[Radiance-general] window glass modeling

[Thomas Bleicher]

On Wed, Dec 29, 2010 at 10:23 AM, Milan Cakanovic
<milan.cakanovic at hotmail.com> wrote:
> Hi Marija,
>
>> To render illuminance on a work plane, no need to input additional desk
>> geometry into the scene. Everything you add in the scene has influence on
>> light distribution and calculation time, so it is better to avoid adding of
>> furniture geometry.
>
> Thank you for advice. I do not plan to add a furniture.

Although it's not quite what you are concerned with right now
there is an alternative method to calculate illuminance values
on a (theoretical) surface thanks to Radiance's "lego bricks"
architecture:

http://sites.google.com/site/tbleicher/radiance/stencil

(Warning: Content may not be suitable for beginners. Guru guidance advised.)


>> You can define sensor point on the workplane area.
>
> My first plan was to define a few sensor points, but when I use rtrace with -ab
> 6 or 8 (for more accuracy), calculation takes a long time. For now, I will
> define just one sensor point.

The value you choose for -ab depends on the complexity of your
scene geometry and lighting conditions. From what you have
written so far you have an empty room with one window and
exterior blinds. I assume there is no outside obstruction to the
window.

Without the blinds you can cover the light distribution within the
room accurately with an -ab value of 3 or 4. If you add the blinds
you will need additional "bounces" to account for the light reflected
from the blinds. I assume you will need at least a value of 6 to
achieve reasonable accurate results.

Before you do your evaluation calculations you should do some
tests to find parameters which are accurate enough for your situation:

1) Use one typical scene setup and a sky definition without sun (only
    skylight and ground) and calculate the illuminance on one point with
    increasing values for -ab. You can use the other parameters as given
    by Marija in her email; it's a good start.
    Your results should show smaller variations the higher -ab is set.

2) With a reasonably accurate setting for -ab use the same scene and
    sky but define a sequence of 10 or more points from the window to
    the back of the room. Calculate these points and make a diagram
    of the results with the distance to the window on the x-axis and the
    illuminance on the y-axis.
    Your graph should show a smooth curve falling in illuminance with
    increasing distance from the window. If you can't make out a clean
    curve in your plot your "-ab" value is too low (or other values need
    adjusting).


Your main concern about using multiple points is the processing time.
However, if you pass all the points to rtrace in one call rtrace will cache
intermediate results from the first point to calculate the next. This will
speed up your calculation time so you won't spend four times as much
time calculating four points. If you call a separate rtrace process for
each individual point you will see no speed up, though.

Regards,
Thomas