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By setting the shutterOpenTime and shutterCloseTime parameters to [0.0], we can make the shutter still open instantaneously, but then spend the time between 0 and 1 closing at a constant speed. This generates the most samples at the beginning of the sphere's motion, at screen left, and gradually fewer samples as it moves across the screen, making the blur fade out to screen right.

Camera "world" "float shutterOpenTime" [0.0] "float shutterCloseTime" [0.0]

Having this shutter spend the entire interval opening and then close instantaneously reverses the image and the blur fades in over the sphere's motion.

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