The Digital Fact Book - Quantel

Parallel processingUsing several processors simultaneously with the aim of increasing speed over singleprocessor performance. It often refers to array processor computer hardware that carriesout multiple, often identical, mathematical computations at the same time. Generally arrayprocessors are designed with specific tasks in mind and so are not suitable forrunning complexoperational software. Due to system administration and the fact that not all processorswill complete their tasks at the same moment, causing waiting time, the increase in speedgained by sharing the task is not likely to be proportional to the number of channels available.Due to the very different structure of a parallel processing computer, software designedto run on a single processor system may well need major changes to take full advantage ofa parallel system. The current expansion of popular PC CPUs to offer two or four processorsgenerally works with established applications by running the various applications on separatecore processors, rather than one application on multiple processors.Parallax (Stereoscopic)This refers to the separation of the left and right images on the projection device or displayscreen. Positive Parallax puts an object behind the screen (on screen objects in the lefteye image are to the left of the same objects in the right eye image). Negative parallax putsan object in front of the screen (on screen objects in the left eye image are to the rightof the same objects in the right eye image).Zero or neutral parallax puts an object on the screen (on screen objects in the left eyeimage are overlaid on the same objects in the right eye image).The only difference between stereo cameras should be parallax or angle between the axesof the lenses, as in Camera Convergence – anything else can disturb the stereo viewingexperience. This requires close attention, so that the cameras are set-up the same and withthe same filters. Color differences, skewing, vertical misalignment, differential weave andhop, lens flares, poor VFX fixes, scratches and dirt can all cause problems.Fast cuts between shots with strong positive and strong negative parallax can be unsettlingin some circumstances. This is because the eyes and brain are being asked to jumpuncomfortably quickly between positions and then make sense of the result. This can bemitigated by the use of ‘handing off’ – dynamically changing the convergence of an outgoingshot in relation to an incoming shot. Another method of dealing with this is trying whereverpossible to cut between shots that are somewhat close in parallax.Vertical parallax is a vertical offset between stereo images and is very uncomfortable towatch – so it is necessary to remove it during post production if there has been cameramisalignment during shooting.Note: The term ‘Parallax’ is sometimes used interchangeably with ‘Congruence’ or ‘Disparity’P139