As was mentioned in the sixth article of this series, depth budget is the single most important parameter in stereoscopic 3D TV. As well as observing the practical and desirable limits to which the depth parameters can be pushed, making a good 3D production required careful management of depth dynamics. This is mostly done in post production to ensure that the viewers’ 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 such as jump cuts that don’t exceed the actual depth budget but which are still objectionable for some of the audience. An example would be three short scenes where the front-most object in shot 1 is not present at all in shot 2, followed by shot 3 in which the most front-most object is a lot further back than in shot 1. Viewers’ eye muscles must be allowed time to shift attention from one shot to the next. It is also recommended that time be given occasionally for viewers' eyes to rest in the course of the programme.
For these and artistic reasons, a depth script can be prepared as part of the storyboarding during preparation for a big production.
With shorter projects or documentaries, a depth plot or depth chart created during post production is more likely to be the only review of these matters. To build a depth chart, you need to go through each clip scene or shot and measure the disparity range used for the principle foreground and background objects and points of attention. Then mark these as coloured lines on your paper chart against timecode. The artistic and technical merits of plotting are given in Brian Gardner’s excellent article at http://magazine.creativecow.net/article/perception-and-the-art-of-3d-storytelling
To cut out the tedium of doing this manually, the Cel-Scope3D stereoscopic analyser has a new logging option which can produce a depth chart automatically to aid programme review. This logs against timecode the maximum and minimum depth values employed together with the range in use. The measurements can be shown either in percentage of screen width or in horizontal pixels. A cursor can be placed on any object of interest or point of attention for a instant measurement of that spot's disparity and hence depth.
Robin Palmer is Managing Director of Cel-Soft. He is currently involved with hardware