One concept that is the foundation to understanding and interpreting 3D images is to comprehend the relationship and differences between Negative and Positive Parallax.
In their simplest forms, Negative Parallax refers to any 3D Stereoscopic image that appears in front of the screen plane, or pulling off the screen towards the audience.
Where as Positive Parallax incorporates images that appear behind the screen plane or rather pushed into the Television set or Theatre screen. Converged refers to subjects with little to no parallax making them appear behind as though they are sitting on the screen plane, just as any 2D image would.
Negative = towards.
Positive = away.
Converged = middle (screen plane)
When stereo pairs of images are overlaid on top of one another the objects and you examine the parallax separation between the two images you will find two basic correlations. When objects are in negative parallax, the left eye image will appear to the right and the right eye image will appear to the left. (See image below)
Where as when an object within the scene lays in positive parallax (behind the point of convergence) the left eye image will appear more to the left and right the right image will appear to the right. (See image below)
Negative parallax requires our eyes to cross, as the objects are moving out from the screen. If you were to move your finger closer to your face it would require your eyes to toe in or cross. Similarly in negative parallax theory, since objects appear on the right for left eye, the left eye will cross over to view it and the right eye will cross over to the left. (See image below)
The opposite occurs when we view an object in positive parallax, as the image on the right is for our right eye and left for our left eye. Depending on the amount of positive parallax our eyes may only slightly toe-out or go as far as parallel, similar to when we view a horizon. This physical on screen separation can and SHOULD NEVER exceed 2.5 inches, which is the average human interocular. All too often the separation in the background, or positive parallax exceeds 2.5 inches and this causes eyestrain, headaches and poor stewardship of the 3D craft. Nothing bothers me more then when filmmakers deliberately push their positive parallax beyond 2.5 inches. Essentially their running everyone, including themselves, out of a job, as it’s awful 3D to watch. If we continually subject the audience to such poor 3D, then they will avoid Stereoscopic pictures altogether.
Next time you are watching footage in Best-Buy or you are at the theatre and your eyes start to bug out, then take your glasses off and look at the object which appears furthest away. If the separation physically exceeds 2.5 inches (in a theatre measure the separation based on the chairs in the front row) then these images are called “diverged” and should be avoided. It’s not worth your time to gaze at them, as they will require your eyes to diverge and go past parallel, which is physically impossible unless you are a Walleye Fish.
Because we cross our eyes to see negative parallax we can typically tolerate a higher degree of separation in the foreground rather then the background. For very brief moments we can push objects in the foreground to extreme amounts of negative parallax. These extreme negative parallax values give the sensation to the audience the object is flying right off the screen at them.