Temporal Artifacts in Simulated Displays

Field-Sequential Artifacts

Motion blur (temporal antialiasing) is used in computer animation to more closely simulate reality. However, presently it is computationally too expensive to be used in VE.

Repeating Frames

This is a problem if the frame graphics update rate is less than the refresh rate. For example, if the refresh rate is 60 hz and the update rate is 20 hz, then moving objects will split into ghosts along the path of motion, with the number of ghosts equal to the refresh rate divided by the update rate (or in this case 60/20 = 3 ghosts). So, we want the update rate to equal the refresh rate.

Interlacing

If the video is interlaced (each scan only refreshes every other scan line) then the required update rate is reduced from 60 hz to 30 hz, but this may cause flickering problems.

CRT based stereo viewing systems usually work by sequentially presenting left and then right eye images. But, for moving objects, the left and right eye images should not be the same and this produces visual artifacts, such as wrong depth cueing This is particularly bad for low refresh rate systems, e.g., it is noticeable at 30 fields/sec/eye, but not at 60 fields/sec/eye.

Color monitors have lower contrast than monochrome monitors because of the shadow mask. The shadow mask also makes the manufacture of small high resolution color CRT's difficult. One method is to use a monochrome monitor to sequentially present each color channel and have a colored filter as a shutter. This may be better than LCD displays that are low contrast. But it leads to a problem similar to the repeating frames discussed above. For example, if a monochrome monitor is refreshed at 180 hz, then it would provide 60 hz for each color channel. But if the update rate is 60 hz, then the object is only in the correct position every third frame, as above. Current graphics systems are incapable of updating at 180 hz.

Another problem is a varying frame rate. That is, the graphics system might update sometimes at 20 hz, other times at 24 hz, and other times at 30 hz, depending on the complexity of the image. This is very disconcerting to the user, and tests have shown that a slower, but constant, frame rate is preferable to a faster, but variable, frame rate.

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