[Radiance-general] sharing the ambient cache

[John Mardaljevic]

> 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
Senior Research Fellow
Institute of Energy and Sustainable Development
De Montfort University
The Gateway
Leicester
LE1 9BH, UK
+44 (0) 116 257 7972
+44 (0) 116 257 7981 (fax)

jm at dmu.ac.uk
http://www.iesd.dmu.ac.uk/~jm


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