Re: Pulfich Effect

[P3D Scott Langill <slangill@xxxxxxxxx>]

In response to the discussion of apparent movement...

> > Try watching TV with half a pair of sunglasses. You will occasionally 
> > be treated to startling 3D when the camera pans left or right while 
> > following an actor or with a title placed over a moving scene. 
> 
> Actually, as long as we're being precise about words, panning is when 
> you rotate the camera about a vertical axis, preferably approximately 
> through the lens.  This won't give you the Pulfrich effect because you 
> won't get a different perspectives.  There is a word for what you're 
> talking about, but I can't remember it.  At any rate, the camera has to 
> translate relative to the objects (or vice versa) to get the effect.
> 
> > It does tend to have the "cardboard cutout" sensation because the whole of
> > an object is usually moving at the same rate. 
> 
> I think the cardboard cutout effect you're seeing is probably due to the 
> usual cause which is poor resolution.  With very poor resolution, you can 
> only see the grossest stereo disparities.  A gross disparity would be 
> between a nearby person and a distant background, not between the tip of a 
> person's nose and his eyes if the person is standing at any distance, for 
> instance.  It's amazing how fine the resolution of the eyes is in stereo 
> mode.  TV sets have rather poor horizontal resolution.
> 
> John B
> 

The Pulfrich Effect utilizes a colored or shaded lens over one eye to
delay the signal from that eye to the brain.  The delay is falsely
interpreted by the brain as representing two simultaneous views
of the same object displaced from one another spatially due to
the presence of depth in the object.  The brain recognizes the
signal as disparity information of the sort used to resolve depth (i.e. in
stereopsis).  The effect is wholely dependent upon movement (e.g. a
pendulum was used in the original experiments).  Factors such as 
resolution, or relative true depth would perhaps interact with the effect
(as would other two dimensional cues to depth), but movement is necessary
for the brains false resolution of "apparent" disparity.  Without movement
there would be no sensation of three dimensional depth perception.  In
other words, the faster the movement the more the depth.  The sensation
would be triggered by real movement across the retina and not by apparent
velocity (e.g. it's in the background so it must be traveling faster).
Apparent velocity is a factor in the two dimensional depth cue of motion
parallax but has no direct relevance to the Pulfrich Effect.  Presumably,
camera movement would also give rise to the sensation of depth.  However,
vertical displacement (or disparity) has been shown experimentally to have
little or no effect on stereopsis and therefore panning or vertical
movement of the camera would have little or no effect.

Two dimensional cues to depth can be very powerful, and as is the case
with most illusions of three dimensional depth perception, they will
interact with and can overpower three dimensional cues (such as with
distorted rooms of the mystery spot variety).  It is important to
differentiate between perceived depth due to the Pulfrich Effect and
perceived depth due to two dimensional cues.

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Scott W. Langill                                   Arbeit mach das leben
Special Edition          slangill@xxxxxxxxx        suesse, aber faulheit
Washington, D.C.                                 staerkt die gliederung.
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