[Radiance-general] rtrace -i -I flags

Bleicher, Thomas T-Bleicher at bdp.co.uk
Fri Mar 2 16:04:22 CET 2007

As Giullio has already explained
-i    will return the value for a 'light meter' at (x,y,z) pointing in
direction (dx,dy,dz)
      Only that one point will be calculated.
-I    will return the value for virtual light meter at (x,y,z) pointing
in direction (dx,dy,dz)
      but it will sample the whole hemisphere around the direction
      Basically rtrace introduces a virtual plane and turns the 'virtual
light meter' around
      to get the value of a point on this plane. You have to increase
-ab by 1 to get the
      same number of effective bounces as with '-i'.
Try a simple scene with both options and low -ab values (0,1,2).
If you print origin and direction of the rtrace ray as well you will see
that '-I' changes
them. Quite confusing if you're not prepared for it.


	From: radiance-general-bounces at radiance-online.org
[mailto:radiance-general-bounces at radiance-online.org] On Behalf Of Jelle
Feringa / EZCT Architecture & Design Research
	Sent: 02 March 2007 14:38
	To: radiance-general at radiance-online.org
	Subject: [Radiance-general] rtrace -i -I flags



	I'm running a simulation with rtrace, where I've set -I to
compute irradiance rather than radiance values.

	Though reading the man pages, I do find the explanations of the
flags somewhat confusing:



	Boolean switch to compute irradiance rather than radiance
values. This only affects the final result, substituting a Lambertian
surface and multiplying the radiance by pi. Glass and other transparent
surfaces are ignored during this stage. Light sources still appear with
their original radiance values, though the -dv option (below) may be
used to override this. This option is especially useful in conjunction
with ximage (1) for computing illuminance at scene points. 



	Boolean switch to compute irradiance rather than radiance, with
the input origin and direction interpreted instead as measurement point
and orientation.


	In what cases should one use the -i flag and in which cases is
-I the preferred option?

	I find it hard to make the distinction from the docs.






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