Re: [Algorithms] vrefresh jumpiness

[Stephen J B. <sj...@li...>]

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 30fps
> 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 chucking
a rock at a rabbit that he fancies for lunch.  As the rabbit runs behind
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 interpolate
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 relates
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 mechanism.

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 object
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 happen
somewhere below 30Hz and above 10Hz (for 99% of people - there are always
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 people
see jerky/blurry motion instead.

On a CRT, if you run the CRT at 60Hz and repaint the image just 20 times
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 quadruple
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.
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.

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