Hi,

I have one question about the glass modifier.

Transmittance depends on both the materials properties and the distance

that light is traveling through the material. This distance depends on

the angle under which the ray hits the surface, and will be shortest if

hit perpendicular. Is this correctly modeled by the glass modifier,

which does not know the thickness (but the refraction of the material)?

Or do I need a 'solid' glass body to model this, with dielectric?

CU Lars.

I remember from some documents (maybe even RwR) that

'glass' is optimised to model the effects of a thin

sheet of glass in one single polygon rather than

two parallel 'interface' polygons. The thickness of

glass panes in buildings is rather standard (4 or 6

mm) so there should be minimal errors due to this

optimisation.

If you want to model exactly how a solid body of

glass would behave you should use other primitives

like 'dielectric' (see the whale sculpture and

anti-matter examples in RwR). Arguably, the Radiance

approach is not the best algorithm for this.

Regards,

Thomas

## ···

On 14 Jan 2009, at 05:41, Lars Oliver Grobe wrote:

Hi,

I have one question about the glass modifier.

Transmittance depends on both the materials properties and the distance

that light is traveling through the material. This distance depends on

the angle under which the ray hits the surface, and will be shortest if

hit perpendicular. Is this correctly modeled by the glass modifier,

which does not know the thickness (but the refraction of the material)?

Or do I need a 'solid' glass body to model this, with dielectric?

CU Lars.

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Hi Thomas, thank you for the reply!

Yes, I know that it is an optimization. Still it would be possible to

account for the angular dependent distance given a single polygon if you

rely on that average of about 6mm. But the internal calculation should

include the angular dependency than, and that is what I am wondering. So

is the fact that a ray entering a glass face at 0 degree (perpendicular)

would travel a much shorter distance through the material then one

hitting it at 45 degree considered in the glass modifier code?

BTW yes I think this is explained in RwR, though I do not know if this

question is answered there. Unfortunately my book is 10.000km from my

current location and the one we ordered weeks ago seems to be not

available.

CU Lars.

Hi Lars,

The 'glass' type correctly computes the angular transmittance and reflectance of any sheet of glass, since the actual thickness drops out of the equation. The only thing it won't do for you is show double- and triple- reflections that are offset by the glazing thickness, as one might notice in a thick pane of glass.

-Greg

## ···

From: "Lars Oliver Grobe" <akilog@nus.edu.sg>

Date: January 14, 2009 2:17:40 AM PST

Hi Thomas, thank you for the reply!

Yes, I know that it is an optimization. Still it would be possible to

account for the angular dependent distance given a single polygon if you

rely on that average of about 6mm. But the internal calculation should

include the angular dependency than, and that is what I am wondering. So

is the fact that a ray entering a glass face at 0 degree (perpendicular)

would travel a much shorter distance through the material then one

hitting it at 45 degree considered in the glass modifier code?

BTW yes I think this is explained in RwR, though I do not know if this

question is answered there. Unfortunately my book is 10.000km from my

current location and the one we ordered weeks ago seems to be not

available.

CU Lars.