Pulfrich Take 2

[P3D Scott Langill <slangill@xxxxxxxxx>]

My  earlier posting "Discounting the joy of Pulfrich 3D" engendered a
number of interesting responses. I have tried to reply to them in the
order in which they were raised. Where my original posting was quoted in
the reply I have placed it in quotes. My apologies for the length of the
post, but I tried to respond to all nineteen responses to my posting
here. Much of the confusion seems to stem from my connotative use of the
word illusion.

> From: P3D Gabriel Jacob <jacob@xxxxxxxxxxxxxx>

> To the brain, it is seeing very real disparity. But in a sense it is
> "apparent", will explain in next paragraph.

The common perceptual reference for disparity would involve non-
corresponding spatial points on the left and right retina. The
Pulfrich Effect is dependent upon the slower transmission and processing
of neural signals from the rod receptors in the dark adapted eye (the
eye with the darkened lens covering it) versus the signals from the cone
receptors in the eye which is not dark adapted. Light falling
simultaneously on corresponding spatial points on the left and right
retina give rise to an inaccurate representation to the brain or an
"apparent disparity".

> Stereopairs, polaroid, anaglyph present both perspectives at the same
> time and different methods are used to isolate the two simultaneous
> images presented...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.

The point is well taken in the typical sense of an illusion being an
erroneous perception. With an anaglyph a 2D object is seen as 3D. I
should perhaps have been more precise and indicated that by illusion I
meant an incorrect representation or distortion. In the classic Pulfrich
experiment a pendulum moving in a straight arc is seen as moving in an
elliptical orbit in depth. Usually a stereophotograph (depending on
viewing distance and assuming it is not hyperstereo, etc.) portrays an
accurate representation of the reality of the subject. This is not the
case with the Pulfrich Effect except under extremely controlled
circumstances where the constraints placed on a filmmaker or
videographer arguably would totally disrupt the storytelling function. 
In a car chase across the screen where one vehicle is outpacing the
other, either the faster vehicle looks unrealistically wide or the
slower vehicle looks unrealistically narrow. I would agree with the
comments by George Themelis characterizing Pulfrich as "artificial"
(i.e. "does not relate to variations in the 3d dimension in the real
object"); but I would go further in the case with the Pulfrich
Effect (i.e. does not relate to variations in the 3d dimension in the
real object and may portray depth which is contradictory or misleading
when compared to variations in the 3d dimension in the real object).


> " 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.

I made the point regarding two dimensional cues to indicate that the
brain will take the misrepresentational depth due to the Pulfrich Effect
and use two dimensional cues to resolve it into a more realistic
depiction of the scene. An extreme example of the brain's power to do
this is the inability of most people to see human faces as hollow when
viewed with prisms reversing disparity.

From: P3D P3D Marvin Jones  <Campfire@xxxxxxxxxxxxxx>

> I can't say I have noticed this unbridled enthusiasm over the Pulfrich
> effect here, but the simple truth is that, given the technical
> limitations of NTSC television and the need for mass-disseminated
> broadcasts to be viewable by everyone, with or without special
> equipment, Pulfrich is about the only game in town. As ABC viewers
> know, anaglyph simply looks like garbage when broadcast. That's a fact
> of life. It always has and it always will. Alternate field images are
> incompatible with FCC rules because they require expensive decoders.
> This pretty much leaves Pulfrich which, for all its many admitted
> limitations, can produce a startling and impressive stereo image under
> the proper conditions. And despite the sniffy comment about "true"
> stereo, the Pulfrich image is every bit as "true" stereo as any slide
> pair shot with the "rock and roll" method. As was well described in
> the above letter, it sends to different perspectives of an image to
> the brain which are fused into an illusion of stereo -- it doesn't get
> any "truer" than that.

As to Pulfrich being the only game in town, I would refer you to the
article in the April 1974 edition of American Cinematographer entitled
"The Video West, Inc. Three Dimensional Photographic System". The still
photographs made with their technique accompanying the article are
clearly viewable in 2D without glasses and in 3D with glasses. The
technique was designed for video and film. Pulfrich is the perhaps the
cheapest game in town. The question is,hHas the disservice done to the
public's 3D enthusiasm by the headaches of poorly made 50s anaglyphic
films been replaced by the pseudoscopic effects of the 90s Pulfrich
television shows?

From: P3D P3D John W Roberts  <roberts@xxxxxxxxxxxxxxxxx>

> " 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)."

> Perhaps you mean that depth cues from *focus* (accommodation) are
> functional out to about 20 feet? Unless you have really bad depth
> perception, you should be able to see depth (without motion) much
> further out than that. Also, humans *do* automatically move their
> heads to get parallax to judge the distance of very distant objects.
> And Pulfrich works in real-world applications - I experimented with it
> about 5-6 years ago looking out of car and airplane windows, and
> others have also done this. The brain already uses the relative
> motion to reconstruct depth - the Pulfrich effect allows another part
> of the brain to pitch in and reinforce the depth perception.

My understanding of experimental evidence is that given the limitations
of stereoscopic acuity, most individuals make judgements relying
upon two dimensional cues when viewing distant objects. The issue is
whether the Pulfrich Effect provides realistic information or an
accurate portrayal of the true depth of objects. A pendulum moving in a
straight arc is seen to move elliptically when viewed with a darkened
lens over on eye, in fact it moves in straight arc.. An anaglyphic film
of the pendulum viewed with anaglyphic glasses shows the pendulum moving
in a straight arc. Which is more accurate or realistic?

> "It is important to differentiate between perceived depth due to the
> Pulfrich Effect and perceived depth due to two dimensional cues or
> stereopsis."

> Why? If your premise is that both Pulfrich and 2D cues are false 3D,
> what is the benefit of differentiating between them?

Again, as I indicated in my response to Gabriel Jones, I made the point
regarding two dimensional cues to indicate that the brain will take the
misrepresentational depth due to the Pulfrich Effect and use two
dimensional cues to resolve it into a realistic portrayal of the scene.

> "Any coincidence between perceived depth due to the Pulfrich effect
> and real depth is (despite the ingenuity of television writers)
> accidental at best."

> I don't think that's a fair statement. If by "coincidence" you mean
> "exact numerical match of apparent depth", then I would agree that
> Pulfrich is hard to "calibrate", but it sounds more like you're saying
> that it's logically impossible for television producers to consciously
> utilize Pulfrich to create an appearance of depth where they want to
> create an appearance of depth.

The issue is whether you can tell an effective story or present an
effective story with all the constraints that compensating for the
misrepresentational effects of movement with the Pulfrich Effect. I
doubt whether they are aiming for an accurate representation of depth, I
think "a jazzy effect, with occasional pronounced depth effects" sounds
more Hollywood probable to me.

> "You mention analogies - I'll attempt one. In the realm of
> "conventional" 3D, there are two groups of people that are
> particularly prone to hostility toward the methods used: those who are
> "stereo blind", who are sometimes antagonistic to the whole idea of
> 3D, and those who are unable to decouple their perception of size from
> degree of convergence - these people sometimes object strenuously to
> the use of hyperstereo because "it makes everything look small". By
> analogy, is it possible that you are "Pulfrich blind"? If you ride in
> a fast moving car during the daytime, looking directly out a side
> window, with a very dark lens in front of the eye nearest to the rear
> of the car, does the depth of the scene not look any different to you?
> Or is it possible that you are one of those people who strongly
> prefers that the degree of convergence in a stereo image be "just
> right", or things don't look the right size to you? If either is the
> case, then I sympathize with your situation, but I can assure you that
> Pulfrich does produce useful perception of depth to many people.

I find the ad hominem attack a little disingenuous. The point is, I
disagree that it is a "useful perception of depth". The television
presenters would have me believe that this is an accurate portrayal of
the depth relationships of the objects and backgrounds filmed, in fact
it is not. I enjoy the Pulfrich effect to the extent it shows me how I
can be fooled into thinking things are happening which in fact are not
happening (something which is moving in a straight line looks like its
moving in a circle.  In the Ames rotating trapezoidal window illusion an
object attached to one side will appear to orbit the entire window. If I
place an object in front of the pendulum moving in a straight arc, the
Pulfrich Effect makes it look like its orbiting the object. Both could
be termed pseudoscopic effects. Pretty neat, but are these accurate
portrayals of the objects. No. A funhouse mirror is entertaining, but
should I promote it as how things really look?

> You appear to be using the term "vectographic" for all stereo films to
> be viewed using polarized glasses. The discussion to date on P3D
> appears to indicate that "Vectographs" are made by a very specialized
> process which puts both views overlapping one another on the same
> frame. I believe most or all polarized films put the two views on
> different frames. Did you inten to refer to polarized-view stereo in
> general?

I assume I stand corrected. In the literature of perceptual psychology
the term vectographic is typically used to refer to "polarized-view
stereo in general" as you put it.

> Given the current technology, I think it's more useful to define "true
> stereo" as something which utilizes the stereoscopic perception of the
> human visual system to create an appearance of depth, with the
> apparent depth being a function of what might be perceived in a real
> scene. Given that definition, a Pulfrich film or broadcast can be made
> which exhibits true stereo. If you try to narrow down the definition
> of true stereo (in a logical manner) to exclude Pulfrich, you're going
> to start knocking out the cherished traditional methods of depicting
> stereo as well.

As indicated in my responses above anaglyphic methods viewable in 2D or
3D, which would much more accurately depict the actual depth
relationships of the objects photographed exist. Without totalling
controlling all movement by the camera, actors, and scenic elements (to
the exclusion of entertaining storytelling) misrepresentational depth
relationships are endemic to the Pulfrich Effect. The television industry
has chosen the cheap thrill approach, but is promoting it as an accurate
portrayal of depth.

From: P3D P3D Larry Berlin  <lberlin@xxxxxxxxx>

> You are right that it is totally dependent on motion, relative
> horizontal motion, and that is why so many of us were disappointed in
> the NBC show. It didn't use as much relative motion as was available
> to the subject and the presentation.

I agree to the extent that I believe that the requisite attention to
motion is a crippling artifact of utilizing the Pulfrich Effect in
television and film presentations.

From: P3D P3D ron labbe  <ron@xxxxxxxxxxxx>

> Well, what about the "one-eyed 3D" explored by Marcel Duchamp?
> If there is interest, I will post a sample drawing that can be printed
> and set on a tuntable- when viewed with ONE EYE there is a distinct
> stereoscopic effect- which is NOT there when viewed with two (hey,
> maybe an web animation would work?!)

Not sure if this was on topic or not but I'd love to see it.

From: P3D various <>

> 'discussion of left vs right, clockwise vs counterclockwise movement'

My understanding is that it should make no difference. Maybe it doesn't,
or perhaps the "handedness"of the effect is related to the "handedness" of
the viewer, or left-right brain hemisphere related.

> Why did the Pulfrich Chicken cross the road?

To pump up the ratings of NBC as inexpensively as possible.

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