[Radiance-general] illum examples

[Jia Hu]

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
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