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Media effects are calculated by sampling the media along the path of the
ray. It uses a method called Monte Carlo integration. The intervals
keyword may be used to specify the integer number of intervals used to
sample the ray. The default number of intervals is 10. For object media the
intervals are spread between the entry and exit points as the ray passes
through the container object. For atmospheric media, the intervals spans the
entire length of the ray from its start until it hits an object. For media
types which interact with spotlights or cylinder lights, the intervals
which are not illuminated by these light types are weighted differently
than the illuminated intervals when distributing samples.
The ratio keyword distributes intervals differently between lit
and unlit areas. The default value of ratio 0.9 means that lit
intervals get more samples than unlit intervals. Note that the total number
of intervals must exceed the number of illuminated intervals. If a ray
passes in and out of 8 spotlights but you've only specified 5 intervals
then an error occurs.
The samples Min,
Max keyword specifies the minimum and maximum number
of samples taken per interval. The default values are samples
1,1.
As each interval is sampled, the variance is computed. If the
variance is below a threshold value, then no more samples are needed. The
variance and confidence keywords specify the
permitted variance allowed and the confidence that you are within that
variance. The exact calculations are quite complex and involve chi-squared
tests and other statistical principles too messy to describe here. The
default values are variance 1.0/128 and confidence
0.9. For slower more accurate results, decrease the variance and
increase the confidence.
Note: the maximum number of samples limits the calculations even if the proper variance and confidence are never reached.
The method keyword lets you specify what sampling method is used,
POV-Ray provides three. Method 1 is the method described above.
Sample method 2 distributes samples evenly along the viewing ray or light ray.
The latter can make things look smoother sometimes. If you specify a max samples higher than
the minimum samples, POV will take additional samples, but they will be random, just like in
method 1. Therefore, it is suggested you set the max samples equal to the minimum samples.
jitter will cause method 2 to look similar to method 1. It should be followed by
a float, and a value of 1 will stagger the samples in the full range between samples.
Sample method 3 uses adaptive sampling (similar to adaptive anti-aliasing)
which is very much like the sampling method used in POV-Ray 3.0's atmosphere. This code was
written from the ground-up to work with media, however. Adaptive sampling works by taking
another sample between two existing samples if there is too much variance in the original
two samples. This leads to fewer samples being taken in areas where the effect from the media
remains constant. The adaptive sampling is only performed if the minimum
samples are set to 3 or more.
aa_level
keyword followed by an integer. You can specify the anti-aliasing threshold by using the
aa_threshold followed by a float. The default for aa_level is 4
and the default aa_threshold is 0.1. jitter also works with method 3.
Sample method 3 ignores the maximum samples value. It's usually best to only use one interval
with method 3. Too many intervals can lead to artefacts, and POV will create more intervals
if it needs them.
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