Reference:Sampling Parameters & Methods

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Media effects are calculated by sampling the media along the path of the ray. The method keyword specifies what sampling type is used. POV-Ray has three different types of media sampling methods.

Note: As of version 3.5 it is highly recommended to use sampling method 3. It is now the default. Sampling methods 1 and 2 have been retained for legacy purposes.

Sampling Method 3: uses adaptive sampling which is very much like the sampling method that was used with the POV-Ray 3.0 atmosphere feature. Another sample is taken between two existing, if the difference between the two end samples is greater than aa_threshold. This leads to fewer samples being taken in areas where the effect from the media remains constant. Keep in mind that true adaptive sampling only occurs when samples is set to 3 or more.

Note: Exclusively with method 3, when using two values for samples, the second parameter is silently ignored.

Implementation of the intervals keyword is also different with this method. Consequently it is usually best to use only one interval, since more intervals will dramatically increase the render time without giving any appreciable improvement. In other words: samples 10 and intervals 3 will render slower than samples 50 and intervals 1.

Additionally these parameters are available as follows:

  • The anti-aliasing recursion depth is set by using the aa_level keyword followed by an integer
  • The anti-aliasing threshold is set by using aa_threshold followed by a float.
  • The jitter keyword followed by a float enables / sets that feature.

Note: The keywords: confidence, variance and ratio are used only with methods 1 and 2.

See also: Media for default values.

Sampling Method 1: uses the intervals keyword to specify the integer number of intervals used to sample the ray. 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. The value for Max may be omitted, in which case the range Min = Max will be used.

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.

See also: Media for default values.

Sampling Method 2: distributes samples evenly along the viewing ray or light ray. The latter can make things look smoother sometimes. If you specify a maximum number of samples higher than the minimum number of 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.

Note: When using multiple medias, it's strongly recommend all use the same sampling method. With the current code there is the possibility of some keyword setting bleed between methods.

See also: Media for default values.