[Radiance-general] rtrace

[Jia Hu]

Thank you very much for your elaboration. The more I learn it, the more I am
attracted by its amazing power.

On Wed, Jul 14, 2010 at 5:14 AM, Thomas Bleicher
<tbleicher at googlemail.com>wrote:

>
> Take for example an origin of (0,0,0.75) and a direction of (0,0,1)
> for up which calculates the horizontal illuminance at 750mm. With "-i"
> rtrace would look from (0,0,0.75)  straight up to the ceiling and
> calculate the light falling onto the spot at (0,0) on the ceiling.
> With "-I" rtrace creates a virtual surface at 750mm with a surface
> normal of (0,0,1) (=horizontal). Then it places the origin at
> (0,0,0.75+1) and looks down onto this surface (new view
> direction=(0,0-1)). The result is the irradiance on a horizontal
> surface calculated at (0,0,0.75). Just what we need for daylight
> factors etc.
>
> You give me a very detailed explanation. Now, my understanding is that
for "-i", it actually measures the incident light of the intersection point
of a
physical surface and line along the view direction (in this example, "-i"
makes rtrace calculate the incident light at spot in the ceiling ,say,
(0 0 3.5).) while, "I" is used to calculate the incident light of the
origin.


> > (2) What is the option "-h" (in rtrace) used for? I run several times but
> do
> > not see any header information.
>
> As I mentioned above you can created images pixel by pixel using
> rtrace. However, a valid Radiance picture needs a header containing
> the resolution and other information. By default rtrace prints this
> header before it prints the results of the calculations. (I assume
> there are historical reasons for that. I'd expect that most uses of
> rtrace switch the header of with "-h".)
>
I see now, thanks,

>
> You probably have never seen the header because your output is piped
> through rcalc which converts it into a bit of number garbage. Just try
> the following commands on your bash prompt:
>
> echo "0 0 1 0 0 1" | rtrace scene.oct
> echo "0 0 1 0 0 1" | rtrace -h scene.oct
> echo "0 0 1 0 0 1" | rtrace -h -w scene.oct
>
> Substitute "scene.oct" with any octree you have around. The difference
> between the first two should be obvious. The last command uses "-w" to
> switch of the warning about "no light sources found", too. That will
> only be printed if you have no sun in your scene, though.
>
> > (3) There are interior blinds installed. If I first use mkillum to make
> the
> > window as a secondary light source, and then run rtrace, in this case the
> > program runs twice to get the illuminance value. Does this way make
> running
> > faster than directly using rtrace (only once)?  I input the model from
> > Revit and thus the window object is connected in the .rad file and
> therefore
> > It is not easy to use mkillum (To use mkillum requires either (1) not
> > to import window from revit and to generate window in Radiance or (2) cut
> > and paste the window .rad to a separate .rad file?)
>
> mkillum can help you saving time by splitting the calculation in one
> part for the exterior (until the light reaches the window) and one for
> the interior. If your exterior is fairly simple there is little gain
> from this extra step.
>
> You will also find it difficult to extract the window glass geometry
> from your *.rad file if it is produced by Revit. The normals might be
> the wrong way around and you may have multiple surfaces instead of
> only one as mkillum requires. I think you're better of not to use
> mkillum at his point. If you have doubts about the accuracy of your
> calculations increase "-ab" by 1 or 2.
>
Yes, it is difficult to extract the window geometry.

>
> > (4) For mutiprocessing, besides the option "-n",  is there any other
> > options I should pay attention to? I use Python script, and should I set
> > other options related to Python itself?
>
> There are no options for Python that would affect Radiance. From Rob's
> page you should know that you need to specify an ambient file ("-af"
> option) to benefit of the "-n" parallel processing. If you have a
> number of options for your geometry you can also just run one rtrace
> process for each scene in a separate shell. Then you don't have to
> worry about parallel processing options.
>
> For my understanding, ambient file is just like a shared ambient caching
to let all the processors access the file and make interpolation of indirect
calculation.
For the same scene with different sky conditions (diffuse and
direct irradation),
if I calculate illuminance at only one point under each sky condition,
the ambient file seems not to be necessary for parellel processing because
the caching values are different for each processor under different sky
condition,
It is no need to share the ambient caching, is that right? By the way,
should I
just specify a name of ambient file at the beginning ? The program will
create the
ambient with the specified name automatically ?




> Cheers,
> Thomas
>
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