Like I was feebly trying to say before, I think it's a hidden coupling with
density being a default of 1 (maybe?) even when not specified at all.  That's
just a guess of course but it seems to fit.
I believe the most correct way to use scattering then is to use a much lower
color value when the volume is greater and vice versa.
Try this out:  multiply White by 10000 when you use the BoxA.  The resulting
render will look much the same as your "wrong" BoxB render.  Divide White by
10000 and use BoxB, it will be all white.  It's still just a volume/scattering
relationship after all but I still think an implied density {rgb 1} is in there
whether used or not.  extinction is probably a part of it all for that very
reason but I'm not the one to say that's absolutely so.

Bob

"Tom Melly" <tom### [at] tomandluf9couk> wrote in message
news:38cfeeff@news.povray.org...
| Many thanks - I'll play with those settings. I must confess that media still
| mystifies me - as soon as I seem to be able to make a model in my head of
| what it does, it goes and bites me in the arse. Why, for example, should the
| extinction value be so significant - particularily as the docs advise that
| the default is the most accurate representation? IMHO If x units thickness
| of media show as white, then so should x+y (when the media is viewed from
| the lighted side). Is this an inaccuracy? (the thick media showing as
| black) - if so why does megapov compound the error?
|
| --
| Tom Melly
| tom### [at] tomandluforce9couk
| http://www.tomandlu.force9.co.uk
|
| Bob Hughes <omn### [at] hotmailcom?subject=PoV-News:> wrote in message
| news:38cfdd64@news.povray.org...
| > I tried it out and although I don't fully know the reasoning behind it I
| found
| > putting  extinction .0025 into the 'scattering' statement fixes it.  The
| logic
| > seems right if there were 'media_attenuation on' in the light but I tried
| adding
| > that in as well with it both on and off, neither way makes a difference.
| > Check it out using 0.0033 for the extinction and it's interesting.  My
| guess is
| > the missing link to 'density', it acts as though there is a  'density {rgb
| 1}'
| > present I believe and if approximately  density {rgb .00025}  (or lower)
| is used
| > then it corrects it (no need for extinction then either).
| >
| > Bob
| >
| > "Tom Melly" <tom### [at] tomandluforce9couk> wrote in message
| > news:38cfc981$1@news.povray.org...
| > | Sorry for lack of clarity - have reposted with full code to
| text.scene-files
| >
| >
| >
|
|

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