[Radiance-general] daylight factor and rtrace cmd

[Greg Ward]

Hi Thomas,

> From: Thomas Bleicher <tbleicher at googlemail.com>
> Date: July 17, 2009 12:05:19 AM PDT
>
> Hi Greg. Still awake or are you traveling again?

Slept, thanks.

> On Fri, Jul 17, 2009 at 7:02 AM, Greg Ward<gward at lmi.net> wrote:
>
>> I'm not sure what you mean, though, about the -I option changing  
>> the effect
>> of -ab.  I suppose it depends on how you think about it.  With -ab  
>> set to 0,
>> only direct illumination arriving at that position is considered.   
>> Setting
>> -ab 1 sends out sample rays over the hemisphere to gather indirect
>> contributions.  Is this other than what you would expect?
>
> Perhaps it's me being extremely blond about the meaning of "-i" vs  
> "-I".
>
> Consider a scene with a sky and a plane just below z=0.
> I am now looking down from z=1 onto the plane and get:
>
> $ echo "0 0 1 0 0 -1" | rtrace -h -i -ab 0 scene.oct
> 2.541776e+01    2.541776e+01    2.541776e+01
>
> $ echo "0 0 1 0 0 -1" | rtrace -h -i -ab 1 scene.oct
> 7.457800e+01    7.457800e+01    7.457800e+01
>
> $ echo "0 0 1 0 0 -1" | rtrace -h -I -ab 0 scene.oct
> 0.000000e+00    0.000000e+00    0.000000e+00
>
> $ echo "0 0 1 0 0 -1" | rtrace -h -I -ab 1 scene.oct
> 2.033156e+01    2.033156e+01    2.033156e+01
>
> Why do I get different values for "-i" for "-ab 0" and "-ab 1"?
> There is nothing in the scene to bounce light off.

The sky dome and the distant ground for a standard sky description  
are considered as part of the "indirect" calculation, so you only get  
those contributions with -ab 1 (or higher).

> And why do I get "0 0 0" for "-ab 0" when i use "-I" but not
> when I use "-i". For "-ab 0" I would have expected both of them
> to be zero.

I'm guessing this is because your sun is above the horizon and your - 
I usage has the normal facing the ground.  Using -i, a ray is sent  
that hits the ground then looks skyward, where it sees the sun  
again.  The sun doesn't need -ab 1 to contribute.

Does this make sense, yet?

-Greg