Difference between revisions of "Documentation Talk:Reference Section 6.1"

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===Sampling Parameters & Methods===
 
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<!--<indexentry primary "intervals">--->
 
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<p>Media effects are calculated by sampling the media along the path of the
 
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ray. It uses a method called <em>Monte Carlo integration.</em> The <code>intervals</code>
 
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keyword may be used to specify the integer number of intervals used to
 
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sample the ray. The default number of intervals is 10. For object media the
 
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intervals are spread between the entry and exit points as the ray passes
 
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through the container object. For atmospheric media, the intervals spans the
 
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entire length of the ray from its start until it hits an object. For media
 
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types which interact with spotlights or cylinder lights, the intervals
 
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which are not illuminated by these light types are weighted differently
 
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than the illuminated intervals when distributing samples.</p>
 
  
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<!--<indexentry primary "ratio">--->
 
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<p>The <code>ratio</code> keyword distributes intervals differently between lit
 
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and unlit areas. The default value of <code>ratio 0.9</code> means that lit
 
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intervals get more samples than unlit intervals. Note that the total number
 
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of intervals must exceed the number of illuminated intervals. If a ray
 
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passes in and out of 8 spotlights but you have only specified 5 intervals
 
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then an error occurs.</p>
 
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<!--<indexentry primary "samples">--->
 
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<p>The <code>samples</code> <em><code>Min</code></em>,
 
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<em><code> Max</code></em> keyword specifies the minimum and maximum number
 
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of samples taken per interval. The default values are <code>samples
 
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1,1</code>. </p>
 
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<!--<indexentry "confidence, media">--->
 
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<!--<indexentry "variance, media">--->
 
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<p>As each interval is sampled, the variance is computed. If the
 
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variance is below a threshold value, then no more samples are needed. The
 
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<code>variance</code> and <code>confidence</code> keywords specify the
 
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permitted variance allowed and the confidence that you are within that
 
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variance. The exact calculations are quite complex and involve chi-squared
 
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tests and other statistical principles too messy to describe here. The
 
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default values are <code>variance 1.0/128</code> and <code>confidence
 
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0.9</code>. For slower more accurate results, decrease the variance and
 
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increase the confidence.</p>
 
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<p class="Note"><strong>Note:</strong> the maximum number of samples
 
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limits the calculations even if the proper variance and confidence are
 
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never reached.</p>
 
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<!--<indexentry primary "method">--->
 
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<p>The <code>method</code> keyword lets you specify what sampling method is used,
 
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POV-Ray provides three. <code>Method 1</code> is the method described above.</p>
 
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<p>Sample <code>method 2</code> distributes samples evenly along the viewing ray or light ray.
 
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The latter can make things look smoother sometimes. If you specify a max samples higher than
 
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the minimum samples, POV will take additional samples, but they will be random, just like in
 
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method 1. Therefore, it is suggested you set the max samples equal to the minimum samples.
 
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<code>jitter</code> will cause method 2 to look similar to method 1. It should be followed by
 
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a float, and a value of 1 will stagger the samples in the full range between samples.</p>
 
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<!--<indexentry primary "aa_level" "aa_threshold">--->
 
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<p>Sample <code>method 3</code> uses adaptive sampling (similar to adaptive anti-aliasing)
 
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which is very much like the sampling method used in POV-Ray 3.0's atmosphere. This code was
 
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written from the ground-up to work with media, however. Adaptive sampling works by taking
 
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another sample between two existing samples if there is too much variance in the original
 
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two samples. This leads to fewer samples being taken in areas where the effect from the media
 
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remains constant. The adaptive sampling is only performed if the minimum
 
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samples are set to 3 or more.</p>
 
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<p class="Note"><strong>Note:</strong> As of version 3.5 the default sampling <code>method</code> is 3. The default for <code>intervals</code> is 1 and <code>samples Min=Max</code> so a single <code>samples</code> parameter is acceptable. Sampling methods 1 and 2 have been retained for legacy purposes</p>
 
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<p>You can specify the anti-aliasing recursion depth using the <code>aa_level</code>
 
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keyword followed by an integer. You can specify the anti-aliasing threshold by using the
 
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<code>aa_threshold</code> followed by a float. The default for <code>aa_level</code> is 4
 
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and the default <code>aa_threshold</code> is 0.1. <code>jitter</code> also works with method 3.
 
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Sample method 3 ignores the maximum samples value. It is usually best to only use one interval
 
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with method 3. Too many intervals can lead to artifacts, and POV will create more intervals
 
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if it needs them.</p>
 

Revision as of 11:04, 11 April 2010

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