# illum examples

Hello experts:

Even if mkillum is used for window, the shadow ray in a point will be still
sent to sun (just pass through the window) to calculate the contribution of
sun (direct calculation). The reason is "illum surface do not interfere with
the shadow testing of others, non illumn sources" (Page 572 in the book

In Page 568-569 of the book, a decorative luminaries is illustrated to show
why to use illum. One reason of using illum is that the shadow ray in
a point may hit the fixture of the lamp and get a shadow which is incorrect
(it is incorrect because other parts of the lamp may illuminate the
point). Using illum will make the lamp a uniform radiator and therefore a
shadow ray hit the illum will return a nonzero value.

In the above first paragraph , as the book said, the shadow ray (that
hitting the sun) will not be interfered with by the illum (window, in this
example) . But for the decorative luminaries example (in the second
paragraph), My problem is that I do not know which of the following is right
as to the ways of treatment of Shadow ray test of this example.

(1) In this way, we do not need to consider the shadow ray test of the
lamp. In orther words, we can think of the new illum enclosing the lamp as
a special light, and there is no light (lamp) inside the illum, and
therefore, only one shadow ray is sent to the illum and NO shadow ray is
sent to the lamp (inside illum)

(2) It is similar with the example (window illum) in the above first
paragraph, In this case, one shadow ray is sent to hit the lamp and one
shadow ray is sent to hit the illum. Both of them will calculate the direct
part of light source. The illum just shows the luminance distribution of
lights reflected by the lamp fixture rather than lights directly from lamp.

Thank you,

Jia

There is a special case for "glow" sources to take care of this. Quoting from the Radiance reference manual <http://radsite.lbl.gov/radiance/refer/ray.html#Materials >:

Glow
Glow is used for surfaces that are self-luminous, but limited in their effect. In addition to the radiance value, a maximum radius for shadow testing is given:
mod glow id
0
If maxrad is zero, then the surface will never be tested for shadow, although it may participate in an interreflection calculation. If maxrad is negative, then the surface will never contribute to scene illumination. Glow sources will never illuminate objects on the other side of an illum surface. This provides a convenient way to illuminate local light fixture geometry without overlighting nearby objects.

···

From: Jia Hu <[email protected]>
Date: July 20, 2010 10:23:31 AM PDT

Hello experts:

Even if mkillum is used for window, the shadow ray in a point will be still sent to sun (just pass through the window) to calculate the contribution of sun (direct calculation). The reason is "illum surface do not interfere with the shadow testing of others, non illumn sources" (Page 572 in the book Rendering with Radiance).

In Page 568-569 of the book, a decorative luminaries is illustrated to show why to use illum. One reason of using illum is that the shadow ray in a point may hit the fixture of the lamp and get a shadow which is incorrect (it is incorrect because other parts of the lamp may illuminate the point). Using illum will make the lamp a uniform radiator and therefore a shadow ray hit the illum will return a nonzero value.

In the above first paragraph , as the book said, the shadow ray (that hitting the sun) will not be interfered with by the illum (window, in this example) . But for the decorative luminaries example (in the second paragraph), My problem is that I do not know which of the following is right as to the ways of treatment of Shadow ray test of this example.

(1) In this way, we do not need to consider the shadow ray test of the lamp. In orther words, we can think of the new illum enclosing the lamp as a special light, and there is no light (lamp) inside the illum, and therefore, only one shadow ray is sent to the illum and NO shadow ray is sent to the lamp (inside illum)

(2) It is similar with the example (window illum) in the above first paragraph, In this case, one shadow ray is sent to hit the lamp and one shadow ray is sent to hit the illum. Both of them will calculate the direct part of light source. The illum just shows the luminance distribution of lights reflected by the lamp fixture rather than lights directly from lamp.

Thank you,

Jia

Hello Greg:

Thank you. I also find a similar thread you replied:

I shall change the luminaires material type to "glow" (with maxrad = 0). In
this case, the shadow rays will only be sent to illum rectangle and not be
sent to the luminaire (inside illum rectangle). This case is just similar
with the window (as sky is "glow" type).

Cheers,
Jia

···

On Tue, Jul 20, 2010 at 2:58 PM, Greg Ward <[email protected]> wrote:

There is a special case for "glow" sources to take care of this. Quoting
from the Radiance reference manual <

*Glow*Glow is used for surfaces that are self-luminous, but limited in
testing is given:

mod glow id
0
0

If maxrad is zero, then the surface will never be tested for shadow,
although it may participate in an interreflection calculation. If maxrad is
negative, then the surface will never contribute to scene illumination. Glow
sources will never illuminate objects on the other side of an illum surface.
This provides a convenient way to illuminate local light fixture geometry
without overlighting nearby objects.

From: Jia Hu <[email protected]>
Date: July 20, 2010 10:23:31 AM PDT

Hello experts:

Even if mkillum is used for window, the shadow ray in a point will be
still sent to sun (just pass through the window) to calculate the
contribution of sun (direct calculation). The reason is "illum surface do
not interfere with the shadow testing of others, non illumn sources" (Page
572 in the book Rendering with Radiance).

In Page 568-569 of the book, a decorative luminaries is illustrated to show
why to use illum. One reason of using illum is that the shadow ray in
a point may hit the fixture of the lamp and get a shadow which is incorrect
(it is incorrect because other parts of the lamp may illuminate the
point). Using illum will make the lamp a uniform radiator and therefore a
shadow ray hit the illum will return a nonzero value.

In the above first paragraph , as the book said, the shadow ray (that
hitting the sun) will not be interfered with by the illum (window, in this
example) . But for the decorative luminaries example (in the second
paragraph), My problem is that I do not know which of the following is right
as to the ways of treatment of Shadow ray test of this example.

(1) In this way, we do not need to consider the shadow ray test of the
lamp. In orther words, we can think of the new illum enclosing the lamp as
a special light, and there is no light (lamp) inside the illum, and
therefore, only one shadow ray is sent to the illum and NO shadow ray is
sent to the lamp (inside illum)

(2) It is similar with the example (window illum) in the above first
paragraph, In this case, one shadow ray is sent to hit the lamp and one
shadow ray is sent to hit the illum. Both of them will calculate the direct
part of light source. The illum just shows the luminance distribution of
lights reflected by the lamp fixture rather than lights directly from lamp.

Thank you,

Jia

_______________________________________________
[email protected]

Hello:

If I set maxrad in the "glow" type of luminaires to a positive value, say,
the diagonal of the luminaire dimension. The shadow testing will happen if
the distance between the luminaire and the point is less than maxrad, right?
If the illum rectangles encloses the luminaire, is there shadow testing if
the distance of the point from luminaire is still less than maxrad (and the
point is outside of the illum rectangle) ?

Cheers,
Jia

Whatever the glow distance is set to, no shadow testing will happen to materials through an intervening illum. This is what it says in the reference manual I quoted to you.

-Greg

···

From: Jia Hu <[email protected]>
Date: July 20, 2010 4:28:27 PM PDT

Hello:

If I set maxrad in the "glow" type of luminaires to a positive value, say, the diagonal of the luminaire dimension. The shadow testing will happen if the distance between the luminaire and the point is less than maxrad, right? If the illum rectangles encloses the luminaire, is there shadow testing if the distance of the point from luminaire is still less than maxrad (and the point is outside of the illum rectangle) ?

Cheers,
Jia

Thank you, master Greg!!

···

On Tue, Jul 20, 2010 at 7:44 PM, Greg Ward <[email protected]> wrote:

Whatever the glow distance is set to, no shadow testing will happen to
materials through an intervening illum. This is what it says in the
reference manual I quoted to you.

-Greg

From: Jia Hu <[email protected]>

Date: July 20, 2010 4:28:27 PM PDT

Hello:

If I set maxrad in the "glow" type of luminaires to a positive value, say,
the diagonal of the luminaire dimension. The shadow testing will happen if
the distance between the luminaire and the point is less than maxrad, right?
If the illum rectangles encloses the luminaire, is there shadow testing if
the distance of the point from luminaire is still less than maxrad (and the
point is outside of the illum rectangle) ?

Cheers,
Jia

_______________________________________________
[email protected]

Jia, keep in mind that any glow inside of the illum box only has the
purpose of illustrating a light source (fluorescent tube) and the
shape and the material of the fitting. The light that is emitted from
the luminaire is already accounted for in the illum material
definition which is created from an IES file. Therefore you do not
want your glow to contribute to the illumination of the scene and
mkillum 'hides' it's light contribution.

Thomas

···

On Tue, Jul 20, 2010 at 9:58 PM, Jia Hu <[email protected]> wrote:

I shall change the luminaires material type to "glow" (with maxrad = 0). In
this case, the shadow rays will only be sent to illum rectangle and not be
sent to the luminaire (inside illum rectangle).

Yes, it did take me some time to understand this. Thanks.

···

On Wed, Jul 21, 2010 at 4:57 AM, Thomas Bleicher <[email protected]>wrote:

On Tue, Jul 20, 2010 at 9:58 PM, Jia Hu <[email protected]> wrote:

> I shall change the luminaires material type to "glow" (with maxrad = 0).
In
> this case, the shadow rays will only be sent to illum rectangle and not
be
> sent to the luminaire (inside illum rectangle).

Jia, keep in mind that any glow inside of the illum box only has the
purpose of illustrating a light source (fluorescent tube) and the
shape and the material of the fitting. The light that is emitted from
the luminaire is already accounted for in the illum material
definition which is created from an IES file. Therefore you do not
want your glow to contribute to the illumination of the scene and
mkillum 'hides' it's light contribution.

Thomas

_______________________________________________