>> You can adjust the EXP variable in the test scene to control the
>> brightness of the sky (it controls how the physical brightness values
>> calculated are converted to POV units). Try increasing it from 4e-5 to
>> 6e-5 or even higher.
>
> Yes I did that indeed, but somehow the scene never comes close to the
> aspect of a regular light and sky_sphere.

It looks like it's because the sun is so bright and so small, the 
radiosity algorithm won't pick it up very often. This would explain the 
bright yellow patches, they're radiosity samples that just happened to 
pick up the sun - most of the samples didn't. I'm not experienced enough 
with radiosity to know a way around this (I use mcpov for most renders, 
which allows for such small very bright objects ok).

Could you ever get a similar scene with a realistically sized and bright 
sun to render with radiosity corerctly? The pigment of the sky_sphere 
should only make a very minor difference when a bright sun is in the sky.

> I added radiosity, and they seem to come from that. Without radiosity
> the landscape remains totally dark whatever the value for EXP and the
> hour of the day.

There was a light_source in my demo scene file I posted, that should 
illuminate the ground - unless you've uncommented the sun sphere that 
happens to be in exactly the same place :-)

 > I think I found the culprit: Your original SunPos() is divided by 1000!

That shouldn't make a difference, because the sun radius is determined 
from the distance. The reason I did that is because SunPos.inc returns a 
massive distance for the sun vector which causes problems in some 
scenes, in reality it only needs to be a few orders of magnitude further 
away than everything else in your scene.

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