tree mechanis=
m.
>
> But if you only update the position 30 times a second - but refresh the
> screen at 60Hz and plot a time-versus-position graph, it looks like a
> staircase - right?
>
> There are two ways to draw a conclusion about the position of the objec=
t
> between frames from that data.
>
> 1) Fit a wavy line or an actual square-edged staircase to those points.
>
> 2) Draw two nice straight diagonal lines - one that goes through
>     the odd numbered points - and another parallel line that goes
>     through the even numbered points.
>
> 99% of humans unconsciously do the latter because rabbits can't change
> their velocity at 30Hz.  We just havn't evolved to interpolate that
> kind of motion data.  So we HAVE to take the two-straight-lines view of
> the world and instead of seeing one object moving jerkily - we see TWO
> objects moving along perfectly smoothly right next to each other.
>
> Hence the double-imaging.
>
> However as you drop the update rate, our brains 'snap' to seeing
> the wavy-line/jerky motion version of events because the motion fits
> better to our mental model of a running prey animal.  This seems to hap=
pen
> somewhere below 30Hz and above 10Hz (for 99% of people - there are alwa=
ys
> a few outliers on that bell-curve).
>
> Cinemas are running somewhere in the middle of that range. 24Hz.
>
> That's why *SOME* people see double-imaging in cinemas - and some peopl=
e
> see jerky/blurry motion instead.
>
> On a CRT, if you run the CRT at 60Hz and repaint the image just 20 time=
s
> a second, some people will see TRIPLE-imaging(!) and others will just
> see a single object moving jerkily.
>
> Personally, my threshold is somewhere between 15 and 20Hz.  I  can
reliably
> see triple-imaging (and cinemas' double-image) - but I can't see quadru=
ple
> imaging at 15Hz no matter how hard I try.
>
> If you run the CRT at 60Hz and repaint at 10Hz, hardly anyone sees SIX
> images moving across the screen - but I have heard of people who see
> exactly that.
>
> The actual Hertz rate when your perception switches depends on a lot of
> factors - ambient light levels, scene contrast, how tired you are...etc=
=2E
> But variation between individuals is a significant factor too.
>
> Hence, one person might see cinema double-imaging whilst another merely
> sees a jerky/blurry image - and you might find that some movies look
> better than others.  If the image is poorly focussed then that would
> have an effect too.
>
> -----------------------------------------------------------------------
> The second law of Frisbee throwing states: "Never precede any maneuver
> by a comment more predictive than "Watch this!"...it turns out that
> this also applies to writing Fragment Shaders.
> -----------------------------------------------------------------------
> Steve Baker                      (817)619-2657 (Vox/Vox-Mail)
> L3Com/Link Simulation & Training (817)619-2466 (Fax)
> Work: sj...@li...           http://www.link.com
> Home: sjb...@ai...       http://www.sjbaker.org
>
>
>
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