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Realization In 3-D Graphics:

Many computer graphic application involve the display of 3-D objects and scenes. For examples CAD

systems allow the

users to manipulate models of machine components, automobile

bodies and aircraft parts. Simulation system present a

continuously moving picture of a 3-D world to the pilot of ship

or aircraft. These applications differ from 2-D applications not

only in the added dimension: they also requires concern for

realism in the display of objects.

Producing a realistic image of a 3-D scene on a 2D-scene on a

2D-display present many problem. How is depth, the 3

rd

dimension to be displayed on the scene? How are pars of the

object that are hidden by other object to be modified and

removed form the image? How is a 3-D world to be modulated

in a computer in a computer so that imagescan be generated?

There are a no. of techniques for achieving realism.

The basic problem addressed by visualization technique is

sometimes called depth cueing. When a 3-D scene is projected

onto a 2-D display scene , information about the depth of

objects in the image tends to be reduced or loosed entirely.

Techniques that provide depth cuesare designated to restore or

enhance the communication of depths to the observer. The

different technique for achieving realism are:

- Parallel projection.

- Perspective projection

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- Intensity cues.

- Stereoscopic views.

- Kinetic depth effect.

- Hidden line elimination

- Shadding with hidden surface removed

- 3-D images.

odelling 3-D scenes: The techniques used to generate

different kinds of 3-D scenes are start from a model of the scene.

The model is needed for two purpose.

1. It is used by viewing algorithm together with information

about the location of the viewer.

2. It is used to modify and analyze the objects in the scene,

activities usually considered part of the application

program.

The information in a model of a 3-D scene can be divided into

two important classes: geometry and topology. Geometry is

concerned with measurements , suchas the location of a point or

the dimension of the object. Topological information records the

structure s of a scene: how points are combined to form

polygons, how polygons form object and how object from

scenes.

3-D objects Representation:

Polygon surface:

- The most commonly used boundary representation for a 3-D graphic object is a set ofsurface polygon

that enclose

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the object interior. Many graphic system stored all object

description a sets of surface polygon. This simplifies and

speed of the surface rendering and display a object scene

all the surface describe with linear equation.

- A polygon representation of a polyhedron precisely

defines the surface of a objects.

Visible Surface detection Method:

The method that are used for identifying those of a scene

that are visible from a chosenviewing position are referred

to as visible surface detection methods.

- Sometimes these methods are also known as hidden

surface elimination method.

Classification of visible surface Detection Algorithm:

- Visible surface detection algorithms are broadly classified

according to whether they deal with object definitions

directly or with their projected images . These tow

approaches are called

- Object space Method.

- Image Space Method.

- An object space method compares objects and parts of

objects to each other within scene definition to determine

which surfaces are visible.

- An image space method visibilityis decided point by point

at each pixel position on the projection plane.

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- Most visible surface algorithmsuse image-space method

although object space method can be used effectively to

locate visible surfaces in some cases.

- Line display algorithm generally use object space method

to identify visible lines inwire-frame display.

Back Face detection:

- A fast and simple object space method for identifying the

back-faces of a poly hedron is based on the “inside-outside”

tests.