POV-Ray : Documentation : Tips on Radiosity
  POV-Ray 3.6 Documentation Online View Adjusting Radiosity   2.4 Objects Tips on Radiosity

Have a look at the "Radiosity Tutorial" in the "Advanced Tutorial" section, to get a feel for what the visual result of changing radiosity parameters is.

If you want to see where your values are being calculated set radiosity count down to about 20, set radiosity nearest_count to 1 and set gray_threshold to 0. This will make everything maximally patchy, so you will be able to see the borders between patches. There will have been a radiosity calculation at the center of most patches. As a bonus, this is quick to run. You can then change the error_bound up and down to see how it changes things. Likewise modify minimum_reuse.

One way to get extra smooth results: crank up the sample count (we have gone as high as 1300) and drop the low_error_factor to something small like 0.6. Bump up the nearest_count to 7 or 8. This will get better values, and more of them, then interpolate among more of them on the last pass. This is not for people with a lack of patience since it is like a squared function. If your blotchiness is only in certain corners or near certain objects try tuning the error bound instead. Never drop it by more than a little at a time, since the run time will get very long.

Sometimes extra samples are taken during the final rendering pass. These newer samples can cause discontinuities in the radiosity in some scenes. To decrease these artefacts, use a pretrace_end of 0.04 (or even 0.02 if you are really patient and picky). This will cause the majority of the samples to be taken during the preview passes, and decrease the artefacts created during the final rendering pass. You can force POV-Ray to only use the data from the pretrace step and not gather any new samples during the final radiosity pass. To do this, use "always_sample no" within the radiosity block inside global_settings.

If your scene uses ambient objects (especially small ambient objects) as light sources, you should probably use a higher count (100-150 and higher). For such scenes, an error_bound of 1.0 is usually good. Higher causes too much error, but lower causes very slow rendering. And it is important to adapt adc_bailout. Adjusting Radiosity   2.4 Objects

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