[Radiance-general] How to generate parallel light

[Jia Hu]

Thanks Greg,  I received an error from server at first post and I thought
the email was not sent


On Thu, Feb 20, 2014 at 4:01 AM, Greg Ward <gregoryjward at gmail.com> wrote:

> Hi Jia,
>
> It seems that you posted this question twice to the mailing list.
>
> The main effect of a "parallel" source is that its intensity does not
> diminish with distance.  The closest you can come to a parallel local light
> source in Radiance is to use the "spotlight" material with a very long
> direction vector, indicating that the virtual source is far behind the
> emitting area.
>
> *Spotlight*Spotlight is used for self-luminous surfaces having directed
> output. As well as radiance, the full cone angle (in degrees) and
> orientation (output direction) vector are given. The length of the
> orientation vector is the distance of the effective focus behind the source
> center (i.e., the focal length).
>
>         mod spotlight id
>         0
>         0
>         7 red green blue angle xdir ydir zdir
>
>
> Something like:
>
> void spotlight long_throw
> 0
> 0
> 7 1e5 1e5 1e5 1.0 0 0 -1000
>
> long_throw ring spot
> 0
> 0
> 9
> 0 0 3
> 0 0 -1
> 0 .1
>
> The above will create a 1-degree spread spotlight with a 1000-meter focal
> length, meaning that the effective emitting position is 1000 meters behind
> the emitting surface (a 10cm disk in the ceiling in this case).  This
> should give you the effect you want, even if the sample rays to the source
> are not always parallel.
>
> If your actual purpose instead is to generate perfectly sharp shadows, you
> can do this by shrinking the size of the emitting area (and increasing the
> RGB arguments of spotlight to compensate).
>
> -Greg
>
> From: Jia Hu <hujia06 at gmail.com>
> Date: February 20, 2014 5:13:57 AM GMT+01:00
>
> Hello,
>
> how could I generate some parallel light like sunlight?  Should I use
> "source" identifier? The parallel light source is located in a room.
>
> What if I want to generate a parallel light beam (emitting from a tube)? I
> am currently using a tube and place a spheric shape light in that tube, but
> it may not be parallel emitting from the tube..
>
>
> Thanks,
>
> Jia
>
>
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>
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