Ray Trace Case
The same stones the Simulator shows, put on a bench. Every cut here is ray traced under four light models and reported as face-up brightness, for the whole stone and for the table alone, then swept through tilt. The geometry is the geometry: these twenty-one cuts are the ones whose rendered solid reconciles with the .asc a cutter would actually work from.
All four windows are the same ray trace read four ways. Light Return sorts the source direction into zones and colours them; ISO, COS and SC2 weight that same direction by how bright the model's dome is there, so white is a facet returning light from the brightest part of that dome and black is a facet returning nothing. None of the four depends on the scene lighting.
What is being counted
Brightness here is energy, not just direction. At the crown a Fresnel share of each ray reflects straight off without entering the stone, and that light still reaches the eye: face-up, the table's own reflection points at the zenith and lands in the head shadow, which is the dip you see near zero tilt. The rest goes through and pays Fresnel again on the way out. Internal bounces are left alone, because they are overwhelmingly total internal reflection, which loses nothing.
That choice is not cosmetic. Measured across this roster, leaving the energy out and counting direction alone reorders 8 of the 21 cuts on ISO, 7 on COS and 10 on SC2, while adding the exit face to the entry face reorders 0, 4 and 5. The model settles; counting direction alone is the outlier. What is still missing is the energy lost at partial internal reflections, which is why these figures sit above a full ray tracer's and why the dip near zero tilt is shallower here than in GemRay.
Face-up brightness
Per cent of the dome's available light returned into the viewing cone, averaged over the stone's face-up image. Whole stone is the full outline; table is the area inside the table facet alone, which is the figure that moves most between a good cut and a poor one. Leakage sits below the rule because it is not a light model: it counts the rays that came back with nothing at all, so it does not move when you change the head shadow.
| Light model | Whole stone | Table only | vs T57 |
|---|---|---|---|
| Measuring… | |||
| Light model | Whole | Table | Whole | Table | Difference |
|---|
Brightness against tilt
The stone tilts under a fixed dome, which is what happens on a hand. Solid lines are the whole stone, dashed are the table alone. A cut that holds its height across the sweep keeps working when it is not being looked at straight on.
What the four models are
Where the light comes back from. Red is the bright band 45 to 75 degrees up, green is low-angle light near the horizon, blue is the zone your own head blocks, black is a leak.
A dome lit evenly from every direction, like a ceiling of light panels. The most forgiving test: it asks only how much light returns, not where it came from.
Bright overhead, fading to black at the horizon. Hardest on shallow cuts, because it starves exactly the low angles they lean on.
A ring of light peaking at 45 degrees, dark overhead and dark at the horizon. Closest to how a stone is actually lit on a hand.