
Hi!

Welcome to the MOTION.EXE demonstration that the human eye can
actually tell the difference between 30Hz and 60Hz.

Someone disputes my claim and says the eye can see only 30fps.
You can tell the difference between 30fps and 60fps _INDIRECTLY_
using a property called "feel of motion smoothness".

THINGS THAT GO 30FPS
~~~~~~~~~~~~~~~~~~~~
- Tomb Raider 3Dfx version on most systems (30 fps, at 60Hz refresh)
- DOOM (actually close, framerate locked to 35fps)
- Movies (actually close, framerate locked to 24fps)
- Virtua Fighter Original in arcades (30 fps)

THINGS THAT GO 60FPS
~~~~~~~~~~~~~~~~~~~~
- Television Sports, News, Sitcoms. (Yes, TV displays 30 fullframes per 
second, but there are 60 _fullscreen_ halfframes per second (interlaced
images), consisting of only the even scanlines, or the odd scanlines. 
The TV camera is actually recording the second interlaced picture
1/60th second after the first interlaced picture, in one frame, so there
are actually 60 different images per second on a television.  You'll
also notice this if you playback this video one field at a time on
a studio quality videotape recorder that can do single-field stepping.)
- Virtua Fighter 2 in arcades (60fps)
- Most of the time during latest new patched version of 3Dfx Descent2 
registered on a fast pentium (60fps)

The eye cannot _directly_ detect high rates, but the eye can 
_indirectly_ detect higher frequencies (motion, flicker) via
motion, for a "feel of fluidity"

An experiment to try: Did you know a common LED alarm clock flicker,
usually at 60 or 120Hz?  You can detect this _indirectly_ using these
instructions:

1. Find an LED digital clock (One of those with glowing red digits)
2. Turn off all lights except for the clock. (Must be a dark room)
3. Pick up the clock and hold it in front of your face, about an
arm's length away.
4. Roll your eyes around in circles quickly.  (Or wave the clock
around very fast but carefully)

You'll notice an effect (an illusion) that does not occur with
a non-flickering light source.  The effect is hard to describe, but
as you roll your eyes around, the digits of the clock becomes 
fragmented.  The clock display looks _very_ distorted while you
roll your eyes around, than if you were staring directly at it.
(It's as if the clock was displaying only part of its display 
only while you were rolling your eyes)

The physics behind human ability to _indirectly_ detect higher 
frequencies can be explained by imagining a flickering indicator
light.  If the flickering indicator light was flashing on and off
60 times per second, and was moving 60 inches per second past your
face while you simply stared ahead, you'll notice a brief "dotted light
path" illusion with the lights being on every 1 inch, and off in 
between the inches.   (This explains the alarm clock phenomenon
illustrated above.)

The human eye has been known to indirectly detect _really_ high
frequencies.  (Imagine a really bright 600 Hertz flashing light 
zooming by at 600 inches per second, you'll still notice a dotted
path.)  In general, the higher the frequency, the faster the
source must be, in order for your eyes to detect the "effect".

When there's no dotted path, the motion is considered "smooth".
The same applies for video game framerates.

NOTE:
Incandescent lights (common light bulbs don't flicker with the 60Hz A.C. 
electric current because incandescent lights are still hot for 1/60th of 
a second and continue glowing even between the 60Hz cycles of the
A.C. electric current).  That's why you can't use the motion flicker
technique to detect whether or not they flicker.

Thanks,
Mark Rejhon
marky@ottawa.com
http://www.marky.com/
