168 Digital Lighting and Rendering
To simulate the most photo-realistic motion blur possible, remember that
slower shutter speeds are often necessary to let in enough light in darker
environments, and very high shutter speeds are usually possible only in
brightly lit environments.
The Comet Tail Myth
Some people imagine that motion blur lags behind a moving object, simi-
lar to how motion lines are drawn behind a running cartoon character, or
a tail of fl ames appears behind a comet. In reality, motion blur is uniformly
bi directional, meaning that if something is fi lmed moving continuously left to
right, it will have the same amount of blur on the left side as the right side.
When you look at a single frame from a movie, the motion blur does not indi-
cate which direction something was traveling. If the subject moved at a con-
stant speed throughout the exposure, then the motion blur will not be bright
in front and fade out in the rear like the tail of a comet, instead will appear
as streaks of a continuous brightness, as you can see in Figure 6.11. Even in a
slow exposure capturing long streaks of motion blur, nothing about the motion
blur would have looked different if the cars were driving in reverse.
[Figure 6.11]
This shot would look just
the same if all of the cars
were driving in reverse.
Motion blur does not fade
o like a comet tail—it
remains constant through
the exposure if the bright-
ness and speed of the sub-
ject remains constant.

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