# glass modifier under different angles

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.

_______________________________________________

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.