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Lighting, Light Sources
There are three light sources predefined for the user to use. The are Observer, Right_Shoulder and Left_Shoulder. They all use a similar kind of light source, i.e., a light source which is an "infinite" distance away from the parts. This effectively means that the light rays are parallel. The different light sources look as follows on our "standard" scene with Left_Shoulder first, Observer second and Right_Shoulder third:
In addition to these standard light sources, the user can create a light source of his/her own design. The general types of light sources available for creation are Point, Spotlight, Floodlight and Sunlight. A summary of the characteristics which can be set for each light source is as follows:
Light Source Types
All light sources follow the inverse-square law of physics, i. e., for every doubling of the distance from the light source, the intensity of the light is one-fourth.
Point Light Source
A Point light source is one that emits light in all directions. It is a point source of light which has no "lamp shade" to restrict the direction in which the light can travel. In the following picture, the Point light source is very close to the corner of the front-most block on the bottom.
Spotlight
A Spotlight is one which does not emit light in all directions. It has a cone-shaped lampshade on it which allows light to be emitted outward in a cone shape. Within the cone at any distance from the light source, the light intensity is constant. The illustrations below shows some Spotlights (the position of which is illustrated with a small white sphere), specifically one with a 20 degree Cone Angle and another with a 60 degree Cone Angle. The angle can be thought of as the total angle at the vertex of the cone from conical surface silhouette to conical surface silhouette. In previous versions of I-DEAS before Master Series, this angle was measured from the center axis of the cylinder to the silhouette of the conical surface. That meant that the range of allowed Cone Angles, which is from 0 degrees to 180 degrees represented a Spotlight being able to achieve the status of Point light with no "lampshade" on it. The redefinition of the meaning of Cone Angle in Master Series indicates that a Spotlight can only emit light up to a hemispherical leve.
The pictures which follow were made with Diffuse = 100% and Brightness = 0%. This was done to avoid a specularity which might hide the characteristic "constant light intensity" within the cone of light. The following displays show a golden-orange block with a small white sphere in front of it. The small white sphere is just to illustrate where the Spotlight is. The three displays represent Cone Angles of 20 degrees, 60 degrees and 120 degrees rendered in Shaded Software.
These pictures seem to lie about everything that was stated above. Now look at this same light with this same golden-orange block with the same three Spotlights, but rendered in Ray Tracing.
Now it is easy to see the very Spotlight-like behavior which was described above. Note that the reason for the "fall off" of the light intensity toward the edges of the block is due to the inverse-square rule, i. e., the light has traveled further to the edges of the block so its intensity is less, but at a give distance from the light source within the Cone Angle, the light intensity is constant.
The reason that Shaded Software does not seem to show the cone-like light behavior is because of the method that is used in Shaded Software to calculate the shaded image. For the block front face, there is only one display polygon (no further tessellation of the surface is required since it is a flat surface). Shaded Software only computes the display model at the vertices of each display polygon and then interpolates that computation to the interior of the display polygon. Since the 20 degree Cone Angle and the 60 degree Cone Angle do not cause light to strike the vertices of the display polygon, the entire polygon is interpolated to have no light from the light-source hitting it. Thus, only Ambient light is making any display of the block. When the Cone Angle reaches 120 degrees, light is finally hitting the vertices of the display polygon. Since each vertex is equidistant from the light source, the same intensity is being "seen" and therefore a constant level of golden-orange is being rendered over the face of the block.
Ray Tracing causes the light model to be calculated for every pixel on the screen, not just at the vertices of the display polygon, thus creating a more accurate rendering of how a Spotlight really acts. This should be a strong statement for using Ray Tracing for all important displays, even if Shadows and Reflections are not desired. More about this topic will be discussed in the section on Shaded Options where Display Quality is discussed.
This can be further reinforced by replacing the block in the above scenes with a sphere. Because the sphere is a curved surface, more tessellation must be accomplished (more display polygons are created) in order to represent the curved surface. This means there are more display polygon vertices at which to compute the display model and more accurate rendering of the Spotlight occurs.
Compare these to the Ray Tracing rendering of this same sphere with the three light sources.
Floodlight
The definition of the Floodlight is exactly the same as the Spotlight except for the fact that the light intensity within the cone of light varies from the brightest along the vector that the light is aimed to the dimmest at the edge of the cone. All other issues are the same. For that reason, all the displays describing Spotlight behavior will not be duplicated here; just Ray Tracing rendering will he presented. The following pictures illustrate two Floodlights, one with a 20 degree cone angle and one with a 60 degree cone angle. Note the differences in light intensity within the lighted circle from the illustrations of the equivalent Spotlights.
Sunlight
Sunlight is always at "infinity" and the user just specifies the direction along which the Sunlight is shining. The following is Sunlight acting along the same vector that is illustrated in the Spotlight and Floodlight examples.
Mixing Them Up
The following is a couple of illustrations showing how light sources can be defined to create many effects:
Other Light Source Characteristics
From the table it is apparent that there are various characteristics of a light source which can be defined by the user. For instance, the color of the light source can be set as in the following example where light sources of colors cyan, magenta and green are being used:
The intensity of the light sources can be set:
For Spotlights and Floodlights, the cone angle can be set:
Their positions can be set:
The direction that they are aimed can be set:
Finally, whether they are set relative to model space (so they don't "move" relative to the model with changes in view; this is similar to lights in a photographic studio):
or they are set relative to the screen (so they do "move" relative to the model with changes in view; this is similar to a flash on a camera) can also be defined.
