Setting up tree using trans material

I am trying to look at the impact of trees on daylight in a room. I am modelling the tree using trans material but sturggling to accurately determine the inputs based on pg 325 of RwR. From the BR209 guidance I am given transparency (% radiation passing) = 0.6 and reflectance = 0.1. What I am unclear on, from reading RwR and previous trans/trees posts, is how is Cr, Cg and Cb calculated from a Rd = 0.1 and how Ts and Td are calculated.

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Hi Eoin,

Welcome to the forum! I’m surprised no one else jumped on this, as it’s a fairly common question. Even so, I had to refer back to RwR myself to make sure I was answering correctly…

If you model your trees with a gray transmittance, then the answer is simple, just use 0.1 for Cr, Cg, and Cb, as well as for Rd. If your diffuse reflection has a different color, then compute:
Rd = 0.265*Cr + 0.670*Cg + 0.065*Cb

The same color above will affect the transmitted light, whether it is specular or diffuse. The actual amount you assign to specular and diffuse transmission and reflection is up to you, and what is appropriate has much to do with the type of trees you are modeling. There was an interesting talk in this year’s workshop from Jessie Pan & @J_Alstan_Jakubiec from the University of Toronto, called Deciduous Trees in Climate-based Daylight Simulations. You might give that a look for ideas.



Hi Greg,

Apologies to jump on a old thread.
I am looking into the exact topic and wanted to clarify a few things.

Firstly, I want to clarify the transmittance only scenario.
I found an old thread of yours (modeling trees in Radiance)
where you have suggested to define the 7 parameters as
7 T T T 0 0 1 1

However, Jakubiec and Balakrishnan on above the link write the 7 parameters as
7 1 1 1 0 0 T 1

I tried to experiment with both options in a simple room scenario with a single face object in front and found that Jakubiec and Balakrishnan’s parameter definition gives higher than expected result. (I compared the with and without the single face.)

Once I understand this, I want to try/understand the scenario including the reflectance.


The first set of arguments, where T is repeated 3 times for RGB, corresponds to a change in total scattering. In the second set, the total scattering is always 100%, divided between diffuse reflectance and specular (pure) transmittance. This is why the values would be higher. I think the first version makes more sense, unless you want some degree of diffuse scattering. You may wish to vary both the transmission amount and the total scattering in that case.

Hope this helps!

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Hi Greg,

Thank you very much for your reply. It makes more sense now.

I managed to understand the scenario with the transmittance and the reflectivity as well. It took me little longer as @J_Alstan_Jakubiec cleverly processed the A1 A2 A3 values further to make it green.

I am very much grateful for your guidance on this occasion and before.