[Radiance-general] Odd values when using gensky and ambient accuracy is zero...

[P George Lovell]

Thanks Greg!

George

On 08/06/2011 11:55, Greg Ward wrote:
> Hi George,
>
> This is an intelligent and well-written question.  Thank you for taking the time to construct such a nice posting.
>
> Setting -aa 0 makes the calculation very sensitive to settings of -lw and -lr.  You should make sure that -lr is negative to enable Russian roulette sampling -- try "-lr -8" for starters.  If this doesn't fix the problem, try reducing the -lw parameter setting by powers of 10 until the discrepancy goes away or become acceptable.
>
> Best of luck,
> -Greg
>
> P.S.  You may find my 2005 presentation on new Radiance features illustrative:
>
> 	http://www.radiance-online.org/radiance-workshop4/cd/website/PDF/Ward_talk1.pdf
>
>> From: P George Lovell<p.g.lovell at st-andrews.ac.uk>
>> Date: June 8, 2011 3:12:43 AM PDT
>>
>> Hi Everyone,
>>
>> Please excuse me if this question has come up before (or if it's simply a dumb question). I'm using mingw radiance with a matlab wrapper to make stereo stimuli for perceptual experiments. Up until now my illuminationhas been made by specifying a hemifield of small lights. I'm beginning to prepare an experiment which will involve the creation of a larger number of images, hence I'm trying to speed up my rendering. One option seems to be to use gensky to specify a more natural illumination field, this seems to halve my rendering times.
>>
>> However, I only get satisfactory reproduction of the shadows when I use low values of ambient accuracy (not surprising). Shadows created with non zero ambient accuracy seem to be interpolated to a certain extent. Because my rendered images form a stereo pair, the differences between the shadows become very noticeable as irregularities in the surface depth, rather than as changes in brightness, this is a problem. I get much better rendering of the shadows when I  specify an 'aa' value of zero. But when Ido this the reflectances seem to get messed up. The attached images include a series of monochrome tiles towards the bottom, these are linearly spaced in reflectance from 0.2 to 0.8. However for the gensky rendered image with aa set to 0 the darkest tile seems too dark. This jump in radiance disappears when I increase aa to 0.1, it also disappears when I change to multiple spherical lights and leave aa at zero.
>>
>> Some example images:
>>
>> http://i1235.photobucket.com/albums/ff431/GeorgeLovell/RadianceProblems/SkyType_sphere200Lights_aa_000_ar_0256.png
>>
>>   - Things seem fine, though the rendering takes 780 seconds
>>
>>
>> http://i1235.photobucket.com/albums/ff431/GeorgeLovell/RadianceProblems/SkyType_gensky_aa_010_ar_0256.png
>>
>>    - Rendering is quicker with gensky but with an aa value of .1 thereare problems with the shadows.
>>
>>
>> http://i1235.photobucket.com/albums/ff431/GeorgeLovell/RadianceProblems/SkyType_gensky_aa_000_ar_0256.png
>>
>>    - Here the darkest tile along the bottom of the image is too dark.
>>
>> ----
>> For reference: the leftmost and middle images form a cross-fuseable stereo pair, i.e. you cross your eyes when you look at them left eye looks at the middle image and the right eye looks at the leftmost image (this can give you a headache!). The rightmost image can be viewed using red/green glasses.
>> ----
>>
>> Does anyone have a clue why setting aa to zero seems to change the brightness of the darkest tile?
>>
>> Thanks,
>>
>> George
>>
>>
>>
>> --
>> Department of Psychology, University of St Andrews
>>
>>      RM 1.05 (The white room).
>>      Phone      (01334) 462090
>>
>>
>>
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-- 
Department of Psychology, University of St Andrews

     RM 1.05 (The white room).
     Phone      (01334) 462090