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OverviewWhen POV moved from 3.0 to 3.1, it's atmosphere and halo were dropped in favor of a single volumetric effect, known as 'media'. While media is much more flexible and powerful than halo and atmosphere, it has one major limitation. It only uses monte-carlo (random) sampling. This sampling leads to a very grainy look. While sometimes this graininess is favorable to aliasing effects, other times it just looks bad. Changesmethod 2 Sample method 2 (invoked by adding the line "method 2" to the media code) uses evenly spaced samples. This was first introduced by Mike (last name unknown) in the 'minipatch'. You can jitter the samples by using the "jitter" keyword followed by a float. Unlike the minipatch, if you specify a max samples higher than the minimum samples, POV will take more samples, but they will be random, just like in method 1. Therefore, I suggest you set the max samples equal to the minimum samples. method 3 Sample method 3 (invoked by adding the line "method 3" to the media code) 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. You can specify the anti-aliasing recursion depth using the "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. The end of the 'Too few sampling intervals' error message. Have you ever had POV stop half-way through a render with a "Too few sampling intervals" error? Well, this made me mad on a few occasions (media renders take a long time without having to re-start in the middle). So, if UVPov determines that you need more intervals than you specify (due to spotlights), it will create more for you automatically. So, I suggest always using one interval with method 3. Two last important notes: 1) It's usually best to only use one interval with method 3. Too many intervals can look bad, and POV will create more intervals if it needs them (see below). 2) Media created with method 3 sometimes looks slightly brighter than media created with methods 1 and 2. I'm not yet sure if this is a bug or just a result of the different sampling method. If you experience this and it causes a problem, simply increase the absorption or extinction for your media as a workaround. ResultsHere is a comparison of POV 3.0 and the three UVPov sampling methods. Each of these images took just over two minutes to render on my P266.
This page was last updated October 24, 1999. If you have any comments, please email me. My email address is Nathan at Kopp dot Com. |