Re: [Algorithms] vrefresh jumpiness

[Thatcher U. <tu...@tu...>]

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.

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 30fps
update on a 60Hz screen, with an object moving at a velocity v, your
eye tries to track the object at velocity v.  At 60fps, the screen
would show:

p0 + v*0/60
p0 + v*1/60
p0 + v*2/60
p0 + v*3/60
p0 + v*4/60
p0 + v*5/60
...

Now if your eye is tracking smoothly to compensate for the object's
velocity, the object's image will hit your retina in the same spot
each frame, and create the illusion of a single object moving at
constant speed.  But at 30fps, the screen shows:

p0 + v*0/60
p0 + v*0/60
p0 + v*2/60
p0 + v*2/60
p0 + v*4/60
p0 + v*4/60
...

In this case, the object image will hit your retina in two alternating
spots, and you percieve a double image.

You can run your own experiments to explore this.  A small white
circle moving fairly quickly on a black background works pretty well.
A CRT shows it better than an LCD, but it's still noticable at 30fps
with an LCD.

At 20fps you should be able to see a triple image; at 15fps you may
even see a quadruple image.  It might help to be in a dark room.  Also
take note of your monitor refresh rate; you can still see this effect
at e.g. 72Hz, but you'll need to display 36fps, 24fps, etc.  I.e. this
effect has little or nothing to do with your eye being accustomed to a
particular refresh rate.

-- 
Thatcher Ulrich
http://tulrich.com