2.6.2.2 Sampling Parameters & Methods
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 have 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
chisquared 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, POVRay 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 antialiasing) which is very much like the
sampling method used in POVRay 3.0's atmosphere. This code was written from the groundup 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.
You can specify the antialiasing recursion depth using the aa_level keyword followed by an integer.
You can specify the antialiasing 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 is 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.
