Game Development Reference
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Figure 8.1 A Zoetrope.
slots cut in the perimeter. A strip of images is placed inside the cylinder.
The cylinder is free to rotate about a central axis (see Figure 8.1). The
viewer looks through the slots. Since they can only see inside the cylinder
when a slot whizzes past the eye, they don't see the strip of images inside
as a blur. The viewer sees the strip as a series of static pictures that
appear to change from one picture to the next as it is viewed first through
one slot then the next as the cylinder rotates. The slots have the effect of
freezing the rotational motion so that we don't see as image one swings
around to the position of image two; instead, we seem to see image one
change into image two. This fast changing of the pictures gives the illusion
of movement.
A Praxinoscope uses mirrors to achieve the same result. The eye can
only see the pictures when they are aligned to the mirror. The result is that
the pictures do not seem to be rotating, they seem to be changing, giving
the illusion of movement.
These early Victorian parlour toys helped lead to the invention of
cinema. The illusion of movement when a film is projected comes from the
film having 24 separate pictures for every second of screen time. This
sequence of pictures is displayed in order by anchoring the film in a static
gate for around one fiftieth of a second. Then a shutter swings across
blocking the light; while the light is blocked the film is released from the
static gate, advanced a frame and re-anchored. Once the film is again
static, the shutter swings back allowing light to shine through the celluloid
again. If this flashing of the light is set at a speed below 12 times a second
then flickering is detected by most viewers. Early films were shot at low
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