Am 11.11.2010 16:10, schrieb Tek:

> After some testing I'm getting the impression that the bokeh filter isn't
> strictly applied. I'm getting square shapes when using a bokeh which is mostly
> black, as if the algorithm gives up after a few tries and just uses the sample
> anyway. Indeed, using a completely black bokeh filter doesn't cause any error
> and I still have blur.

This is indeed the case; the design of the algorithm was subject to the 
following constraints:

* allow the use of an arbitrary user-specified bokeh (as opposed to a 
set of hard-coded bokehs)

* allow the use of POV-Ray's pattern mechanism (as opposed to requiring 
an image file)

* produce the same output image brightness regardless of the bokeh 
pattern's average brightness (for instance a circular black-and-white 
"mask" would typically have an overall brightness of pi/4, while a 
regular hexagonal one might have an overall brightness of 3*sqrt(3)/8)

* use importance sampling to get more quality out of a given number of 
samples

All in all this calls for a monte-carlo approach at choosing sampling 
points, with some variation to make it robust against pathological cases 
- not only the (impossible-to-detect) case of an all-black bokeh 
pattern, but also e.g. an almost-all-black pattern where the combined 
choice of sample-candidate stream (in this case 2D Halton) and monte 
carlo pseudo-RNG happens to never find a suitable sample.

The current solution to the robustness issue is to monitor how well the 
algorithm is getting along at choosing samples, and adaptively brighten 
up the pattern a bit.


> This is an understandable limitation of the bokeh filter implementation, but it
> limits my ability to use it to randomize blur samples.

Not really: Any pattern utilizing high-frequency random noise with an 
overall brightness around 0.5 (in the regions that should be part of the 
bokeh) should do the trick; for instance, randomly-chosen slices of a 
scaled-down "bumps" pattern should do (modulated of course by the 
pattern you really want to achieve).

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