> I did a brilliant round cut a few years ago, but was never satisfied 
> with the results.  They just never had the level of sparkle that I see 
> in real diamonds.  Looking at the light traces, I suspect that my 
> parameters may be suboptimal.  (I got the parameters from a Web site 
> that appears to have passed on.)
> 
> I also suspect a bug (feature?) in dispersion: renders seem to be darker 
> than without dispersion.  It just occurred to me that the multiple 
> reflections and refractions in a cut gem could possibly exacerbate the 
> problem.
> 
> I know Chris B. will show up to buttonhole us any second now, so I'll 
> just say that I've been shy about submitting this, because of my 
> dissatisfaction of the sparkle, but also because I fear that, with just 
> the one cut, the module lacks generality.
> 
> 

Just a few remarks:
You may need a stronger dispersion, and probably more dispersion_samples.
Your specular highlights look like they are not sharp enough.
If using phong, try to multiply phong_size by around 100.
If using specular, try dividing roughness by around 100.
If you use a look_like for your light_source, try yo mage it smaller, or 
put the light farther away.

Your transmit or filter value may be set to low, make it much closer to 
1. Filter + transmit should add to 1 or almost 1.

Add some variable reflection and don't forget fresnel and conserve_energy.

When you add dispersion, the intensity get spread out. So, it's only 
normal that the result looks darker. A solution is to mage the 
light_source slightly stronger.

Adding photons can also add a great deal in realism.


Alain

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