On Mon, 10 Aug 2015 23:09:44 +0200, clipka <ano### [at] anonymousorg> wrote:

> Am 10.08.2015 um 09:21 schrieb Anthony D. Baye:
>> In an effort to speed the render of the scene I'm working on, I reduced  
>> the
>> radiosity settings to default, but wound up with large patches of pure  
>> white
>> with jagged edges.  I managed to fix some of it by increasing the  
>> number of
>> pretrace steps, but there are still large ambient areas in the render.
>>
>> Can anybody tell me what causes this, and which settings I might adjust  
>> to fix
>> it? I can't find anything about this in the documents or tutorials.
>
> An image might help to better diagnose the issue. Are the "patches of  
> pure white light" isolated splotches, or do you have a general problem  
> with jagged edges in the radiosity gradients?
>
> Generally, jagged edges are an indication that a lot of radiosity  
> samples are taken during the main render.
>
> Whenever a sample is taken during the main render, some nearby pixels  
> have already been rendered with the presumption that the new sample  
> doesn't exist, using a different set of samples to interpolate between  
> than any pixel rendered later, so you get a discontinuity in the  
> gradient where the additional sample was taken. This isn't much of a  
> problem if the set of samples to interpolate between is large already,  
> as the discontinuity will be low in amplitude; but as the percentage of  
> samples taken during the main render grows, so does both the total  
> number /and/ amplitude of such discontinuities.
>
> Among the report POV-Ray displays after finishing a render, there is a  
> table listing how many radiosity samples were taken during which pass at  
> which recursion depth, like this:
>
>      -----------------------------------------------------------
>        Pass     Depth 0           Total
>      -----------------------------------------------------------
>        1            148             148
>        2            583             583
>        3           2067            2067
>        4           2028            2028
>        Final       2571            2571
>      -----------------------------------------------------------
>        Total       7397            7397
>        Weight     0.282
>      -----------------------------------------------------------
>
> As a rule of thumb, the number of top-level samples ("Depth 0") taken  
> during the main render ("Final" pass) should be less than half the  
> number of top-level samples taken in total.
>
>
> If the number of samples taken during the main render is too high, this  
> indicates that your pretrace coverage is too low, and you may need to do  
> the following:
>
> - Set "always_sample" to "off" (this is the default if "#version 3.7" or  
> higher is specified); this will cause POV-Ray to only take samples  
> during the main render if it absolutely needs them. If you instead set  
> this to "on", artifacts are almost guaranteed.
>
> - Set "nearest_count" to a reasonably large value; this will cause  
> POV-Ray to aim for a higher pretrace coverage. A value of 5 is ok, but  
> 10 is better, and I personally always use 20 except for test renders.
>
> - Decrease "low_error_factor"; this will also cause POV-Ray to aim for a  
> higher pretrace coverage, while at the same time making it less picky  
> when it comes to re-using samples during the main render. (Note that  
> this comes at the cost of "blurring" the radiosity effect; but when a  
> low pretrace coverage is your problem, then that's probably your least  
> concern.)
>
> - Increase the number of pretrace steps by decreasing "pretrace_end"  
> ("pretrace_start" usually has only a small impact on the pretrace  
> coverage, as long as it's reasonably large). If the number of pretrace  
> steps is too low, POV-Ray will be unable to achieve the pretrace  
> coverage as requested by the other parameters.
>
>
> If "pretrace_end" is already at the lowest effective value (the inverse  
> of the image size) and the number of pretrace steps is still lower than  
> necessary to achieve the pretrace coverage prescribed by the other  
> parameters, the following settings may also help; they all affect the  
> pretrace coverage POV-Ray needs, but also the one it aims for:
>
> - increase "error_bound" to reduce the required and aimed-for coverage  
> in general.
>
> - Increase "minimum_reuse" to reduce the required and aimed-for coverage  
> near corners.
>
>
> *BUT*
>
> if your issue is isolated bright splotches, then the main problem is  
> most certainly entirely different, namely that you have some  
> comparatively small but very bright object in your scene. In that case,  
> you only have two viable options: Up your radiosity "count" parameter,  
> or try to replace that bright object with a conventional point or area  
> light source (maybe with a looks_like object).
>

This should be added to the docs.

-- 
-Nekar Xenos-

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