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by jerf
4363 days ago
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It depends on your "bullet-time" model whether it's red-shifted or not. Much "bullet-time" is explicitly just a special effect, i.e., the idea is that the action is still occurring in real time but we're having it slowed down so we can see it, understand it, and enjoy it. In that case we are simply shooting the scene with a high-FPS camera. "Technically" we should indeed get a dimmer scene, just as real high-speed cameras do, but it's not that unrealistic that it's compensated for, again, just as in real life when the Mythbusters go to a high-speed camera shot they've already compensated for that. If you have a "real" time slow-down, exactly what happens depends on your model, especially where the photons cross whereever the effect is occurring. It is true that one model could be that a photon crossing into the effect keeps the same "real" Hertz, in which case you'd see it red-shifted. However, since in the end the entire idea of such sharply-localized time variance without a corresponding gravity field is complete nonsense, the truth is that no matter how you slice it the result is gibberish, so it's just as sensible to let the field "keep" the old photon's frequency too. However, I'd point out that "real" time slowing in the movies is fairly unusual. Mostly it's the first kind. There's a number of sci-fi books with a variety of "real" time slowing effects, though. |
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a) Compensating with shutter so that the integration time is constant means the quality of motion blur changes during the ramp. For example a shot that starts at 24fps and ramps to 120fps keeping a shutter time of 1/240th would start out with really crisp-looking "skinny shutter" motion, and you might see flickering and strobing if there's much camera motion. The end of the ramp would have normal looking amounts of motion blur. If there are CRTs or fluorescent lights in shot they'd flicker at different rates throughout the ramp. It can be a dead giveaway that something's about to happen if a shot starts with really tight motion blur like that.
b) Compensating with lens aperture is much less common because the depth of field changes, but starting with everything in focus, the focus becomes much tighter with the background way out of focus at high speed. Reportedly, mechanical problems tend to manifest as the aperture overshooting its mark or bouncing back leading to brightness variations at the end of the ramp.
c) Compensating with ISO/gain is possible these days, which would avoid the above problems but would end up with a noisier image for the high-speed end.
Recent bullet time shots are either fully CG, shot with an array of cameras, or cheated by hanging everything on wires, but it's interesting to check if any of these things from the days of 16mm skate films with the classic hang-time dips are re-created :)