Unidirectional pixel tracking

hi guys, i'm looking possibility of using arduino tell me when object (any object) moves frame from, say, left edge. know it's not ideally suited sort of processing, if able it, cost savings substantial seems. we'll have mass of stuff going rapidly from, say, right left, , slower moving objects going left right, or stopping. using ir capable web cam or such sensor, possibly looking at, say, small subset of pixels. has attempted such thing? much 

just getting web cam's video arduino feat in itself. best can off load video acquisition onto external circuit , have arduino access circuit's memory.

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