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Rotoscoping  is an animation technique in which animators trace over live-action film movement, frame by frame.
Cheating..?  Maybe.   Effective..?  Very!   It's a huge time-saver & helps create a more realistic sense of movement in the sequences.

Originally, recorded live-action film images were projected
onto a frosted glass panel and painstakingly re-drawn by
an animator.  This projection device is called a rotoscope.

The device has been replaced by computers in recent years.
In the
FX industry, the term 'rotoscoping' refers to the technique
of manually creating a matte for an element on a live-action
plate, so it can be composited over another background.

roto -  An abbreviation for rotoscope, or for the animated
masks produced by the rotoscoping process.  
The basic concept of tracing pre-existing imagery as a time saver
is really nothing new, since fine artists have been using hand-drawn
'cartoons' to trace their designs onto walls as a template for painted
fresco murals since antiquity.  A perfect example is the Italian artist,
Michelango Buonarroti, who did this for the Sistine Chapel in 1508-12.  

Likewise, the Dutch painter,
Johannes Vermeer, is thought to have
used a device called a
Camera Obscura (the forerunner of the modern
camera) to achieve his incredibly life-like compositions in the mid 1600's.
in  a  Handful  of  Classic  2-D  Films
The Rotoscope
was invented by
Max Fleischer

he patented
it in 1917
Examples & a Half-Baked History of   ROTOSCOPING  
Blurring the line between live-action & animation...