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

[Robert Franke]

Hi Greg,

I found the Fresnel correction on this paper, thanks for the hint:

http://radsite.lbl.gov/radiance/refer/materials.pdf

How does RADIANCE calculate the index of refrecation of plastic?

Thanks,
Rob

Am 28.06.2012 01:30, schrieb 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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