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Unexpected experiences with stereo pictures

Introduction

If there is a mismatch between the stereo image (a simulation) and the real-world experience; the difference in brain expectation can cause some unexpected experiences (even eye strain). This experience is a brain-eye communicaion issue; the brain tries to compute the scene, possibly strengthens the eye muscle to get the right experience, but does not succeed and thus some eye strain which can escalate in stress/pain.

For instance a convergence/focus mismatch could be a possibility reason for unexpected experiences, as it is a difference between the normally experienced world and the fake stereo image. A real 3 dimensional viewer might solve this (and that is also why stereo is not the same as 3D).
A lot of unexpected experiences can, IMHO, be overcome by exposure time. Like if you wear spectacles that reverse up and down; in the beginning one will have a hell of an eye stress, but after some time one sees everything normal again.
If you have spectacles, you know the effect when you get a new prescription; When going from single to vari vocal, it can take the brain-eye combination many months (in my case some 4 months) to adjust to that new experience.
So all these stresses are not only due to the eye, but IMHO; the combination eye and brain.

In the below overview, possible reasons for unexpected experiences are gathered.

Viewing method
All viewing methods (anaglyphs, parallel, cross, viewer, screen, etc.) have a difference in experience for the same picture pair. Every method has a slightly different optimum.
Technical errors
Vertical deviations (and many errors relate to this like (Ferwerda [1990]); barrel distortion, pincushion distortion, focus difference, etc.), divergence/convergence angle (key stoning) errors, color/brightness differences and horizontal deviations (incl. window violations) can give unexpected experiences (as these are not normal in every day live also). The origin of the errors could be due to:

Mounting errors
Recording errors

Non-orthostereo
A relation between base of the stereo camera and the focus length can be different then the eye separation of the viewer. The location of the main object related to the stereo window depth might be important here.
Convergence/focus linkage
It is worth investigating if this causes uncomfortable expereinces; as it is the only one that is really different if we have no technical errors. Ideas how to test/demonstrate this are welcome.
Concentrated attention
Excessive depth is sometimes provided to show off stereo, this can cause uncomfortable expereinces.
Controlled environment
A physical viewer device controls/restricts the viewing of a stereo pair and this might reduce uncomfortable expereinces.
Stereo experience
If someone gains experience with a certain viewing method the uncomfortable expereince is reduced. Also the time one looks at a scene determines the amount of stress (can be in both directions;-).
Eye properties
If spectacles are worn, this means that the viewer has some aspect of the eyes that are not optimal (like short/long sighted, astigmatism, etc.). Is can also lead to eye stress.
Difference between people
Everyone might react differently on one of the above. So the thresholds for uncomfortable expereinces are different. There is also a hysteresis when getting the uncomfortable expereinces and when getting rid of it.

Viewing methodology and calibration

IMPORTANT: If you see this web page for the first time, it is essential to check the points in this paragraph (otherwise the showcases won't be presented properly).

To properly view the stereo pictures; use at least Internet Explorer 5.5 or Netscape 6.2 (or compatables).
One must allow JavaScript (to optimize the pictures for parallel viewing), cookies (to remember stereo viewing method when returning to this site) and pop-windows (to use the separate window for the YouTube video).
To optimize the viewing of the stereo pictures for your screen (screen pixels per inch [dpi]) and your eyes (eye separation); please calibrate the picture sizes and eye pupil distance (eye separation) using this page.

Unexpected experiences

A few types of unexpected experiences can be witnessed when watching stereo pictures:

a different experience than expected

rivalry between right and left image,
smaller/bigger than usual scene,
changed depth experience,
ghosting; can happen with most viewing methods where the left and right picture are superimposed; like anaglyph, projection, interlacing, etc.
etc.

eye strain
head/eye/nose ache

Please report what you experience.

Showcases

Parallel viewing
Important: Make sure you at least once calibrated your setup when doing parallel viewing!
The showcases are presented with viewing method:
and using a window with:

Technical errors

Mainly due to mounting

Mainly due to recording

Non-orthostereo
...

Other viewing methods than parallel viewing
One can use the normal YouTube web pages, and select the wanted 3D View Style (pull down menu below the video frame). As I am not able to control the size of the YouTube window; comparing experience of people might be more difficult. Please provide in your feedback the size in millimeters [mm] of the YouTube video frame.

Technical errors:

Feedback is very welcome

It would be great if you can provide feedback when you experience eye strain (or head ache) in the above showcases.
Use the above showcases and follow the below procedures to report your experience:

Click on a certain showcase
Press Play button and directly the Pause button in the YouTube video; you will see a red bar, to show that the video is being downloaded on your workstation. Wait until that is done.
While looking at the stereo pair (using parallel method); slide the time ruler to the right. At some moment you will experience something unexpected. Note the time (which is the first number right of the time ruler and before the '/').
Continue to move the time ruler to the right; at some moment this unexpected experience will vanish; note again the time.
Note the properties of your spectacles (if any): do you have an emmetropic eyes, are you long/short sighted, do you have cylindric lenses (astigmatism), eso/exo-phoria, etc. Also note what type of spectacles you have: single-, bi- or vari-focal. Also tell if you were wearing your glasses when looking at the stereo pair.
For non parallel viewing methods: What is the resolution of the display (pixels per cm): measure the horizontal width of screen in cm and provide the pixel width of screen (right click desktop -> Properties -> Settings -> Screen resolution).
Include in this response also the approximate time you spend on doing this showcase (as people might get used to some errors after some time).
Send the above noted items for which showcase you did.

Tools used

The following tools have been used:

VRML

Cortina3D Viewer 6.0 (a pity of the watermark; might be due to freeware version): VRML viewer
Internet Explorer 8: embedding Cortina3D

PSP 8: for grabbing the scene from Cortina3D
SPM 4.11: resizing corrected pictures, pincushion/barrel distortion
Magix MEP 14: deviations, rotations, making video, brightness, hue
YouTube: providing video storage
JavaScript: aligning the video properly (looking at screen resolution and eye separation).

Acknowledgments

I would like to thank the following people for their help and constructive feedback: Olivier Cahen, Betty Drinkut, Bill Glick, gl, Linda Nyrgren, Mathew Orman, JR, George Temelis and all other unmentioned people. Any remaining errors in methodology or results are my responsibility of course!!! If you want to provide constructive feedback, let me know.

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Major content related changes: Sept 4, 2009