Introd To Comp. Anim. (1st yr)

Introduction to Computer Graphics & Animation

by Alan Dorin

Animate - breathe life into, enliven through...

Movement
Character / Personality
Feeling / Emotion
Etc. etc. etc.

Movement is a powerful medium through which to convey a message to your audience.

Gertie the Dinosaur was (perhaps) the first real animated character. Gertie was created by cartoonist Winsor McCay in 1914. Unfortunately, due to copyright restrictions, Gertie cannot appear in person on this site. But you can read about her and check out some pictures elsewhere on the web.

Gertie Dinosaur

Nowadays, computers may be used to produce photo-realistic animated dinosaurs. Of course, if dinosaurs aren't to your liking, you can animate just about anything else too!

Animation is the process of creating images one at a time to be displayed rapidly in sequence...

Persistence of vision is the blending together by the eye/brain of rapidly displayed sequential images, giving the illusion of movement.

Individual images forming an animation are called frames (after the frames on cinefilm).
Using the computer, synthetic images may be created and played back rapidly in sequence to give the illusion of moving characters and scenery.
When linked to a soundtrack, digitally produced cinema is just a few (billion) CPU cycles away.

Three tasks for computer animation

Once the storyline, scenery, characters etc. have been developed, the production of computer images falls roughly into three major areas:

	Modelling - the process of specifying the geometric properties of an object. Usually performed through a GUI allowing the user to select basic primitives (cones, spheres etc.) and modify their geometry by various means as well as decorate them with textures and connect them in hierarchies. At this stage synthetic lights are also added to the model for the rendering process.
	Animating - the process of specifying the time varying properties of a model. Usually performed by a keyframing process where the model is posed at various times and the computer mathematically determines where the model ought to be positioned between these key frames.
	Rendering - the process of synthesizing images of a model. Usually a computationally expensive process where imaginary light rays are bounced off the imaginary model at each time an image is required, and for each pixel in the image, to determined the colour visible to a viewer at that point in space.

Modelling - How is the model represented in the computer?

A model object may be stored within the computer by its:

Vertices - specified in cartesian co-ordinates as P(x,y,z). The model may be stored in an array of data-structures such as:


typedef struct {

	double x,y,z;
	
} vertex;

Edges - specified as an array of line segments between two vertices. A single edge may be stored in a data-structure such as:


typedef struct {

	vertex endPoint1;
	vertex endPoint2;
	
} edge;

Instead of two vertices, two indices into an array of all vertices of the model may also be stored to form and edge.

Polygonal faces - specified as an ordered array of vertices or indices into an array of vertices.

The vertices of the face are ordered in an anti-clockwise direction when looking at the face's front side.

A single polygonal face may be stored in a data-structure such as:


typedef struct {

	int    numberOfVertices;
	vertex pointArray[numberOfVertices];
	
} polygon;

Graphics Libraries

If you build software with the above data-structures, you may use a library of routines for graphics (a graphics library) such as OpenGL to display your data on the screen in 'realtime'.
OpenGL has routines for displaying points, lines, surfaces, textures, lights and a few other things on the screen.
This is achieved by making calls to C routines which take parameters which are often just the vertices of your points, lines and surfaces.

Non-Realtime Renderers

If you don't want to display your graphics directly to the screen as you calculate it, you can write out a model file.
This is usually just a text file which contains all the geometry for the scene you are producing.
You can then render this model using a share/free-ware renderer such as POV Ray (POV-RAY is a raytracer (see below)).
The renderer can usually save the picture of your model as a bitmap image to the disk.

Animation - How does the model change over time?

A few simple transformations may be applied to the geometry of a model (such as Phillipa fish). These form the basis of much computer animation.

Translation - a movement through space. This may be specified by changes of position along each of the three Cartesian axes. For example: P(+3, +1, +1)
Rotation - re-orientation, often by some specified angle about each of the Cartesian axes in turn.

A typical rotation might look like: R(30.0, -45.0, 230.5) specifying a rotation of 30 degrees about the x axis, -45 degrees about the y axis and 230.5 degrees about the z axis.

The order of rotations is significant. Usually they are carried out first about the X,Y then Z axis.
Scaling - re-sizing by some amount either uniformly along each co-ordinate axis or non-uniformly (different size changes parallel to each axis).

A typical scaling operation might look like: (-1.0, 0.5, 1.0). (This has nothing to do with the scales on Phillipa!)

To use these operations in the production of a computer-animated sequence, the computer stores a time line of the appropriate moments to apply an operation to a model and thereby change its characteristics.

Rendering - How do the pictures get to the screen?

There are a number of ways the geometry of a model may be drawn on the screen. Some of these are based around a set of routines stored in a graphics library. Graphics libraries often include procedures for rendering lines given their endpoints and surfaces given their vertices (see above).

Here are a few ways objects may be rendered to the screen.

Bounding box - objects are rendered as cubes which surround the more detailed geometry. This method is mostly for rapid preview of models, it is not useful for much else.
Wireframe - objects are rendered as if they were built out of wire or drinking straws attached at the vertices of the geometry. This may often be performed with or without hidden-surface removal.
Flat shaded - objects are rendered with facets as if they were made out of sheets of flat card. Each card is shaded in a uniform colour depending on the relative angle of its surface normal, light source and view point.
Gouraud shaded - objects are rendered with facets which are shaded non-uniformly in such a way as to help the surface of which they are a part look smooth.
Phong shaded - as for Gouraud shading, only the algorithm for shading facets on the surface is superior and gives an excellent appearance of smoothness. No amount of shading tricks in Gouraud or Phong shading can disguise a surface's polygonal/facetted nature along its silhouette.
Raytraced - surfaces which are supposed to be smooth are rendered as perfectly smooth. Since the surfaces are not rendered as facets, silhoutte edges are perfectly smooth also. Raytracing works by bouncing rays of light from objects in the scene to the viewpoint. This can be a very time-consuming process, even for computers. Raytracing allows renderings of reflective and transparent objects to be created. The more transparent or reflective objects there are in a scene, the slower the raytracing process will be.

Here are a couple of close ups. Note the silhouette edge on Phillipa in each image.

How have these images been rendered?

Looking for further information?

What you look at will depend on your aims. If you wish to start learning about 3d animation as a computer programmer, have a look at:

Angel, Edward, Open GL: A primer , Addison Wesley, 2002
Angel, Edward, Interactive computer graphics – a top down approach with Open GL (3rd edition), Addison Wesley, 2002
Hill, F.S. Jr., Computer Graphics Using Open GL (2nd Edition), Prentice-Hall, 2001
The PDF version of the GLUT manual may also be handy.

Whatever your aims, go and visit the library. There are many books there, and they're all available to you for no cost on top of the amount the government sees fit to charge you for your education!

You can also check out the web, including another more detailed lecture: introducing animation or one of the lectures listed for students of multimedia and the WWW.