THREE-DIMENSIONAL DISPLAY METHODS

 
                                          To obtain a display of a three-dimensional scene that has been modeled in world coordinates. We must first founded a coordinate reference for the "camera". This coordinate reference defines the position and orientation for the plane to the camera film, which is that the plane want to use to display a view of the objects within the scene. Object descriptions are then transferred to the camera reference coordinates and projected onto the chosen display plane. we will then display the objects in wireframe (outline) form, or we will apply lighting and surface rendering techniques to shade the visible surfaces.

PARALLEL PROJECTIONS

                                          Parallel projection discards z-coordinate and parallel lines from every vertex on the item are extended till they see the read plane. In parallel projection, we've a bent to specify a direction of projection instead of center of projection.
In parallel projection, the space from the center of projection to project plane is infinite. during this type of projection, we've a bent to attach the projected vertices by line segments that correspond to connections on the initial object.
Parallel projections are less realistic, however they're smart for actual measurements. during this type of projections, parallel lines stay parallel and angles don't seem to be preserved. varied varieties of parallel projections are shown within the subsequent hierarchy.

PERSPECTIVE PROJECTION

                                         In perspective projection, the space from the center of projection to project plane is finite and also the dimensions of the item varies reciprocally with distance that appearance plenty of realistic.

The distance and angles don't seem to be preserved and parallel lines do not stay parallel. Instead, all of them converge at one purpose noted as center of projection or projection point of reference. There are three varieties of perspective projections that are shown within the subsequent chart.

• One purpose perspective projection is easy to draw.
• Two purpose perspective projections offers higher impression of depth.
• Three purpose perspective projections is most troublesome to draw.

DEPTH CUEING

                                         With few exceptions, depth information is very important in order that we will easily identify, for a specific viewing direction, which is that the front and which is that the back of displayed objects. There are several ways during which we will include depth information within the two-dimensional representation of solid objects. a straightforward method for indicating depth with wireframe displays is to vary the intensity of objects in step with their distance from the viewing position. The lines closest to the viewing position are displayed with the very best intensities, and contours farther away are displayed with decreasing intensities. Depth cueing is applied by choosing maximum and minimum intensity (or color) values and a variety of distances over which the intensities are to vary. Another application of depth cueing is modeling the effect of the atmosphere on the perceived intensity of objects. More distant objects appear dimmer to us than nearer objects thanks to light scattering by dust particles, haze, and smoke. Some atmospheric effects can change the perceived color of an object, and that we can model these effects with depth cueing.

VISIBLE LINE AND SURFACE IDENTIFICATION

                                        We can also clarify depth relationships in an exceedingly wireframe display by identifying visible lines in how. the best method is to focus on the visible lines or to display them in an exceedingly different color. Another technique, commonly used for engineering drawings, is to display the invisible lines as dashed lines. Another approach is to easily remove the invisible lines. But removing the hidden lines also removes information about the form of the rear surfaces of an object. These visible-line methods also identify the visible surfaces of objects. When objects are to be displayed with color or shaded surfaces, we apply surface-rendering procedures to the visible surfaces in order that the hidden surfaces are obscured. Some visible surface algorithms establish visibility pixel by pixel across the viewing plane; other algorithms determine visibility for object surfaces as a full.

SURFACE RENDERING

                                       Added realism is attained in displays by setting the surface intensity of objects in step with the lighting conditions within the scene and in step with assigned surface characteristics. Lighting specifications include the intensity and positions of sunshine sources and therefore the general background illumination required for a scene. Sur- face properties of objects include degree of transparency and the way rough or smooth the surfaces are to be. Procedures can then be applied to get the right illumination and shadow regions for the scene. Surface-rendering methods are combined with perspective and visible-surface identification to get a degree of realism in an exceedingly displayed scene.
 
EXPLODED AND CUTAWAY VIEWS

                                      Many graphics packages allow objects to be defined as hierarchical structures, in order that internal details is stored. Exploded and cutaway views of such objects can then be accustomed show the interior structure and relationship of the thing parts. Figure 9-8 shows several varieties of exploded displays for a mechanical design. another to exploding an object into its component parts is that the cut away view, which removes a part of the visible surfaces to point out internal structure.