Before computers started playing their indispensible roles in the animation industry, everything was done manually by animators, who were essentially artists. They would create a series of slides having images on it, where each slides image is the continuation of the previous one in the sequence. For example if an animator wanted to simulate a ball falling down, they would create a sequence of slides where first slide would portray the ball at the top. The next slide will show the ball, may be 1 cm lower than that in the first slide. In the next one, again lower and so on, till the last slide shows the ball hitting the ground. When the whole sequence of slides are shown in front of the viewer in a fast rate, it creates the feeling of the ball falling down.
The whole process was tedious and time consuming. When computers came into play, the frame redrawing works had been minimised since, copying and pasting duplicate elements between successive frames was very easy with the computers aid. The artist has to make only the necessary changes that should exist between successive frames. As technology advanced, softwares evolved that again minimised the work of a 2d animator, in such a way that several things started getting automated. Using motion tweening and other techniques, an animator can set the initial position or shape of an object and then its final position and shape and the computer would generate the intermediate frames automatically. The artist even has the freedom to make corrections to that.
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