Animated Three-D Graphic Scene

by D.W. Hyatt

Using Basic GLUT Solids

Although in basic OpenGL all 3D objects are created from simple polygon regions, there are a number of basic solid types provided by the GLUT Library. Among them are:

glutSolidSphere()
glutSolidCube()
glutSolidCone()
glutSolidTorus()
glutSolidTeapot()

These are actually complex sets of commands that include the generation of the triangles that form the surface of the objects as well as the calculation of normal vectors that are used for lighting. For our first program involving 3D visualization, we will use these basic objects. Use your textbook on OpenGL to refer to the required parameters for these basic objects.

Using Transformations

There are three main transormations that we will use to modify the objects in our scene. Usually, these transformations are used in conjunction with glPushMatrix() and glPopMatrix() in order to isolate complex movements. The basic transformation functions are:

glRotatef(degrees, x-axis, y-axis, z-axis)
This command will rotate the point of view a certain number of degrees around the specified axis. Usually a 1.0 is placed for the axis around which the programmer wishes to rotate, with values of 0.0 for the other axes.
glTranslatef(x-distance, y-distance, z-distance)
This moves the point of view the proper distance in the specified directions. A value of 0.0 is used for any dimensions where no movement along the axis is desired, yet any appropriate floating point value can be used for the other dimensions.
glScalef(x-amount, y-amount, z-amount)
This enlarges (or shrinks) the specified dimension. To keep one of the original dimensions the same, it is necessary to make the parameter a value of 1.0, and then values above 1.0 will elongate, whereas numbers less than 1.0 will shrink the dimension.

Animation and Double Buffering

In order to create an animation, the objects in a scene must be erased and then redrawn. If were not for some advanced techniques, the animation would seem very choppy since as soon as one frame in the animation is drawn, the entire scene must be erased and then redrawn with the objects in the next position. This creates a very choppy animation because most of the scene is incomplete as each frame is displayed.

The technique known as Double Buffering allows for a smoother animation because two areas of display memory are used instead of one. As the viewer is observing one frame of the animation, the other display memory area that is not visable is used to create the next frame. Any images on that frame are erased, and the objects are redrawn. As soon as the frame is complete, the command glutSwapBuffers() is called and the new area of memory becomes the visable screen.

Properties of Light

In the real world, there are not really three types of light to consider, but in order to generate a realistic scene in computer graphics, people usually play around with three separate terms, ambient, diffuse, and specular light. By adjusting these three values, various effects can be achieved. Each of these light properties are expressed as four-component vectors in the form RGBA which means that the first three components control the amount of Red, Green, and Blue light coming from the source. The fourth or Alpha component is used for deciding how multiple light sources interract, and is typically given a value of 1.0 in simple models.

Ambient Light - GL_AMBIENT
Ambient light is the general light in the room that seems to come from nowhere. It is actually the light that bounces off the various surfaces in the room such that even though the back of an object may not be in direct line of the light source, it is not entirely black but still reflects some light.
Diffuse Light - GL_DIFFUSE
Diffuse light is the general directional light provided by such a light source as the flourescent lights in a room. It comes from one direction, and will cast shadows, but it does not reflect sharply. If a surface is perpendicular to the light source it will be brightly lit, whereas if the light hits the surface at an angle it will be less strongly lit.
Specular Light - GL_SPECULAR
Specular light refers to light that comes from a specific direction but reflects well in another direction. A flashlight beam that reflects off of a shiny surface is an example of specular light. A glass bottle with strong highlights would be highly specular.

Normal Vectors

In order to determine how much light will be reflected from objects in a scene, the objects are divided up into small polygon regions and the normal vector, a directional vector perpendicular to the surface, is calculated. The angle between the normal vector and the light source is calculated for each polygon, and then degree of lighting is determined by the rendering machine. If the light is directly overhead and the normal points in the direction of the light, the full value is applied. If the light is at an angle to the normal, less lighting is rendered. If the light is at a 90 degree angle or the vector points away from the light source, this polygon will be in the shadows and will thus be drawn with a darker color. We will discuss normal vectors in more detail later, but if you want to look ahead, the web page on 3-D Lighting and Surface Normals.

Fortunately, the predefined objects used in this first 3D program already have their normal vectors calculated, so there is no need to take this into consideration. We will calculate our own normals in the next program when we generate a 3D surface.

Material Properties

Materials need to have their properties defined much as the light sources do. These properties can be defined for each face and may be applied or removed from specific objects drawn in the scene through the use of glPushMatrix() and glPopMatrix() functions. In this example, the properies are defined at the beginning and apply to all objects in the scene. For most objects the ambient and diffuse parameters would have the same values, but the specular property could be adjusted to make objects look shiny with strong reflections.

The Source Code

Take a look at the source code: scene.c

Your Assignment

Your first assignment in this unit will be to create a simple scene of 3D objects, and then use mouse or keyboard routines to navigate through it in some way. Be creative, be original. Be sure you include your own comments in the source code of your program to explain different portions of your design. Everyone's program in this class should be diferent.