This tutorial guides you through the PlayPen, an interactive tool which lets you explore how the various aspects of a gemstone design affect its performance. The PlayPen provides a user-friendly interface to Gemray. You do not need to know or understand GemRay, GemCad or Optimization to operate and benefit from the PlayPen. For many users, it may be the first and only part of BOG that they use.
The PlayPen lets you see raytraces of a gemstone while interactively:
- adjusting pavillion and crown angles
- varying refractive index and dispersion
- changing tilt angle and direction
The PlayPen also lets you store the current results for direct comparison with a different combination of angles, refractive indices, gem materials, etc., or even with a different file.
The PlayPen has three modes of raytracing: Normal, Tilt Performance, and Colour Raytrace . Normal Mode produces a "standard" raytrace of the gem for the current set of parameters. In Tilt Performance Mode, the PlayPen will evaluate and plot the brightness of the gem over a range of tilt angles. Finally, the Colour Raytrace mode uses the gem material colour and the dispersion to produce a full-colour facsimile of the gem's appearance.
|Version 1.2 offers two types of Tilt Performance evaluation. The orginal BOG criterion measures the variation of brightness with tilt (less variation is better - you don't want the stone to darken when tilted). The USFG Tilt Performance option evalutates the sum of ISO brightness over a range of tilt angles. Needless to say, there has been considerable discussion about the best way to measure tilt performance, but the difference is not terribly important. Designs tend to do well in both or poorly in both.|
This tutorial assumes that you have just completed the Quick Start tutorial and are still running the BOG program. Don't worry if this is not the case. Just read and follow the instructions in the box below.
If you aren't continuing from the Quick Start tutorial, please read and follow the instructions in this box.
1. Click Select .GEM File in the main BOG window. Choose the file called BOGTUT.GEM and click Extract Angles. Choose the 41 degree pavillion angle and the 28 degree crown angle from the lists then click Ok.
2. Then click the Modify Values button in the panel labelled "Set Optimization Angles and Gem Material" panel. Enter the angles as shown below.
3. Under "Specify Refractive Index," choose the Select Gem Material option and choose Tourmaline from the scrolling list. You have now told BOG which file, angles, and gem material to use in the PlayPen. Click Ok to return to the main window
The Story So Far...
From the main BOG window, click the PlayPen button to begin. The PlayPen window should appear. Take a moment to familiarize yourself with the layout of the various sub-panels.
The PlayPen window is dominated by six image boxes, which will contain the raytraced output of Gemray. From left to right, these will be the Random, Cosine, and ISO raytraces. The random raytrace shows the gem design under random lighting conditions and most closely approximates the real-world appearance of the stone. The cosine raytrace shows the gem's response to lighting conditions that are brightest directly above the stone, and the ISO image portrays the stone under uniform lighting. The gemray manual contains a complete description of the various lighting models, and further information appears here. The upper three boxes, labelled Cur, refer to the current gem configuration. The lower three boxes, labelled Ref will contain the stored raytraces of the Reference design (more on using the Reference later in this tutorial).
To the right of the raytrace images lies the Tilt Performance panel. This will contain graphics and text clarifying the brightness variation of the gem with tilt angle (more on this later in the tutorial).
The Gem Parameters panel is directly below the raytrace windows. This panel contains six boxes allowing interactive entry of the current gem configuration. If you just completed the QuickStart tutorial, the optimized angles should already be entered for you. If not, please type in the values shown above. Note that each text box has a rectangle directly below it. Like the raytrace windows, these lower boxes are used to store values for the Reference gem. The small Gem Materials button (with the list symbol, next to the Refractive Index entries) allows you to select gem materials (i.e. refractive index and dispersion) and stone colour from a window.
To the right of the Gem Parameters panel is a small panel listing the current File being examined and, if appropriate, the filename of the Reference gem. The small button with a folder icon lets you load a different file for comparison. Note, however, that changing the file here does not change the file being optimized (the one shown in the main BOG window).
Below the file panel is another small panel which shows the ISO and COS Brightness of the Current and Reference gems.
|In addition to Cosine and ISO brightness, GemRayX reports the Dispersion brightness of the gem. The bottom box will contain this value or "N/A" (not applicable) if you are using GemRay. You can learn more about the various brightness measures here.|
The Raytrace Options panel shows the current mode. We will try all three modes during the course of this tutorial.
The main command buttons are located below the Raytrace Options panel. Raytrace initiates the Gemray calculations for the current gem parameters and mode. Cancel aborts the PlayPen and returns you to the main BOG window. The Save Raytraces as BMP button creates Windows Bitmap (.BMP) files of the current gem raytraces. You can use standard image editing programs to view and manipulate these images, and they can be included in other documents such as Microsoft Word files and web pages. The Make Ref button stores the current parameters and output for comparison. Finally, the Cur<=>Ref button allows you to rapidly exchange the current and reference configurations. This can be a big help when trying to understand the effects of angle and gem material changes.
|The PlayPen command buttons are slightly re-arranged in version 1.2 of BOG. More importantly, v. 1.2 adds the Cur<=>Ref button, which streamlines your work with this tool.|
Click the Raytrace button. The DOS window should show Gemray busily calculating, and soon, three images of the raytraced gem should appear. Note also that the PlayPen reports the ISO and COS values in the Brightness panel (and the dispersion brightness, if you are using GemRayX).
Store the current gem as the Reference. Click the Make Ref button. Notice that identical copies of all the relevant images and values have been copied to the Reference boxes below each item. Reference values appear in blue text for easier identification.
Try tilting the stone. Enter 30 degrees in the Tilt Angle box and 1 in the Tilt Direction box. Click Raytrace again. Now, the upper images show the gem tilted 30 degrees toward the 12 o'clock direction. Note the reduced ISO and COS values in the Brightness panel.
Let's try the design in diamond! Enter the refractive index for diamond (2.42) into the Gem Parameters panel (ignore the dispersion for now). Alternatively, click the Gem Material button (next to the Refractive Index box) and select Diamond from the list of materials. Click Raytrace again. Because of added internal reflections and trapped rays, the calculations will probably take somewhat longer. Even without optimization, the higher refractive index of diamond gives it excellent tilt performance. Too bad that it is pretty expensive and hard to polish. Re-enter the refractive index and dispersion values for tourmaline (1.63, 0.017) before proceeding (you can also grab these values by clicking on the small gem material button on the lower left).
Our optimized gem has excellent face-up brightness, but relatively poor performance when tilted 30 degrees - the ISO brightness drops from 96% to 56%. Select the BOG Tilt Performance option in the Raytrace Options panel. The Tilt Angle and Direction should still be 30 degrees in direction 1. Once again, click Raytrace and watch what happens. The program raytraces the gem for three orientations: zero, fifteen, and thirty degrees in the 12 o'clock direction. In general, this will always be inclinations of zero, half the Tilt Angle , and the full Tilt Angle. In the Tilt Performance panel, a plot appears showing the ISO brightness at each angle. Click the Change Plot Limits button if you want to zoom in on the trace.
Below the plot, the program reports the average and variation of the ISO brightness. The Avg ISO value is simply the numerical average of the ISO brightness at the three angles. The Var number is the average of the difference between each brightness value and the Avg ISO value. A gem showing no variation in brightness with tilt will have Var=0. In this example, we see that the variation of brightness with tilt is almost 14% on average. More information on tilt calculations appears here.
Have we really improved the gem? Recall that running the optimizer based on ISO brightness alone changed the crown angle from approximately 28 to 19 degrees and improved the ISO value by 2% - but what did this do to the tilt performance? We can easily compare our optimized gem with the original. Click Make Ref to store the tilt performance evaluation of the optimized gem. Then enter the original pavillion and crown angles in the Gem Parameters panel. You can drag the PlayPen window around to reveal the original values (41 and 28) in the main BOG window, but don't Cancel at this point. Click Raytrace again to evaluate the original design. The plot window should now contain two traces: a black one associated with the current gem (i.e. the original design), and a blue one associated with the Reference (i.e. the "optimized" gem).
Note that the original design with the higher crown has better tilt performance. Not only is the average ISO brightness somewhat higher, but also the variation of brightness with tilt angle is less.
Clearly, there is more to optimizing a gem design than simply maximizing face-up ISO brightness! In fact, experience has shown that tilt performance is at least equally important in determining the impact of a gemstone design.
In the next part of the tutorial (follow this link), we will explicitly include tilt performance as an optimization target.
|USFG Tilt Performance|
In 2004, the United States Faceters Guild held an online contest for gem design. The organizers established a criterion for judging submitted designs based on the ISO brightness of the stone over a range of tilt angles. More information about the contest appears here. (CHECK LINK) (and congratulations to GemCAD and GemRay author Robert Strickland for his victory in the competition!)
At the request of participants in the contest, the USFG Tilt Performance criterion was added to BOG in a special release of the program. Version 1.2 continues this support. The screenshot below shows the PlayPen output for this mode. Note that the Tilt Performance panel has been replaced by a text listing of the relevant ISO brightnesses and their sum.
Before leaving the PlayPen, you should try the the Colour Raytrace mode. Although less "scientific" than the rest of the PlayPen and BOG, this option can be fun, and it can give at least an approximate impression of the final appearance of a stone in real life.
Select Colour Raytrace in the Raytrace Options panel then click the Raytrace button. Notice that the program calls Gemray three times, and that the images change slightly each time. BOG uses the Dispersion value to calculate and tell Gemray the exact refractive index of the material for red, green, and blue light. It then combines the three resulting Random raytraces into a 24 bit image, which it presents against a black background in the upper left of the window. If you look carefully at this image, you should see some flashes of colour corresponding to regions in the stone where dispersion or "fire" is occurring.
Back to diamonds! Click the Make Ref button then enter the Refractive Index (2.42) and Dispersion (0.044) for diamond in the relevant boxes (or use the Gem Materials button). Click Raytrace to see another reason why diamonds are so costly. For fun, try the Dispersion of Psychadelium (0.5).
The Colour Raytrace option can also simulate the body colour of a gemstone. The body colour was set in the "Specify Angles and Materials" window accessible from the main BOG window. Alternatively click the Gem Material button (next to the Refractive Index value) to bring up the Gem Material selection window. Click on Select Colour to see a Windows-standard colour picker. Choose an appropriate colour then click Ok. Back in the PlayPen, click Raytrace to see the result. Note that the colour image includes both body colour and fire.
Please note that this is only an approximation of how light and colour play in a real stone, but it can be fun and useful:
BOG PlayPen rendering of the "Trouble"gem design (and BOG's icon!)
A 15 mm, 8 carat Amethyst cut in the "Trouble" design.
We have learned that additional factors besides face-up ISO brightness can affect the performance of a gemstone design. Follow this link to the next part of the BOG tutorial, in which we explicitly include tilt performance in the optimization procedure.
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Documentation maintained by Tom Herbst. Last modified 10-Aug-2005