We are trying to do a daylight analysis that includes some frosted glass or “kalwall” style skylights which diffuse the light into the space.  We can get the VLT (Visible Light Transmittance) values easy enough.  However, is there some way to accurately (or semi-accurately) account for the rays being dispersed and spread through the frosted glass? 

Simulating frosted glazing in 3ds Max Design for lighting analysis is doable.  You however need to know how to do it properly.  Here is how:

Some useful background information:

First, as opposed to Radiance, the A&D Material has a few internal “things” going on that you need to be aware.  The most important one is that the A&D Material performs internal energy conservation as follow:

Transmissivity wins over Specular Reflectivity which wins over Diffuse Reflectance.  On top of that, the Transmissivity is weighted against a Specular / Diffuse factor.  This factor is ruled by the Translucent color / weight controls in the interface.

In contrast, in Radiance, one can specify a material that is reflecting 100% diffuse and 100% specular while transmitting 100% of the light, leading to “creating” energy.  This is why the parameters of the Radiance materials cannot be plugged “as-is” in the A&D Material.

Translucent Panels:

We compared translucent glazing simulation in mental ray against radiance and measured data and got convincing results (see image) with the following settings:

Desired Diffuse Transmittance:  0.1621  (16.21% Diffuse – Diffuse Transmittance)

• Treat surface as a single polygon in the model
• A&D Diffuse Level:  0.0
• A&D Diffuse Color:  pitch black  (so no weighting is given to the diffuse reflectance)
• A&D Reflection | Reflectivity Level:  1.0
• A&D Reflection | Reflectivity Color:  pure white (the color is a multiplier, we need it to be 1.0 1.0 1.0)
• A&D Refraction |  Transparency Level:  1.0
• A&D Refraction | Transparency Color:  pure white (the color is a multiplier, we need it to be 1.0 1.0 1.0)
•  A&D Refraction | Translucency Checkbox : ON
• A&D Refraction | Translucency Weight: 1.0  (we want it fully translucent)
• A&D Refraction | Translucency Color:  0.1621 0.1621 0.1621 (the color is a multiplier, we need it to be set to the desied transmissivity level “as-is”, equally for all RGB components)
• BRDF | Custom Reflectivity Function: ON
• BRDF | 0 Deg Refl:  0.0
• BRDF | 90 Deg Refl:  1.0
• BRDF | Curve:  ~5  (we need to approximate a typical Fresnel curve)
• Advanced Rendering Options | Thin-Walled : ON

Some important notes: 

• While the illuminances will carry through properly on light meters, the glazing appearance may not look “natural” in “pretty picture renderings”.   It seems that there is currently a limitation with the appearance of the surface when it is hit by light:  its resulting luminance won’t be correct (AFAIK) so glare analysis based on luminance measurements won’t be convincing.
• The following image shows a graph comparing 3ds max, radiance and measured data.  Ignore the “3ds max 2009 SP1 Initial Submission” curve, this is representing a case where our material settings in 3ds Max where wrong, which we corrected later on – in fact, we forgot to turn off a layer so we had 2 panes of glass on top of each other…).  The green curve is what we need to look at….

This group maintains a database and publishes this data via a program called Optics 5. From Optics 5 you can then export a Radiance Material (*.rad file), which can be interpreted
as mental ray A&D Material parameters.

To convert Optics 5 data into A&D Material suitable for lighting analysis, export a glazing or glass definition as Radiance (*.rad) from Optics 5. You will find this command under File | Export to Radiance File.

Once the file is exported on disk, open it in Notepad and search for a section that looks like this:

void BRTDfunc B530_front
10
0.245 0.281 0.340
0.169 0.197 0.187
0 0 0

The color coefficients (RGB) for the ideal specular reflection corresponds to 0.245 0.281 0.340. The color coefficients for the ideal specular transmission corresponds to 0.169 0.197 0.187. Those values will need to be used as a basis for the mental ray A&D Material.

How to use the provided Microsoft Excel spreadsheet to convert to the mr A&D Material.

To correctly convert specular reflection and transmission from a Radiance material to a mr A&D Material, we need to take into account internal energy conservation methods that are built in the mr A&D Material that are not factored by the Radiance material. In other words, numbers can’t be “plugged-in” as is.

To help you with this task, we developed a Microsoft® Excel® software spreadsheet that will let you do this precisely. The spreadsheet can be downloaded here.

Limitations:

• The Radiance Materials exported from the Optics 5 database does not take into account angular dependency: A Fresnel falloff curve is assumed so metallic-coated glazing systems may be less precisely simulated.
• The Optics 5 database contains optical data measured spectrally. The exported Radiance materials and the A&D materials use RGB colors which are a crude approximation of the visible light spectrum. Therefore, lighting simulations are done within limitations of RGB colors.

It will also control Text objects to display the current Time and Date of the Daylight System:

Here is a video demonstrating its usage:

You can download the tool here.

1. In Revit, go in the 3D view you will export to FBX
2. Edit the Graphic Display Options
3. Specify a Location and Time that matches your project settings, for a single day:
4. Export to FBX
5. Import in 3ds Max

Your Daylight System should now match the Revit model in Location and Time.

]]> http://www.pfbreton.com/2009/10/translating-project-location-from-revit-to-3ds-max-via-fbx/feed/ 0 http://www.pfbreton.com/2009/07/lightfair-2009-lecture-high-dynamic-range-imaging-for-lighting-design-applications/ http://www.pfbreton.com/2009/07/lightfair-2009-lecture-high-dynamic-range-imaging-for-lighting-design-applications/#comments Sat, 25 Jul 2009 08:57:26 +0000 Pierre-Felix Breton http://www.pfbreton.com/?p=308 Hand Out Document (PDF – 1.5Mb)

HDRI and Photography

With today’s typical hardware, it is not possible to capture a lighting design project in a single shot and get it represented accurately on screen “as-is”.  Accurately meaning “how the eye perceived the project at the time the photo was taken”.  Typical digital display devices cannot display the entire range of luminance that a human eye can perceive. Cameras can’t capture that range of luminance at a same time either.  In technical terms, that is the complex (and how interesting) world of Tone Reproduction or Exposure.

For example, your eye adapts rapidly to a variety of contrasts and your brain condenses all this into a unique image, as if you would apply different brightness and contrasts ratios to individual parts of the image.  By opposition, exposure functions of cameras are applied to entire image at once – so the two worlds don’t match very well.

 The future is in what is called “High Dynamic Range” in computer language, where all luminance levels can be captured, stored and displayed into (expensive) devices.

The lecture describes methods to photograph lighting design projects in such a way that results are as close as possible to the human eye perception. Workflows involving image manipulation programs are discussed and demonstrated. 

The attendee learned how to digitally capture the full range of luminance of any given scene (in our case, lighting design projects) with affordable hardware.  He also learned how to manipulate this data with affordable software to create a digital picture that is closer to his perception of the scene.

HDRI and Qualitative Lighting Simulation

HDR Imaging does not only improve our ability to photograph design projects.  It also allows us to improve our efficiency in lighting simulation workflows.  One can use a 3D rendering software to create one image per light source and combine them together in a compositing application.  The advantage of doing this is that the designer can interactively change the intensity of each layer in real time without re-rendering the model each time.

Typical techniques for achieving this workflow are demonstrated as well.

HDRI and Quantitative Lighting Analysis

HDR Imaging also gives us the possibility to perform advanced Daylighting studies of architectural and urban spaces.  We can use this technology to understand glare issues, yearly or monthly cumulative irradiances and more.

Application examples are shown in this lecture.

The exterior twisted steel ribbon representing a race track created a potential of glare for pedestrians and cars passing near by. My mandate was to help the design team understanding if this was a real issue or not.

With the use of computer simulation, several videos where created demonstrating the effect of the reflection of the direct sunlight on the surroundings of the building to identify glare sources. Simulations for the solstice and equinoxes where performed.

Lighting Consultant: One Lux Studio
Computer simulations: Pierre-Felix Breton

I have participated in the process and contributed to the authoring of the following paper: 3ds Max Design Exposure Validation (pdf – 5835Kb) – which describes the process and results.

MacroScript files for 3ds Max Design 2009 created to improve the workflow of lighting analysis projects.

The first one will be on the topic of Daylight Simulation  with 3ds Max Design 2010. The second one will be about strategies to estimate optical characteristics of surface finishes for accurate renderings.

Daylight and Lighting Analysis with Autodesk® 3ds Max® 2010:

• How to predict lighting levels in a scene lit with daylight or electric light
• Finding correct values of reflectance and transmittance for the building materials you create and using these values in a lighting render of an existing building
• Importing and using the correct photometric information for a particular manufacturer’s light fitting
• Creating animated daylight simulation studies and experiencing your building in a dynamic way
• Limitations of lighting analysis programs
• Using MAXScript to perform advanced lighting analysis tasks

Capture and Acquire Appearances of Real World Materials for Improved Accuracy in Renderings:

• Rendering engines and color
• Gamma Correction and Linear Workflow
• Color acquisition devices comparison
• Measuring optical characteristics of materials in an affordable way
• Using this data correctly in Physically Based rendering engines

 
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