[Radiance-general] Trans Oddities Part Deux

[Rob Guglielmetti]

Jack de Valpine wrote:
> Hey Rob,
> 
> According to RwR (p. 325), actuallly the one place Rd has impact is on 
> Argument 6 of the trans parameters. Based on some reverse calculations 
> to figure out what you might have been doing, I also came up with 30% 
> diffuse reflectance for the 10% shade and 20% for the 5%, 1%, 0.1% and 
> 0.5% shades, thus my question. Based on my calculations here is what 
> what you get:
> 
> 10% shade at 30% reflectance:
> Rd = 0.3
> Rs = 0
> Td = 0.005
> Ts = 0.095
> thus the following trans parameter line
> 7 0.4 0.4 0.4 0 0 .25 .95
> 
> 10% shade at 20% reflectance:
> Rd = 0.2
> Rs = 0
> Td = 0.005
> Ts = 0.095
> thus the following trans parameter line
> 7 0.3 0.3 0.3 0 0 .33 .95

You didn't have to reverse engineer my parameters, just ask!  Here is a 
bit of the materials, as defined in my spreadsheet I created 
specifically to apply Chas' formulae from RwR.  I make assumptions in 
the left column (Cr, Cg, Cb, Rs, Sr, Td, Ts), and the spreadsheet does 
the math to generate A1-A7.

Material Name: 				shade.10
Assumptions						Trans Parameters
Diffuse reflectance, RED		Cr	0.3000	A1	0.400000
Diffuse reflectance, GRN		Cg	0.3000	A2	0.400000
Diffuse reflectance, BLU		Cb	0.3000	A3	0.400000
Reflected Specularity			Rs	0.0000	A4	0.000000
Surface Roughness			Sr	0.0000	A5	0.000000
Diffuse Transmissivity			Td	0.0050	A6	0.250000
Transmitted Specularity			Ts	0.0950	A7	0.950000
Photopic Average refl. (calculated)	Rd	0.3000		
Total transmission		10.00%		
Percent specular		95%		

Material Name: 				shade.05
Assumptions						Trans Parameters
Diffuse reflectance, RED		Cr	0.2000	A1	0.250000
Diffuse reflectance, GRN		Cg	0.2000	A2	0.250000
Diffuse reflectance, BLU		Cb	0.2000	A3	0.250000
Reflected Specularity			Rs	0.0000	A4	0.000000
Surface Roughness			Sr	0.0000	A5	0.000000
Diffuse Transmissivity			Td	0.0025	A6	0.200000
Transmitted Specularity			Ts	0.0475	A7	0.950000
Photopic Average refl. (calculated)	Rd	0.2000
Total transmission		5.00%		
Percent specular		95%		

 > FYI:
 > A6 = (Td+Ts)/(Rd+Td+Ts)
 > A7 = Ts/(Td+Ts)

Well there it is, in black & white.  Rd is in the equation for A6.  It 
makes a difference.  Shoot.  Shoulda looked there in the first place.

> I hope that I have done all my calcs correctly here. I expect others can 
> point out if I am incorrect. I am not sure if this ends up pushing your 
> study in the right direction. I will be interested to hear.

I will test it out tomorrow, for sure.  Methinks that -ab 5, -av 0 0 0 
and all the shades having the same reflectance will generate numbers 
that make a lot more sense.  I hope so, anyway.

> Also, I am quite interested in hearing about your methodology and 
> experience measuring the glazing materials.

Not until you tell me all the parameters you used to make this 
(http://www.visarc.com/visarc/projects/images/bu_ab03.jpg) in fifteen 
minutes!  =8-)

Seriously tho, I'm about to blast outta here for the evening, but 
tomorrow I will try and piece together a cohesive explanation of what we 
did to measure the glass.  Something more detailed than "asked Greg for 
help", that is.

----

      Rob Guglielmetti

e. rpg@rumblestrip.org
w. www.rumblestrip.org