Are you out on the range

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The main parameters to worry about in any stereoscopic scene are the most negative and positive disparity values. These numbers are usually expressed in percentage terms rather than in actual screen pixels. The distance between the numbers is the depth range. The depth budget is the maximum value this range can be allowed without being too uncomfortable for the viewer.
The current BSkyB specification, which is becoming more widely accepted by other authorities as a good standard to work to, is 3% of screen width. But the same spec also limits negative (in front of the screen plane) to -1% and positive (behind the screen plane) to +2%.
Adding those numbers up, what is the point of the 3% maximum range number? Are they not the same thing? No, because the BSkyB spec also allows temporary excursions outside the -1% +2% limits to go as far as -2.5% and 4% provided the 3% range rule is not exceeded. This allows for more dramatic scope in using the depth space without asking the viewer's eyes to resolve too wide a range of disparities that could lead to eye strain or headaches.
These rules mean you could have a foreground object as far as -2.5% forwards provided that any visible background or objects are no more than at +0.5% to make a range of 3%. Similarly, with a foreground object as far back as +1%, any visible background or objects could be no more than at +4% again a range 3%.
These are special extreme cases not intended for continuous viewing but they illustrate the allowable temporary excursions within the stipulated range of 3%. Cel-Scope3D can show the analysed depth range numerically or with a bar-graph and more completely with a depth histogram including the specified limits (see image).
As well as observing the limits to which the depth parameters can be pushed, to make a good 3D production the dynamics of depth should be managed. This is mostly done in post-production to ensure that the viewer’s eyes can track and accommodate for the action in the depth dimension. This is particularly important for the apparent movement of the point of attention which the eyes will be attempting to converge onto. It is possible to have scene events that don’t exceed the actual depth budget but which still cause objectionable jump cuts. The viewer's eye muscles must be allowed time to shift attention from one scene to the next..
For these and artistic reasons, a depth script can be prepared as part of the storyboarding for a big production. To cut out the tedium of doing this manually, the Cel-scope3D stereoscopic analyser with its logging option can produce a depth chart automatically to aid programme review. This logs against timecode the maximum and minimum depth values used together with the range in use.
Robin Palmer is Managing Director of Cel-Soft and is currently involved with software solutions for 3D & TV quality control and measurement technology.

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