When I read the tutorial material and textbook and refman, I am still unclear about the material and geometry definition of sky and ground.
First of all, for the sky and ground material, I have the following questions:
What is the function of red green blue values here, are the RGB reflectance or just the RGB radiance of sky and ground? Based on [Radiance-general] sky definition, it seems that they are the RGB radiance of skyglow and groundglow for the sky and ground color.
As for the setting of the RGB value, the link above suggests that just " Use a color that makes sense and don’t over saturate it, and make sure its weighted average equals 1". As for ground reflectance, the -g of gensky will modify it and the RGB of groundglow has no influence of the reflectance. Am I right the P1 and P2?
But how does sky RGB radiance interact with the skyfunc， because in the Rendering with Radiance P355 shows that "Skyfunc multiples RGB radiance of skyglow to determine the RGB radiance of the sky ".
Maybe it would be best not to create a new topic for every question you have related to sky definitions. Since these are more discussions than simple questions and corresponding answers, it would feel less frantic to have just one thread rather than multiple ones going at the same time.
The output of gensky does not specify colors, which can vary with sky conditions and ground cover. However, if you want a match between your sky radiance/luminance values, you need to balance whatever color you specify for the glow such that:
0.265*R + 0.670*G + 0.065*B == 1
The above formula computes a Y-like channel from RGB, and making sure it’s unity keeps from changing the absolute values calculated in gensky.
Do the same thing for the ground color if you don’t have a gray ground. The -g option to gensky sets the average (Y) ground reflectance, so if you have an actual ground surface as well, it should be diffuse and use this -g setting times your groundglow color. In the example above, if you had the default “-g 0.2” setting in gensky, you can multiply 0.2 by the groundglow color to get the material for your ground plane:
You shouldn’t make the ground plane too large – just big enough to capture any building shadows. Past the limit of the ground plane, you will see the groundglow in your description, so make sure that has the same color as your ground geometry.
I am really sorry for creating some confusion earlier. I was feeling a bit “crazy” at the time(haha).
These questions came from issues I encountered while working through the first six chapters of the book for daylight simulation. I initially thought I would understand them better after finishing the chapters, but I still found certain parts unclear. Since these questions touched on different aspects of sky definition, I thought splitting them up might make it easier for others to search for similar issues on the forum in the future.
That said, you are right, and I will make sure to consolidate questions related to a single topic into one thread going forward. Thanks for pointing this out, and I really appreciate your patience and support.
Maybe my previous question is unclear, as for the skyglow, the radaince_tutorial indicates that the sky material needs to be defined by the user. For the definition of skyglow, besides making sure RBG values weighted average equals 1, I am confused about how should I determine the exact RBG values. That I why I want to make clear how the RGB radiance of skyglow interacts with Skyfunc because I want to know if the RGB values of skyglow and groundglow will influence the interior illuminance prediction.
Based on your reply and my understanding, the RGB values of skyglow and groundglow don’t influence the luminance prediction and they only impact the color of the sky and ground. Am I right?
Well, there can technically be subtle interactions between colored illumination and colored surfaces that may affect the calculated illuminance, but these will be on the order of a percentage point or two for unsaturated colors like the sky.
The best way to obtain the color of the sky or the ground of course is to measure it. If that is not possible, then you may be safest specifying a gray (1 1 1) color if your goal is not a rendering with a clear, blue sky. I am sorry there are not better guides for the color selection process, other than using your eyes to judge what looks right to you.
If you really care about colors, then Taoning Wang’s new genssky tool will compute a sky spectrum for a given set of atmospheric conditions. It does not currently work from global and diffuse irradiance measurements or TMY data, but that will be corrected at some point.
Thank you for your explanation. I want to clarify whether the color of glowsky and glowground will impact the calculated illuminance because I am currently performing hourly indoor daylight analysis for over a year, taking weather conditions into account. The color is not my primary concern, so as you pointed out, specifying a gray (1 1 1) color of sky and ground might be the safest approach for this purpose. I truly appreciate your insights and guidance.