windows: illum useage?

radiance-general
1

Hi list!

Once again I have to ask a rather basic question, still I could not find an answer so far.

I have a model of a building which is illuminated solely trough windows consisting of a grid of small rectangular glass panes in stone framing, the whole window has an arc. So far I didn't model the panes, and the light just came in through the frames, which gave not the best results in the rendering, but I didn't care so far. Now that I will start modeling the panes, I have to think about how to use the illums later. As I don't want to keep the artefacts caused by the small openings in my final rendering, and as I want to speed up rpict, I need to use illums.

- The first question is how to define the illum geometry.

a) I could define one polygonal surface covering the whole window from the inside, and using mkillum to "see" the effect of the frame which is "outside" than. The pro is, that I have to model just two surfaces per window (one for the illum, and one glass pane which could intersect with the frame), and I have few light sources in the scene (one per window). The negative point is that the mkillum is necessary, although I am not really interested in the outside, so I get some overhead. It will also be not to easy to define a polygon for the surface, as the round outline is already converted into a polygon from the cad, and both have to fit exactly. However, this is what I would conclude from the advices in Rendering with Radiance (complex fenestration).

b) I could model each pane, place it in its correct location, and than simply put the gensky pattern on it. No mkillum needed any more (I actually don't want to look through the windows). The drawback is that I get a lot of small lightsources (around 40 per window), and I have to model lots of panes... don't laugh, modeling a rectangular surface is not that difficult, but I have to make sure that the surface normals point to the inside, and that is not that easy - I found some problems in formZ regarding normals and dxf export.

So which way to go? As I noted, I have also have to think about rendering speed and memory - the model is large, and I am supposed to produce animations later. And I found the following on the list, which makes me tend to the b) way:

> Illum's, like all light sources in Radiance, are preferred as rectangles or (at least)
> 4 or more sided convex shapes. Since Radiance samples a rectangle with
> equivalent area to the source shape, triangles are a particularly poor choice.
> I recommend replacing any windows you plan to use as illum's with rectangles
> for that reason. (Greg Ward)

- A second question, would it make sense to remove the gensky output except the sun after the illums have been generated if I use procedure a)? If all openings are covered with illums, I don't need the diffuse sky and ground any more, as they will be blocked by the illums, right? Could that give some additional performance? This could also reduce errors caused by "leakage" in the model.

TIA+CU Lars.

···

--
Lars O. Grobe
[email protected]

2

Hi Lars,

I recommend your "choice a)", and suggest you first try creating a rectangle that just covers the opening from the inside, then use mkillum to average it's output. Keep your gensky command in the scene (let "rad" automate things for you if you like), whether you want to look out the window or not. The largest source of error from applying skyfunc directly to fenestration is the lack of the building shadow on the ground plane, leading to overestimation of the upward component from the window in cases where the sun is on the other side of the building. Applying mkillum avoids this error, besides accounting for the framing of the window panes and the circular shape of the opening.

-Greg

···

From: "Lars O. Grobe" <[email protected]>
Date: July 1, 2004 3:50:01 AM PDT

Hi list!

Once again I have to ask a rather basic question, still I could not find an answer so far.

I have a model of a building which is illuminated solely trough windows consisting of a grid of small rectangular glass panes in stone framing, the whole window has an arc. So far I didn't model the panes, and the light just came in through the frames, which gave not the best results in the rendering, but I didn't care so far. Now that I will start modeling the panes, I have to think about how to use the illums later. As I don't want to keep the artefacts caused by the small openings in my final rendering, and as I want to speed up rpict, I need to use illums.

- The first question is how to define the illum geometry.

a) I could define one polygonal surface covering the whole window from the inside, and using mkillum to "see" the effect of the frame which is "outside" than. The pro is, that I have to model just two surfaces per window (one for the illum, and one glass pane which could intersect with the frame), and I have few light sources in the scene (one per window). The negative point is that the mkillum is necessary, although I am not really interested in the outside, so I get some overhead. It will also be not to easy to define a polygon for the surface, as the round outline is already converted into a polygon from the cad, and both have to fit exactly. However, this is what I would conclude from the advices in Rendering with Radiance (complex fenestration).

b) I could model each pane, place it in its correct location, and than simply put the gensky pattern on it. No mkillum needed any more (I actually don't want to look through the windows). The drawback is that I get a lot of small lightsources (around 40 per window), and I have to model lots of panes... don't laugh, modeling a rectangular surface is not that difficult, but I have to make sure that the surface normals point to the inside, and that is not that easy - I found some problems in formZ regarding normals and dxf export.

So which way to go? As I noted, I have also have to think about rendering speed and memory - the model is large, and I am supposed to produce animations later. And I found the following on the list, which makes me tend to the b) way:

> Illum's, like all light sources in Radiance, are preferred as rectangles or (at least)
> 4 or more sided convex shapes. Since Radiance samples a rectangle with
> equivalent area to the source shape, triangles are a particularly poor choice.
> I recommend replacing any windows you plan to use as illum's with rectangles
> for that reason. (Greg Ward)

- A second question, would it make sense to remove the gensky output except the sun after the illums have been generated if I use procedure a)? If all openings are covered with illums, I don't need the diffuse sky and ground any more, as they will be blocked by the illums, right? Could that give some additional performance? This could also reduce errors caused by "leakage" in the model.

TIA+CU Lars.
--
Lars O. Grobe
[email protected]

3

I was curious about this too...

Is there any advantage to making an illum plane for each of the
windows? Or will one large rectangle accomplish the same thing? I was
under the impression there was enough averaging that you would not want
to go the large rectangle route.

Mark

Message: 2

···

Date: Thu, 1 Jul 2004 20:21:26 -0700
From: Greg Ward <[email protected]>
Subject: Re: [Radiance-general] windows: illum useage?
To: Radiance general discussion <[email protected]>
Message-ID: <[email protected]>
Content-Type: text/plain; charset=US-ASCII; format=flowed

Hi Lars,

I recommend your "choice a)", and suggest you first try creating a
rectangle that just covers the opening from the inside, then use
mkillum to average it's output. Keep your gensky command in the scene

(let "rad" automate things for you if you like), whether you want to
look out the window or not. The largest source of error from applying

skyfunc directly to fenestration is the lack of the building shadow on

the ground plane, leading to overestimation of the upward component
from the window in cases where the sun is on the other side of the
building. Applying mkillum avoids this error, besides accounting for
the framing of the window panes and the circular shape of the opening.

-Greg

4

It depends of course on the scene, but I doubt from Lars' description that it would be worth the extra cost in terms of rendering time to make each pane an illum object, and the problems he'd face with the non-rectangular panes at the top would only add to his frustration.

-G

From: "Mark de la Fuente" <[email protected]>
Date: July 2, 2004 6:39:35 AM PDT

I was curious about this too...

Is there any advantage to making an illum plane for each of the windows? Or will one large rectangle accomplish the same thing? I was under the impression there was enough averaging that you would not want to go the large rectangle route.

Mark

5

and the problems he'd face with the non-rectangular panes at the top would only add to his frustration.

Hi Greg,

in fact, that was one of the easier things, as the small panes are generally all rectangular (and have the same dimensions). I have put a drawing to the net (very small, <20KB): http://www1.architektur.tu-darmstadt.de/lars/window.pdf

CU Lars.

···

--
Lars O. Grobe
[email protected]

6

building. Applying mkillum avoids this error, besides accounting for
the framing of the window panes and the circular shape of the opening.

Hi,

to recall what this is about, Greg suggested to use one illum surface covering the whole of a window made of small panes fixed in stone frames (I sent an URL pointing to a mini-pdf showing that in detail).

Now I wonder... afaik the window frames will disappear in the indirect calculation, as the average over the whole covering surface is used as source, right (of course the frames are counted when calculating this average)? So shadowing from these frames will only be visible when windows are hit by a dircet source (the sun), while those frames e.g. on the north walls won't cast shadows. The only way to change this would be to use subdivisioning of the illum surface.

Am I right here? I have to decide that modeling topic now, and I am still not really sure about it. If I have to use subdivided illum surfaces, I think that manually defining each pane as ONE illum surface (so that subdividing would not be necessary any more) is better than making a subdivision that doesn't exactly fit the panes.

Thanks+CU Lars.

···

--
Lars O. Grobe
[email protected]

7

Lars O. Grobe wrote:

building. Applying mkillum avoids this error, besides accounting for
the framing of the window panes and the circular shape of the opening.

Hi,

to recall what this is about, Greg suggested to use one illum surface covering the whole of a window made of small panes fixed in stone frames (I sent an URL pointing to a mini-pdf showing that in detail).

Now I wonder... afaik the window frames will disappear in the indirect calculation, as the average over the whole covering surface is used as source, right (of course the frames are counted when calculating this average)?

Yepp, right.

So shadowing from these frames will only be visible when windows are hit by a dircet source (the sun), while those frames e.g. on the north walls won't cast shadows. The only way to change this would be to use subdivisioning of the illum surface.

Am I right here? I have to decide that modeling topic now, and I am still not really sure about it. If I have to use subdivided illum surfaces, I think that manually defining each pane as ONE illum surface (so that subdividing would not be necessary any more) is better than making a subdivision that doesn't exactly fit the panes.

The other way round, if you do the subdivision manually, what reason should there be to deviate from the original window geometry ?

The other question is, if the whole effort pays off. The frames look small in comparison to the 'window' openings in your sketch, so if this structure is hit by diffuse daylight, you probably won't see any distinct shadowing effect from the frames in RealLife(TM) either, but rather a bright, blurry diffuse patch on the floor, except perhaps in the extreme vicinity of the frames. It might suffice to model the bottom row or two rows separately to deliver a bit of the effect a viewer expects to see (regardless if it is really there or not..)

-cb

···

Thanks+CU Lars.
--
Lars O. Grobe
[email protected]

_______________________________________________
Radiance-general mailing list
[email protected]
http://www.radiance-online.org/mailman/listinfo/radiance-general