RENDERING THE MODELS-
Making It Look Good

As this site has developed, I have received questions on how to get the gem models to render with all the sparkle and flash of the real thing. This section will deal with the means necessary to adjust the models, it's materials and it's environment in order to create a gem of an image.

STEPS TO IMPROVING YOUR GEMSTONE RENDERS.
1. UNSMOOTHING THE MODEL
2. ADJUSTING THE MODEL TO FIT THE IOR
3. CREATING THE ENVIRONMENT

(more sections to come as time allows)

 

STEP 1. UNSMOOTHING THE MODEL

The first step comes when importing the model into your 3D scene. The various programs used usually have some means of 'smoothing' or 'unsmoothing' models. Smoothing means that the polygons are
visually blended during rendering to obscure the divisions between them. This is *only* a visual trick. If you can view the rendered object and compare the interior to the perimeter, you would see that the model still has those sharp edges. (Illus. 1)

Illus. 1: "Smoothing" vs. "Unsmoothing"

Now, while this may be a desired thing when rendering curves and organic objects, it is definitely *not* wanted with faceted gems--here you want each polygon (facet) to be distinct and separate. So, using whatever method provided by your program, ensure that the render process keep the facets sharp. (I will try to compile a full listing of the particular methods for each program as I gain access to them)

 

STEP 2. ADJUSTING THE MODEL TO FIT THE IOR

This is an optional step, depending on just how technically correct you want your image to be. The models provided were designed to fit the best angles for a specific IOR (I have noted the appropriate IORs next to each models' preview image). If you wish to use a different IOR, a little adjustment should be made. If you don't? The result is often what the lapidary trade calls 'fish-eye', an area of light loss or darkness usually in a circle, or 'bow-tie', an area of darkness resembling, (yup, you guessed it) a twinned triangle or bow-tie. Both are the result (in real life) of not cutting the gem to the optimal angles for the stone's IOR and/or it's desired cut. (Illus. 2)

 
"Fish-eye"
"Bow-tie"
Illus. 2

How do these dark areas occur? When improper angles are cut, light entering the gem will exit through the lower areas of the gem, and is 'lost'. (Illus. 3)

Cut is too shallow-light is lost. Cut is just right-maximum return of light. Cut is too deep-light is lost.
Illus. 3 --Lost Light

As you might guess from the preceding illustration, the solution for 3D artists to this problem is to stretch or shrink the model on the 'Y' axis (relative to the model, that is), making it 'taller' or shorter.

Illus. 4--'Y' axis adjustment.

How much is needed? Surprisingly, not a lot. The critical angles are in a small range, from about 25 degrees to about 45 degrees, with most being in the mid-30's. Not a lot of difference there. A very minute tweaking should do the trick. For us 3D folk, a quick tweak-and-test render is a lot easier than the mathematical hoops that some gemstone cutters go through to recalculate angles (tangent ratio adjustment).

For more detailed articles on these topics, Bob Keller has several clear, but detailed, explanations available at his site. While these are geared toward actual lapidary situations, they do contain information which can be applied to 3D rendering. Some of the more relevant ones are:

PROPERTIES OF COMMONLY FACETED MINERALS AND MATERIALS

REFRACTIVE INDEX AND CRITICAL ANGLE

GETTING RID OF THOSE BOW-TIE BLUES

GEMSTONE DESIGN CONVERSION USING THE TANGENT RATIO METHOD

Of course, there are those who have different opinions on the subject, as in Jeff Graham's answer to the question shows:

WHAT IS TANGENT RATIOING? WHAT IS YOUR OPINION?

 

(I hope these first two sections have been helpful. I plan to add more concerning how the 3D environment effects the rendering of your gemstone models, and how to 'fake' those optical qualities of a gemstone that aren't handled by most rendering engines. It will just take a little time to compose them, and create appropriate illustrations and diagrams, so be patient.)