[Radiance-general] calculation of one reflection (rtrace)

[Greg Ward]

Hi Robert,

There is a Fresnel correction that puts more energy into the specular component at low angles, and even though 45 degrees isn't a very low angle, your differences are in the 4th significant digit.  I think this is the source of the discrepancy.

-Greg

> From: Robert Franke <robertfranke at gmx.de>
> Date: June 27, 2012 2:41:26 PM PDT
> 
> Hi,
> 
> I try to understand how RADIANCE calculate the radiance values, but the specular value for one reflection isn't clear.
> I understand, that  the diffuse value is independent of view. On point P_r=(0 1 1)  I got from rtrace for the red channel 0.22361, I calculated:
> 
> R_r_light=100
> sphere_radius=0.5
> distance=2.2360
> rho_surf1=0.1
> cos(63,43) = light incoming angle
> 
> R_r(0,1,1) = R_r_light * (sphere_radius)² / (distance)² * cos (63,43) * rho_surf1 = 0.223606797.
> 
> Ok, this value is fine, but what will happen if I look from (0 -1 1) to (0 1 -1) at point (0, 0, 0), I thought  this, but it didn't work:
> 
> distance=3
> rho_mirr=0.3
> diff=0.9
> spec=0.1
> cos(45) = reflected light - observer angle
> P_r=0.22361
> 
> R_r(0,0,0) = R_r_light * (sphere_radius)² / (distance)² * rho_mirr * diff + spec * P_r * cos(45) = 0.765811388
> 
> rtrace = 0.76517
> 
> Can somebody explain how the specular component is calculated on my test scene? I do not get it...
> 
> Here is my test scene:
> 
> ######materials.rad######
> void plastic mirr
> 0
> 0
> 5 .3 .2 .1 0.1 0
> 
> void plastic surf1
> 0
> 0
> 5 .1 .1 .1 0 0
> 
> void light lighty20
> 0
> 0
> 3 100 150 200
> 
> ######test.rad######
> mirr polygon plate1
> 0
> 0
> 12
> -1 -1 0
> 1 -1 0
> 1  1 0
> -1  1 0
> 
> surf1 polygon plate2
> 0
> 0
> 12
> -1  1 0
> -1  1 3
> 1  1 3
> 1  1 0
> 
> lighty20 sphere light_s
> 0
> 0
> 4 0 0 3 0.5
> 
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