[Radiance-general] window glass modeling

[Milan Cakanovic]

Hi Lars,

> Hm, a lot of work. A first hint, think about how to organize that, so that
> you can later repeat the renderings when you have to change something. People
> have been using Makefiles, bash scripts, rad as a specialized tool, ...

My plan is to use a bash script.

> It will still be useful to render some views (plan view, perspectives) to
> understand what is going on. Artefacts due to ambient settings not adapted to
> e.g. you fenestration become evident when you look at a splotchy image, but
> may be difficult to understand from some few illuminances.

Thank you for advice. This is useful.

> You need the transmissivity for a material definition of a glass modifier.
> Calculating the transmissivity from visual light transmittance is discussed
> in Axel's Radiance tutorial, p. 47.

Formula:  tn=1.09*Tn (Axel's RADIANCE Cookbook, p. 21) is simplification of
formula on p.20.

> The glass object is not only affecting the transmission through the window
> opening by simply multiplying, it is also reflective and depends on the angle
> of incidence. The sun close to the zenith would lead to large portions of
> irradiance on the glass pane be reflected for example.

I agree with that.

> You can (if it makes sense) scale the brightness of the sky either by
> changing the return value or the glow material that is modified by the
> brightfunc. Scaling the zenith and ground plane brightnesses should work, but
> is little transparent (and in fact you change the input values here for the
> skybright.cal script). Instead, it would be better to scale the glow. Of
> course, if you scale by smaller then 1, you will find 0.265*R + 0.670*G +
> 0.065*B < 1 . Still I think you will need a glass pane in your model to care
> for angular dependance and reflection.

Ok. Idea with scaling is obviously out.

> You need only one glass pane. Same as in real. The only pitfall is that,
> without mkillum, you would get a lot of noise at acceptable ambient settings,
> or never ending rendering times. So just use a mkillum surface at your glass
> pane, and on the inner side of the blinds.

I know that I have to use mkillum surface at my glass pane, I just forgot to
write it. Thank you anyway. But, how can I use mkillum surface on the inner side
of the blinds? I checked, when the blinds is down (angle=-90) the surface normal
of slats points into the office.
To create the distribution for the window:
mkillum -ab 0 < objects/window.rad scene.oct > tmp/iwindow.rad
Can I use this to create the distribution for the blinds:
mkillum -ab 0 < blinds.rad scene.oct > tmp/iblinds.rad

> No. Radiance uses a built-in material model for glass. There is a file called
> materials.pdf as part of the Radiance distribution explaining how materials
> are modeled in Radiance.
> You do not need all that. Try to model the scene as the real world is - and
> avoid scaling the universe just as you would not have this option in real
> life neither ;-)

:-)

> The only cheat here is that you can use a single surface in place of the
> window glass, which in fact consists of two glass bodies separated by air.

I plan to use a single surface in place of the window glass.

> This is the only optimization you need, together with helping out Radiance by
> pre-calculating the distribution behind the shading using mkillum.
> I'd propose that you just start with a simple case, e.g. the glazing without
> blinds, set up the rendering procedure (e.g. a script or Makefile), render
> illuminance on a work plane (maybe you put a desk surface into your model)
> using rpict -i and get the sensor points' illuminances using rtrace -I. Once
> you have completed that, there will be a better understanding of the more
> complicated tasks, and you will know how Radiance works in your case. It is
> difficult to solve all questions in advance, better get the hands on this
> nice simulation toolbox and your model :-)
> And always feel free to ask if you face unexpected behaviour by the software.

Please, can you explain me this: "render illuminance on a work plane (maybe you
put a desk surface into your model) using rpict -i".

Cheers, Milan