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bsdf file and actual geometry

asked 2017-08-31 13:03:10 -0600

xchen gravatar image

I'm trying to model light louver for daylight autonomy.

I was reading the presentation about BSDF files and it mentioned that it works with and without the geometry of the element (like louvers or blinds,etc).

In the xml file I have for the light louver,

    <Thickness unit="meter">0.068</Thickness>
    <Width unit="meter">0.300</Width>
    <Height unit="meter">0.025</Height>

I'm a bit confused whether I can apply this xml file to the window that I have, since the dimensions are different. And looks like this file is just for one slat, does it automatically multiply according to the window width and height? (I don't see anything for distance between slats in this xml).

This might be a question for whoever created this xml file. But I want to know if the bsdf file usually includes dimension data, and what is the proper process to assign this to a window for daylight analysis if dimension data is included.

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Hi, I have the same question as you. So have you figured out the answer now?

Fan gravatar image Fan  ( 2021-02-25 14:06:46 -0600 )edit

Fan - the user that posted the question identified the best answer below. You can see it with the green check. If it doesn't satisfy you're question then you should post a new question.

Andyrew gravatar image Andyrew  ( 2021-02-25 15:57:01 -0600 )edit

Thank you Andyrew. I just posted a new question.

Fan gravatar image Fan  ( 2021-02-26 12:32:09 -0600 )edit

2 Answers

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answered 2017-09-02 09:46:34 -0600

I agree with Rob. If your purpose is primarily daylight autonomy calculations, then you do not need the louver system geometry. The dimensions are not needed for that, so their values are not important. If you did wish to include geometry, you could follow the method outlined in Andy's presentation to insert it into your Radiance scene, being sure to "sandwich" it between proxy polygons modified by a "BSDF" primitive referencing the appropriate XML representation. Again, the dimensions reported in the file would not be required.

There's also an earlier presentation from 2011 that mentions the pkgBSDF program, which extracts the geometry from the XML file if present and creates the proxy sandwich for you. In most cases, the geometry is put in the XML file by genBSDF from a complete Radiance description of the system, which someone must provide. This would also enable you to generate a tensor tree BSDF for glare analysis and so forth, but as Rob alluded, it's not for the faint of heart.

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Thanks Greg. I feel I need to dive deeper into radiance to get a better picture of this. I'll test it out with the new Honeybee + module in grasshopper.

xchen gravatar image xchen  ( 2017-09-05 12:07:57 -0600 )edit

answered 2017-08-31 17:20:22 -0600

The easiest way to use this (and most) BSDFs for daylighting analysis is to apply the BSDF distribution to a plane representing the window in a 3-phase workflow. In this case, you'd use Radiance to create a daylight matrix (relates flux between the window and the sky dome) and a view matrix (relates flux from the window to the calculation point or view pixel), and the BSDF is the third matrix in between those two, managing the transfer of flux through the Lightlouver. You'd place a single polygon just inside the window and trace your rays from that, in each direction. The Radiance tool rfluxmtx handles all of this for you, as long as you set up the window geometry file header(s) correctly. (Incidentally, OpenStudio handles all of this for you automatically and includes a BSDF for Lightlouver as an example daylight redirection device.)

When you do it this way, you can use the 3-phase method to fairly quickly create an annual daylight distribution dataset of building geometry that uses specular redirection devices. This is cool. Note I said this is the easy way. If you want to use BSDFs for glare evaluation or in a 5-phase workflow to calculate ASE, you need to obtain a variable-resolution tensor tree BSDF for your device. Such BSDFs do exist for Lightlouver and other products, but you are on your own using them in an annual simulation context.

I'll leave it to other experts to pick up your question on using the geometry info in the BSDF; there is an example of this in the slide deck you cited, but I have no practical experience with that methodology. That's generally used more for qualitative analysis and glare evaluation of specific views (renderings).

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Thanks Rob. I'm new to radiance but your comment is really helpful for me to understand the scene behind. I will test it out with the new Honeybee +. I believe there are both 3-phase and 5-phase example files. I'll reporting interesting findings here if any.

xchen gravatar image xchen  ( 2017-09-05 12:05:59 -0600 )edit

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Asked: 2017-08-31 13:03:10 -0600

Seen: 495 times

Last updated: Sep 02 '17