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Computer animation

Computer animation is the art of creating moving images. It is a subfield of computer graphics and animation. Often created by means of 3D computer graphics.

To create the illusion of movement an image is created on the computer screen then quickly replaced by a new image that is similar to the previous image, but shifted slightly.

A simple example

The screen is blanked, then a filled red circle is drawn in the centre of the screen. Next the screen is blanked, but the red circle is drawn slightly to the right of its original position. This process is repeated, each time moving the circle a bit to the right. If this process is repeated fast enough the red circle will appear to move smoothly to the right. This basic procedure is used for all moving pictures in films and television.

To trick the eye and brain into thinking it is watching a smoothly moving object the pictures must be drawn at about 30 frames a second, or faster. (A frame is one complete image.) Above 70 frames a second no improvement in perceived realism is gained with faster speeds. Below about 30 frames per second most people can detect the flicker associated with the drawing of new images which detracts from the illusion of realistic movement. Above 70 frames per second no improvement is seen due to the way the eye and brain process images. The reason no flicker is seen at higher speeds is due to “persistence of vision”. From moment to moment, the eye and brain working together actually store whatever you look at for a fraction of a second, and automatically smooth out minor jumps.

In 2D computer animation moving objects are often referred to as “sprites”. A sprite is a small image that has a location associated with it. The location of the sprite is changed slightly each fame then displayed to make the sprit appear to move. The following pseudo code makes a sprite move from left to right;

 Int x, y;

 X = 0;

 While ( x < SCREEN_WIDTH )
	 DrawSpriteAtXY(X, Y);  // draw on top of the background

    X=X+5;  // move to the left

Modern (2001) computer animation uses sophisticated mathematics to manipulate complex three dimensional polygons, apply “textures”, lighting and other effects to the polygons, finally rendering the complete image. Let's step through rendering (render: create one complete frame on the computer screen) a simple image of a room with flat wood walls with a grey pyramid in the centre of the room with a spot light shining on it. Each wall, the floor and ceiling is a simple polygon, in this case a rectangle. Each corner of the rectangles is defined by three values: x, y and Z. X is how far left and right the point is. Y is how far up and down the point is, and Z is far in and out of the screen the point is. The wall nearest us would be defined by four points: (in the order x, y, z)

 (0, 10, 0)                        (10, 10, 0)

 (0,0,0)                           (10, 0, 0)

The far wall would be:

(0, 10, 20)                        (10, 10, 20)

(0, 0, 20)                         (10, 0, 20)

The pyramid is five polygons, the rectangular base, and four triangular sides. To draw this image the computer uses math to calculate how to project this image, defined by three dimensional data, onto a two dimensional computer screen.

First we must also define where our view point is, that is, from what vantage point will the scene be drawn. Our view point is inside the room a bit above the floor, directly in front of the pyramid. First the computer will calculate which polygons are visible. The near wall will not be displayed at all, as it is behind our view point. The far side of the pyramid will also not be drawn as it is hidden by the front of the pyramid.

Next each point is perspective projected onto the screen. The parts of the walls ‘farthest’ from the view point will appear to be shorter due to perspective than the parts of the wall nearest the viewpoint. To make the walls look like wood a wood pattern will be drawn on them. To accomplish this a technique called “texture mapping” is often used. A small drawing of wood that can be repeatedly drawn in a matching tiled pattern (like wall paper) is stretched and drawn onto the walls final shape. The pyramid is solid grey so we just fill in its walls with grey. But we also have a spot light. Where its light falls we lighten colours. Where the pyramid blocks the light we darken colours.

Next we render the complete scene on the computer screen. If the numbers describing the position of the pyramid were changed and this process repeated, the pyramid would appear to move.

This short article has only introduced the most basic elements of modern computer animation.

Notable movies and TV shows

External links:
Blue Sky Studios (http://www.blueskystudios.com) (Ice Age)
Mainframe Entertainment (http://www.mainframe.ca) (Beast Wars, Reboot, Shadow Raiders)
PDI Dreamworks (http://www.pdi.com) formerly Pacific Data Images (Shrek)
Pixar Animation Studios (http://www.pixar.com) (Toy Story, Monsters Inc., A Bug's Life, Finding Nemo)

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