Dear All,
I am trying to calculate the direct light from the sky for a reference grid within a number of rooms. I am sure there is a sophisticated manner in which to achieve this, but I have cobbled together an approach in the following way:
delete the ground source glow from the sky.rad description
use rtrace to calculate the values for the points, but set -av 0 0 0 and use a single material for all surfaces (except apertures!) which is excluded from the ambient calculation using -ae.
There are some points within the rooms, which I am pretty certain do not have a direct view of the skydome, and therefore should return a zero value, but they do not.
Any thoughts?
regards
Nick
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--
Nick Devlin
XCO2
1-5 Offord Street
London N1 1DH
Tel: 020 7700 1000
Fax: 020 7700 4455
Engineering Sustainability
Dear All,
I am trying to calculate the direct light from the sky for a reference grid within a number of rooms. I am sure there is a sophisticated manner in which to achieve this, but I have cobbled together an approach in the following way:
delete the ground source glow from the sky.rad description
use rtrace to calculate the values for the points, but set -av 0 0 0 and use a single material for all surfaces (except apertures!) which is excluded from the ambient calculation using -ae.
You have to set "-ab 0" as well or ...
There are some points within the rooms, which I am pretty certain do not have a direct view of the skydome, and therefore should return a zero value, but they do not.
... you get indirect contributions from the sky.
If I do understand the discussions on the list correctly you have to take into
account _only_ direct components for ie. "Right of Light" and other legal stuff.
In reality the indirect components do count as well.
Thomas
···
On 22.09.2005, at 10:00, nick devlin wrote:
Thomas,
thanks for that. I had tried it in the past but rtrace returns values of zero!!
I was confused about this, until I remember that for a uniform, or overcast sky (the types I am testing), the sky is a glow source, which I believe is not tested in the direct calculation.
which means I am back to the drawing board.
I thought about subtracting the ambient values from the total illuminance values, but this is no help either as sky glow falls in the ambient file.
Any thoughts?
regards
nick
Message: 8
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Date: Thu, 22 Sep 2005 11:36:57 +0200
From: Thomas Bleicher <[email protected]>
Subject: Re: [Radiance-general] Compute direct component from sky only
To: Radiance general discussion <[email protected]>
Message-ID: <[email protected]>
Content-Type: text/plain; charset=US-ASCII; delsp=yes; format=flowed
On 22.09.2005, at 10:00, nick devlin wrote:
Dear All,
I am trying to calculate the direct light from the sky for a reference grid within a number of rooms. I am sure there is a sophisticated manner in which to achieve this, but I have cobbled together an approach in the following way:
delete the ground source glow from the sky.rad description
use rtrace to calculate the values for the points, but set -av 0 0 0 and use a single material for all surfaces (except apertures!) which is excluded from the ambient calculation using -ae.
You have to set "-ab 0" as well or ...
There are some points within the rooms, which I am pretty certain do not have a direct view of the skydome, and therefore should return a zero value, but they do not.
... you get indirect contributions from the sky.
If I do understand the discussions on the list correctly you have to
take into
account _only_ direct components for ie. "Right of Light" and other
legal stuff.
In reality the indirect components do count as well.
Thomas
You could probably replace your windows with illums
using mkillum and a higher -ab value. For your final
rtrace calculations you can use -ab 0 again and only
get the "direct" contribution of the window illum.
I have to admit that I don't know what causes your
problems and therefore this approche may fail as well.
Thomas
···
On 22.09.2005, at 13:19, nick devlin wrote:
Thomas,
thanks for that. I had tried it in the past but rtrace returns
values of zero!!
I was confused about this, until I remember that for a uniform, or overcast sky (the types I am testing), the sky is a glow source, which I believe is not tested in the direct calculation.
which means I am back to the drawing board.
Nick,
The direct sky component should be a snip. I guess your points are in a horizontal plane, so the spawned rays will not see the ground glow and you can leave it in the description. Set the following:
-ab 1 You need the first level of hemispherical sampling so that rays are sent to the 'glow' sky.
-aa 0 This switches off interpolation and is likely to give you more precise results (generally not advised when ab > 1).
-ad 1024 then 2048, if there's a significant difference, proceed to 4096 and so on...
-as 256 or thereabouts.
-av 0 0 0
-ar - doesn't matter because aa is 0
No need to bother with the ae or aE parameters for the direct sky component.
And, of course, you have rtrace set to predict irradiation.
I would (strongly) advise against using mkillum to predict the direct sky component. You can get arbitrarily high accuracy in your predictions with -aa 0 and cranking up -ad until the results have converged to whatever degree you think is sufficient.
Shameless plug for CIBSE Daylight Group session of possible interest:
Wednesday 12th. October 2005
Rights to Light: A short-paper/discussion session to be lead by John Mardaljevic (DMU) and Paul Littlefair (BRE) with contributions from Andrew Thompson (Wilks Head & Eve), Delva Patman and Alistair Redler (Delva Patman), Alex Schaunowski and Ian Absolon (Schatunowski Brooks), Kaivin Wong (Malcolm Hollis) and Lance Harris (Anstey Horne).
14.00 for 14.30 at University College London, Torrington Place, London.
(With a line-up of that size, there is good chance that the speakers will outnumber the audience.)
-John
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-----------------------------------------------
Dr. John Mardaljevic
Senior Research Fellow
Institute of Energy and Sustainable Development
De Montfort University
The Gateway
Leicester
LE1 9BH, UK
+44 (0) 116 257 7972
+44 (0) 116 257 7981 (fax)
[email protected]
http://www.iesd.dmu.ac.uk/~jm