rtrace -i -I flags

radiance-general
1

Hi,

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:

-i

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.

-I

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.

Thanks,

-jelle

2

option -I

you want illuminance at 0 0 0 looking up (0 0 1), you sensor is at 0 0 0
and has the photocell up.

option -i

you want to measure that object you can see in direction (1 1 1) from
point 0 0 0. The meter is on the object you are looking at.

(similarly to when you run rpict -i, you get the illuminance over what
you see)

hope this makes is clearer...

G.

···

________________________________

From: [email protected]
[mailto:[email protected]] On Behalf Of Jelle
Feringa / EZCT Architecture & Design Research
Sent: 02 March 2007 14:38
To: [email protected]
Subject: [Radiance-general] rtrace -i -I flags

Hi,

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:

-i

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.

-I

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.

Thanks,

-jelle

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3

Jelle.

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
vector.
      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.

Thomas

···

________________________________

  From: [email protected]
[mailto:[email protected]] On Behalf Of Jelle
Feringa / EZCT Architecture & Design Research
  Sent: 02 March 2007 14:38
  To: [email protected]
  Subject: [Radiance-general] rtrace -i -I flags
  
  Hi,

  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:

  -i

  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.

  -I

  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.

  Thanks,

  -jelle

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4

I guess the man page is pretty confusing. The '-i' option sends a ray into the scene, just as rpict would for generating an image or rtrace would for generating a radiance value, but then computes the incident irradiance at that surface point rather than the radiance. In other words, it makes most surfaces (with the exception of glass materials) look perfectly diffuse, with a normalization factor that computes irradiance.

The '-I' option, which is only supported by rtrace, changes the notion of the input to be interpreted as an illuminance measurement point as Guilio said, rather than a ray origin and direction. This makes it convenient for placing virtual irradiance meters anywhere in your scene.

-Greg

···

From: "Bleicher, Thomas" <[email protected]>
Date: March 2, 2007 7:04:22 AM PST

Jelle.

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 vector.
      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.

Thomas