[Radiance-general] Problem with getting correct irradiance from sky dome

[Greg Ward]

Hi Claus,

You need to specify the sky as a "glow" source -- sources that large  
are not accurately modeled in the direct calculation.  See:

	http://radsite.lbl.gov/radiance/digests_html/v2n5.1.html#SKY

I suppose the renderer should issue a warning or something, but  
Radiance is kind of like a big pile of rope to hang yourself with.

Best,
-Greg

> From: "Claus B. Madsen" <cbm at cvmt.aau.dk>
> Date: November 22, 2007 6:48:34 AM PST
>
> Dear List,
>
> I have problems getting trustworthy irradiances in a scene where I  
> am using a 180 degree light source as a sky dome. For testing  
> purposes I have created the following very simple setup: a  
> groundplane with normal (0 0 1), and a 180 degree source with  
> direction vector (0 0 1), the radiance of the source is RGB=(1 1 1)  
> (unity radiance). I set the camera to look straight down on the  
> groundplane and render using rpict -i to get irradiances. In theory  
> such a source should result in an irradiance at the groundplane of  
> exactly pi, but when I render I get approx.RGB=(4.77  4.77  4.77)
>
> Any ideas for explanations? Any ideas for how to correct this?
>
> Best,
> Claus
>
>
>
> The scene.rad file:
> ------------------------------------
> # Materials
> void plastic perfectlambert
> 0
> 0
> 5 1.0 1.0 1.0 0 0
>
> #Geometry
> !genbox perfectlambert groundplane 10000 10000 0.5 | xform -t -5000  
> -5000 -0.25
>
>
> # Illumination environment: a 180 degree unity radiance skydome
> void light skyradiance
> 0
> 0
> 3 1 1 1
>
> skyradiance source TheSky
> 0
> 0
> 4 0 0 1 180.0
> ---------------------------------------------------------------------- 
> -----------
>
>
> The render parameters:
>
> rpict  -i -x 1024 -y 1024 -ab 4 -aw 1 -ps 1 -dt 0.0 -dr 3 -dp 8192 - 
> ds 0.01 -dj 0.9 -dc 0.8 -u -vf scene.vp
>
>
>
>
> And the scene.vp file:
> ---------------------------------
> rview -vtv -vp 0.0 0.0 10.0 -vd 0.0 0.0 -1.0 -vu 0 1 0 -vh 45 -vv 45
> ----------------------------------------------------------------------
>
>
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