In this article I will explain in simple terms, what goes behind the 3D animation that you watch in movies and what makes it different from conventional two dimensional animation.
What is that extra 3rd dimension? Take a piece of paper and sketch a simple figure on it (a cat, a dog or anything that comes to your head). Lets say it is a cat and it is facing you from that sheet of paper. So you have the front view of the cat in front of you. Suppose if you feel that you want to see the cat from a side, will it help if you rotate the paper or flip it? No. Why? It is simply because the sketch you have drawn has the 3rd dimension missing.
Every real world object that you see around has a 3rd dimension and that is the reason why you can take it and rotate it to watch it from different angles. The sketch you have drawn had a length and a width, since the paper you used to draw also had a length and a width. But it lacks a thickness (3rd dimension) and hence your sketch also didnt have that extra dimension.
Suppose instead of sketching your imagination down on the piece of paper, you decided to sculpt it on a handful of clay. Since the medium you used (clay) had volume, you had to define the cats shape from all angles during the sculpting. Hence you unknowingly added that 3rd dimension to it and that is the reason you have the freedom to rotate it any way you want.
How conventional 2D animation works:
Before computers started playing their indispensable 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.
What was missing in 2D animation?
The 2D animation always lacked the essence, since all the real-world sceneries and objects are 3D and when they gets transformed to 2D, they lose their reality. Later stage cartoons started to simulate the 3D effect by using gradients, and varying highlights, but it required huge extra effort from the part of the artist.
How 3D Graphics works:
The stages in 3D animation are more in number compared to the 2D animation. The first part of 3D animation starts with character sketching and 3D modeling. In the next stage the characters are rigged for animation. In the next stage they are animated. This is in fact a too compact form of what happens in the background. Lets see each of them in a little detail.