The I-DEAS Display Model Mapped to the Physics - Appearance...

If you invoke the "Appearance..." command for a part or surface, the following form will appear:

You can see one of the physics terms used in the form, i. e., "Diffuse Light". The value for Diffuse Light represents the percent of light hitting an I-DEAS surface which should act in a diffuse manner as defined in the previous section.

For the specular characteristic of display, the Brightness value is used. The value represents the percent of light hitting an I-DEAS surface which should act in a specular manner as defined in the previous section.

It may not be obvious either from the above form or from the physics discussion in the previous section, but the sum of Diffuse Light and Brightness must be less than or equal to 100%. This is a very important fact that seems lost on our own defaults for these values. The above form shows the default settings as established by I-DEAS. Notice that the default values for Diffuse Light and Brightness are 85% and 80% respectively.

A review of the physics shows that there are three ways that light can act; diffuse, specular or absorbed. That means for:

Diffuse Light + Brightness = 100%

All the light which is hitting the I-DEAS surface(s) is "accounted for" and is being sent back to the outside world in either a diffuse or specular fashion.

Diffuse Light + Brightness <= 100%

All the light which is hitting the I-DEAS surface(s) is not being sent back to the outside world; some is being absorbed.

Diffuse Light + Brightness >= 100%

This defies the law of the conservation of energy. How can a surface which has no energy creating capability "emit" more light than is impinging upon it? That is impossible. What this means to I-DEAS is that the data as supplied by the user must be normalized before being used to create a display and that includes the I-DEAS defaults. So what do you really get?

Diffuse Light = 85% and Brightness = 80%

Diffuse Light (actual) = 85% / (85% + 80%) = 52%

Brightness (actual) = 80% / (85% + 80%) = 48%

Diffuse Light and Brightness represent two of the three most important values for the shaded display. The third is Glossiness. This value is a number between 0% and 100% which represents the "microscopic smoothness" of the surface. The number can be considered a probability statement, i. e., what is probability that a microfacet (as illustrated in the previously presented graphic on reflecting a photon from a microfacet) is a surface which is the same as the NURB surface? A 0% probability indicates that the surface is as rough as it can be without showing visible texture. A 100% probability indicates that the surface is as smooth as it can be, i. e., there is a 100% probability that the microfacet is the same as the NURB surface or it is as smooth as the defining NURB surface.

Illustrating "The Big Three" in the Appearance... Form

The above is all well and good, but to really understand what all this means to making a display that is desirable requires invoking the old cliché, "A picture is worth a thousand words". The following is a picture which is intended to assist in understanding the relationship between Diffuse Light, Brightness and Glossiness. It is a "cube of spheres", where each sphere has a different value for each of "the big three". The "cube of spheres" represents a three axis chart with Diffuse Light varying from 100% at the center to 0% toward the right and Brightness varying from 0% at the center to 100% toward the left. These two quantities must add up to 100% or less, so some of the portions of the cube are actually normalizing the settings, while some are using the actual settings. The demarcation between these two areas of the cube are noted. Glossiness varies from 0% at the bottom of the illustration to 100% at the top. The image was rendered using Shaded Software with Display Quality set to Fine to improve the image slightly.

The following is the same "cube of spheres" rendered using Ray Tracing with Display Quality set to Fine, Pixel Grid set to 1, and Shadows and Reflections turned off. You can see that this is even a better, more precise rendering of the spheres. (These characteristics will be discussed at a later time in this document.)

This "cube of spheres" is actually a "solid cube of spheres", that is, there are spheres inside the cube which have different combinations of Diffuse Light, Brightness and Glossiness. Somewhere inside the cube might be the settings which achieve a specific desired display, but showing all spheres inside the cube is too great a task for this document However, there is one "slice" through the "cube of spheres" which is worth looking at more closely and that is the "slice" where the spheres have a Diffuse Light and Brightness which sum to exactly 100%. That "slice" is shown in the next illustration. This illustration was created with Shaded Software with Display Quality set to Fine.

Diffuse Light varies from 100% to 0% left-to-right while Brightness varies from 0% to 100% left-to-right. This makes each sphere have a sum of 100% for Diffuse Light and Brightness. Glossiness varies from 0% to 100% from bottom-to-top. The following is the same illustration rendered using Ray Tracing, with Display Quality set to Fine, Pixel Grid set to 1, and with Shadows and Reflections turned off.

These displays should be a starting point for guessing some values for Diffuse Light, Brightness and Glossiness which will come close to the goals that the user has for a particular display.