[Radiance-general] sharing the ambient cache

[Greg Ward]

Hi John,

Just goes to show, when it comes to the older algorithms, it's best  
to follow my old advice rather than my new advice...  Clearly, I  
thought about this more carefully before, and there are good reasons  
for not changing the -ar parameter in subsequent runs.  I think the  
artifacts you're seeing are due to the mix of new values and older  
ones, both of which get included in the interpolation step.  The new  
values cover a wider area than the old ones (and wider than they  
should).  The splotchy look is due to the way the interpolation  
algorithm works, and is even worse than I'd expect from reducing the - 
ab value.

-Greg

> From: John Mardaljevic <jm at dmu.ac.uk>
> Date: December 22, 2005 11:18:29 AM PST
>
>> Jack de Valpine or John Mardaljevic may know more about this than  
>> I do...
>
> Actually, I can't really expand much on what is in the book (p387):
>
> Having created the ambient file with the “overture” calculation,  
> you can, with
> caution, relax some of the ambient parameters for the larger  
> renderings. The
> parameter revisions could be one or both of the following:
> • Reduce -ad and -as by about 50%
> • Slightly increase -aa (i.e., by 0.05 or 0.10)
> The other ambient parameter settings should not be changed. If you  
> do decide
> to change any of the -ad, -as, or -aa settings after the “overture”  
> calculation, you
> should be aware that the modifications will not be reflected in the  
> header of the
> ambient file. Thus, you need to track both the picture and the  
> ambient file headers
> to obtain a complete record of the parameter settings for an image.
>
> I seem to recall that Greg reckoned it was not a good idea to  
> reduce the ambient resolution (ar) parameter when reusing the  
> ambient cache.  But I can't quite remember the reason why.  I was  
> going to end my contribution at this point.  But then I did some  
> tests.  Load up:
>
> http://www.iesd.dmu.ac.uk/~jm/pdfs/relax.pdf
>
> What we have here is a test scene: a ridged box by a window and  
> illuminated by a diffuse (i.e. glow) sky.  The ar parameter limits  
> the extent to which ambient sampling can convincingly shade the  
> grooves.  I populated the ambient file artest.af in creating the  
> image at the top which only sees half the box.  I then reused the  
> ambient file with lower resolution settings to create wide views of  
> the box -- I thought that seeing the "boundary" between the already  
> populated hi-res ambient settings and the low-res might be  
> interesting.  It was.  Note that for each of the three low ambient  
> resolution images I reused the *original* artest.af (i.e. I kept a  
> copy).
>
> In the left and bottom images I relaxed the aa and ad parameters  
> respectively.  The results are what most of us would probably  
> expect.  For the image on the right I relaxed the ar parameter.   
> Now, for this image, the "new" part on the left-hand-side is what  
> would be expected from a fresh run without any pre-existing ambient  
> file -- ar 64 doesn't allow close-enough sampling and the grooves  
> get some "flat" ambient shading [1].  The interesting effect is of  
> course on the right -- the pre-existing ambient samples on this  
> side have somehow produced this lovely frog-spawn pattern.  Why? [2]
>
> -John
>
> PS. Dontcha just love this "I'll never quite master it" side to  
> Radiance?
>
> [1] When av is zero, as it was here, how is the ar-limited shading  
> value arrived at?
>
> [2] I have this vague feeling that I sort of might know the reason  
> why.  But it is all too hazy for me to attempt to put it into words.
>
> -----------------------------------------------
> Dr. John Mardaljevic
>