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

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===Sampling Parameters & Methods===
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Regarding the note at the end of http://wiki.povray.org/content/Documentation:Reference_Section_6#Media
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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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The casting of a shadow depend on the kind of media concerned.
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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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Absorbing media always cast a shadow.
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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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Scattering media also cast a shadow unless it's extinction is set to zero.
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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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Emissive media never cast shadow.
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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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I would suggest reordering the 3 sampling methods descriptions.
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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>You can specify the anti-aliasing recursion depth using the <code>aa_level</code>
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Start with method 3.
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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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Then, put the warning followed by methods 1 and 2.
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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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* the above issues have been addressed --[[User:Jholsenback|jholsenback]] 23:50, 11 April 2010 (UTC)
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with method 3. Too many intervals can lead to artefacts, and POV will create more intervals
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if it needs them.</p>
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A few minor suggestions:
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Section 6
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Transformations apply only the density statements
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->
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Transformations apply only to the density statements
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There should be a consistent capitalization for
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the first word in a "Note"
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Section 6.1
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The emission keyword specifies a color of ...
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->
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The emission keyword specifies the color of ...
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Although we say they "emit" light, this only means that they are visible
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without any illumination shining on them. They do not really emit light that
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is cast on to nearby objects.  ->   Particles which emit light are visible without
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requiring additional
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illumination. However, they will only illuminate other objects if
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radiosity is used with media on.
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a scattering media -> scattering media
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, additionally samples Min=Max so a single samples parameter is acceptable.
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->
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This doesn't really seem to belong in the box. Rather, when describing
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samples Min, Max below, add "The value for Max may be omitted, in which
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case the range Min = Max will be used".
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a max samples higher than the minimum samples
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->
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a maximum number of samples higher than the minimum number
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of samples
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Generally, commas are set a bit sparsely. Here are some which
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should probably be there for grammatical correctness:
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After "for example" (2x)
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After "However" (3x at beginning of sentence)
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Before "i.e."
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After "For object media"
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After "In addition"
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After "Typically"
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[[User:Froesccn|Froesccn]] 21:18, 19 April 2010 (UTC)
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:* Thanks for reviewing these pages. The changes have been done --[[User:Jholsenback|jholsenback]] 12:17, 20 April 2010 (UTC)

Latest revision as of 12:17, 20 April 2010

Regarding the note at the end of http://wiki.povray.org/content/Documentation:Reference_Section_6#Media

The casting of a shadow depend on the kind of media concerned.

Absorbing media always cast a shadow.

Scattering media also cast a shadow unless it's extinction is set to zero.

Emissive media never cast shadow.


I would suggest reordering the 3 sampling methods descriptions.

Start with method 3.

Then, put the warning followed by methods 1 and 2.

  • the above issues have been addressed --jholsenback 23:50, 11 April 2010 (UTC)


A few minor suggestions:

Section 6

Transformations apply only the density statements -> Transformations apply only to the density statements

There should be a consistent capitalization for the first word in a "Note"

Section 6.1

The emission keyword specifies a color of ... -> The emission keyword specifies the color of ...

Although we say they "emit" light, this only means that they are visible without any illumination shining on them. They do not really emit light that is cast on to nearby objects. -> Particles which emit light are visible without requiring additional illumination. However, they will only illuminate other objects if radiosity is used with media on.

a scattering media -> scattering media

, additionally samples Min=Max so a single samples parameter is acceptable. -> This doesn't really seem to belong in the box. Rather, when describing samples Min, Max below, add "The value for Max may be omitted, in which case the range Min = Max will be used".

a max samples higher than the minimum samples -> a maximum number of samples higher than the minimum number of samples

Generally, commas are set a bit sparsely. Here are some which should probably be there for grammatical correctness:

After "for example" (2x) After "However" (3x at beginning of sentence) Before "i.e." After "For object media" After "In addition" After "Typically"

Froesccn 21:18, 19 April 2010 (UTC)

  • Thanks for reviewing these pages. The changes have been done --jholsenback 12:17, 20 April 2010 (UTC)
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