Power Tip

From CADD Master Ray

Shedding Some Light

on the Light Source
Probably the most exciting addition to Generic's new release of Generic 3D (version 1.1) is light source shading. Part of the new Rendering function, this feature gives you the ability to color the surfaces of your model with light and dark shades, based on the location of a user-positioned light source.

Placing the light, however, requires a little experience and a lot of experimentation. Similar to selecting a point of view for perspective viewing, the light source is specified by locating two points. The first is a point toward which the light is directed, while the second is the location of the light source. Because most of us have experience looking at real objects, this same process is somewhat intuitive for specifying a point of view. However, not too many of us have been a light source before, and really don't know how to behave like one.

In trying to understand exactly how shading can be controlled by the location of the light source, I came up with an alternative paradigm for specifying the light source which employs amount of light rather than location.

If you draw a simple unrotated cube, you will see that the three visible surfaces each face toward one of the positive axes. In isometric view mode, the surface on the right faces positive X, the surface on the top faces positive Y, and the left surface faces positive Z. I will call these the X, Y, and Z surfaces.

When you place the light source using the SS (Set Source) command, make sure that you have Manual entry Relative turned on by typing MR. When asked for the point that the light is pointing to, type 0,0,0 (actually, any point will do). Get ready, my tip is next.

Instead of thinking of the next point (the light source) as a location in space, treat it as the relative amount of light that you want to shine on each surface of a simple cube. To keep it simple, I like to imagine that I have about 6 units of light to distribute among the three surfaces. For example, to put the most light on the X surface and the least on the Z surface, while illuminating all surfaces to some extent, I specify 3,2,1.

With this system I can also use a zero if I want NO light to fall on any surface. For example, the specification 4,2,0 will put twice as much light on the X surface as on the Y surface, and none at all on the Z surface. If you specify 6,0,0, ALL of the light will be on the X surface, and the others will be dark.

In addition to simplicity, the use of about 6 "lighting units" reminds me that there is only so much light to go around, anyway. You cannot make all surfaces bright by specifying a large number for each. Both 100,100,100 and 1,1,1 will produce the same basic unshaded cubes, as all surfaces are receiving an equal amount of light. The amount of "fine tuning" that you can do by specifying values like 7,15,19 is limited unless you have 256 colors available. For most of us, this would be exactly the same as specifying 1,2,3.