Hollywood producer and director James Cameron has been the leading proponent of 3D and his movie Avatar is arguably the best use of the technology to date.

Recently the 3D@Home Consortium posted Cameron's 10 Rules of good Stereo to help everyone from directors and post-production professionals, to commercial cinema owners and everyone in between deliver a good 3D experience to the public.

Cameron's tips address a number of points that include the subjectiveness of the 3D effects, to how to properly troubleshoot a 3D system.

Here are the tips which were provided to 3D@Home Consortium by 3D CineCast and Chuck Comisky:

• There is no screen: "Whenever somebody starts talking about 'stuff coming off the screen,' ignore them," says Cameron.  "They are charlatans. The brain does not think there's a screen there at all. It is fooled into thinking there is a window there; a window looking through into an alternate reality. In fact, the brain is barely aware of the boundaries of the that window or of how far away that window is, which is why objects which break the frame edges may be shot at distances closer than the actual screen plane, which classical stereography texts will tell you won't work. Not only does it work, it is essential to doing good narrative 3D that this old rule be broken as frequently as possible."

• Stereo is very subjective: No two people process it exactly the same says Cameron. It's important to get a group consensus.

• Analyzing stereospace on freeze frames can be misleading: You can work this way, but the final judgment needs to be done with the shots flowing, and ideally in the actual cut. Generally they look worse stopped than moving because the eye gets depth cues from motion as well as parallax. However, excessive strobing caused by the 24P display rate may actually worsen the comfort factor in some shots advises Cameron.

• Convergence Cannot fix stereospace problems: This is critical to remember Cameron warns. Correct convergence does two things: It allows the eye to fuse quickly when cutting from one shot to another. It can also be used to reduce ghosting caused by bleed through of the glasses on high-contrast subjects in the background depth planes. The eye will fuse a given object in frame in direct proportion to how closely converged it is; more converged, faster fusion. You can only converge to one image plane at a time [and when its done] make sure it is the place the audience--or the majority of the audience--is looking.

• Convergence is almost always set on the subject of greatest interest: This follows the operating paradigm for focus ... the eyes of the actor talking. If focus is racked during the shot to another subject, then convergence should rack. An exception to the rule of following focus exactly is a shot with a strongly spread foreground object which is not the center of interest such as in an OTS in which case a convergence-split may be used (easing the convergence forward slightly, to soften the effect). This should be combined with control of interocular to yield a pleasing result. Convergence splits are limited by high contrast edges at the plane of interest, which may cause ghosting in passive viewing systems.

• Interocular distance varies in direct proportion to subject distance from the lens: The closer the subject, the smaller the interocular. The farther away the subject is, the larger the interocular distance. A shot of the Grand Canyon from half a mile away may have a 5-foot interocular. A shot of a bug from a few inches away may have a 1/4-inch interocular. Interocular tolerance is subjective, but there is a constant value of background split which cannot be exceeded.

• Interocular and convergence should both vary dynamically throughout moving shots

• In a composite, the foreground and background may want to have different interoculars: Cameron provides an example such as in an OTS, the stereo space between the two foreground characters may be compressed, and the stereospace in the background may not be compressed. Conversely, in a problematic greenscreen comp where the interocular was baked in too wide, the background may be brought closer to some extent by shifting one eye horizontally relative to the other. These fixes only work in shots with an empty mid-ground between the foreground elements and the nearest objects in the background. This technique can be used or abused.

• When stereo looks bad to the eye it is important to eliminate the possible problems sequentially:
  
  1. Synch: the number one killer of young eyeballs
  2. Reverse Stereo: This will look equally egregious. Some shots may actually appear to almost work as stereo, but foreground objects will look "cut out" as if you are looking through a window. Turning the glasses upside down is the test. If it improves, it's reverse stereo.
  3. Zoom Mismatch: Technically it's focal-length mismatch and it's characterized by a radial interference pattern when left and right images are viewed overlaid.
  4. Vertical Alignment: The eye can tolerate a lot of horizontal alignment mismatch--this is equivalent to incorrect convergence--but very little vertical misalignment.
  5. Color or Density Mismatch: The brain is more sensitive to density mismatch than color, but both should be matched. It's also important to note that with linear polarization there will always be a slight magenta/cyan shift between the eyes. This should NOT be corrected in the color timing of the master because some systems use circular polarization, which doesn't have this shift.
  6. Render Errors or element drop-outs between eyes: Some elements, including objects, shadows or lighting artifacts are missing from one eye.
  7. Specular Highlights: Because the angle of reflection is different for glossy or mirror surfaces as viewed from left or right eyes highlights may exist in one eye but not the other.
  8. Lens Flare & matte box shadows: These problems typically affect one lens.
  9. Image Warping: This problem can happen at the edges of frame with certain lenses and it can happen with warped beamsplitters.
  10. Movement or vibration which is different in left & right (L-R): This shows up in some camera systems Cameron explains. It takes a lot of jiggle between eyes to become apparent. Only when all these possible sources of brain-shear (Cameron's term for bad stereoscopic content) have been eliminated, should interocular be re-examined he adds.

• Some shots just can't be fixed: If they are photographic shots with the interocular baked in they must be re-done or they must be left in the film as non-stereo shots (L-L). If they are CG [computer-generated] shots, the interocular can be reduced to a very low value to give a sense of some stereospace even though it is inconsistent with the rest of the sequence. Cameron notes that in the dramatic flow of the film the low value will work.