Re: Discounting the joy of Pulfrich 3D

[P3D Gabriel Jacob <jacob@xxxxxxxxxxxxxx>]

>     I wholeheartedly agree. In a sense you were seeing an illusion of
>depth, but you were not seeing depth or 3D.

Well, respectfully I would have to disagree. I wondered about the reference
about Pulfrich being an "illusion" and not being true stereoscopic 3D a
while back. The description you gave, I have read in numerous places but
still puzzles me, because I come to different conclusions.

Using Dr.T's own words, slightly twisted, am I the only one that sees
things differently. I would agree that alot of illusions are overpowering
and very hard for the brain to see objectively. For the most part, people
that don't have experience in percieving 3D in different mediums, (other
than of course real life) I would say "maybe" it's an illusion. But I and
others that have considerable experience in viewing 3D and I don't believe
what I am seeing is not stereoscopic.

>     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).

So far, we are in complete agreement.

>The
>effect is wholly dependent upon movement (e.g. a pendulum was used in
>the original experiments). Factors such as relative true depth or
>apparent velocity (i.e. motion parallax) would perhaps interact with the
>effect (as would other two dimensional cues to depth), but real movement
>across the retina 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.

Again correct, but I take issue with the word "apparent". To the brain, it
is seeing very real disparity. But in a sense it is "apparent", will 
explain in next paragraph.

>     Moving objects have depth, static objects do not, faster objects
>have more depth, objects moving perpendicular to the observer's line of
>sight have more depth than those moving angularly at the same real
>velocity, and moving objects in the distance have depth even though
>stereopsis is normally functional out to about twenty feet. This does
>not bear much resemblance to the real world (despite what the 3D
>Frequently Asked Questions File would lead you to believe).

After this paragraph, you have completely lost me. How do you conclude
that the Pulfrich effect is an illusion and doesn't use true stereoscopic
retinal disparity? It's true that Pulfrich uses motion to achieve the
so called "illusion" but other techniques also resort to some sort of modus
operandi to achieve their goal. Example just because Pulfrich resorts
to motion and when it stops, the 3D effect is lost, means nothing.
If one stops LCS displays from stopping the rapid altering between the
two perspectives, all 3D is also lost. What does that mean? All it means
is that there has to be some sort of way to present to the eyes 
different perspectives at the same time. There are differing systems
used to present two differing views to the eyes in isolation.

Stereopairs, polaroid, anaglyph present both perspectives at the same
time and different methods are used to isolate the two simultaneous
images presented. In Pulfrich, as in LCS displays, the different
perspectives are not simultaneous, thus some way of presenting them
at different times is required. That is the key to the problem.
Of course movement is required in Pulfrich but so what! When it boils
down to, is that it utilizes retinal disparity to achieve stereopsis,
be it by movement. 

So again I raise the question, why is it a so-called illusion????
If it is, it's no more illusion than ALL other forms of 3D.

>     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).

Agreed, but this is not the case with Pulfrich, it is not an illusion
IMHO.

>It is important to
>differentiate between perceived depth due to the Pulfrich Effect and
>perceived depth due to two dimensional cues or stereopsis. Any
>coincidence between perceived depth due to the Pulfrich effect and real
>depth is (despite the ingenuity of television writers) accidental at
>best.

This is where we disagree and the basis of your conclusion does not support
this. So still unresolved.

>     It is unfortunate that a group as influential as the readers of and
>contributors to the photo-3d mailing list and 3D Frequently Asked
>Questions file are so enthusiastic about the use of the Pulfrich Effect
>on television. 

Sorry to hear you feel this way, but glad that you have posted your
thoughts on this matter, since I want to get this clarified. Thus
it's not unfortunate because this forum will hopefully address this
issue. Some other experts want to shed some light on the differing views.

>It would seem apparent that bandwidth would better be
>spent attempting to convince television networks and producers to use
>the available and true stereoscopic techniques.

Well I guess after this is resolved, at least in my mind, maybe we can
better spend this bandwidth.

Many regards
Gabriel back in full de-lurking mode!


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