A test scene for refraction and dispersion, run on genuine POV-Ray 
3.7.0.beta.34.

 From left to right:

(1) no refraction

(2) refraction at a single interface between air (camera side) and a 
medium with ior of 1.5 (scale/background side), tilted by 30 degrees

(3) ditto, but with dispersion (1.013, 20 samples)

(4) ditto, but with stronger dispersion (1.027) and only 2 samples

(5)-(8) ditto, but with 3 to 6 samples respectively

Common setup for each section is as follows:

- A fixed self-illuminated white scale mounted behind the refracting 
interface, with 1-degrees divisions (numbered) and 0.5- and 0.1-degrees 
subdivisions.

- A diffusely reflecting white background screen.

- Two self-illuminated white indicator shapes, one mounted in fixed 
position in front of the refracting interface (left triangular shape), 
another mounted at the scale (right triangular shape) at a precomupted 
position so that it should line up with the other indicator.

- A spotlight source close to the camera, with photons enabled, which 
should be visible as a blurred grey disk around the indicators' position.


Things to note in this render (which took 74 seconds on a Core i7 machine):

- The shortest-wavelength (extreme violet) sample is invariably 
invisible, and the longest-wavelength (extreme red) almost so as well.

- Extremely low sample counts lead to wrong hues.

- The perceived "center" of the reflected image is slightly off when 
using dispersion.

Conclusion: POV-Ray 3.7.0.beta.34 gets dispersion wrong. (Note however 
that the same scene, rendered with POV-Ray 3.6, gives virtually 
identical results, so the problem has been around a while.)


(Aside from the problems demonstrated here, POV-Ray 3.7.0.beta.34 also 
has additional problems when more than one interface is involved - such 
as with prisms - which POV-Ray 3.6 does not have. I'll demonstrate those 
in a follow-up.)

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