"muyu" <lsy### [at] gmailcom> wrote:
> I am simulating outdoor scene using radiosity. Regarding the light source, I
> used one parallel light to simulate the beam. However, how could I simulate the
> hemispherical diffuse radiation? Thanks in advance.
>
> Shouyang

Create a hemisphere (perhaps with an opaque flat back) and assign a light source
to it.

From the [F1] documentation

3.4.3.1.7 Looks Like

By default a light source has no visible shape. The light simply radiates from
an invisible point or area, however there are cases where this is not desired.
Using looks_like is as an easy way to override this behavior. There is an
implied no_shadow so that light is not blocked by the object, without it the
light inside a non-transparent object could not escape. The object would, in
effect, cast a shadow over everything.

When using looks_like there are a few important things to consider:
1.the object should be positioned at the origin
2.it's generally easier but not necessary to declare the object beforehand
3.works with point and spot lights not parallel lights
4.use a union instead if you want the object to block light and remember to make
some portion of the object transparent

See the following examples:
#declare My_Lamp_Shape = sphere { <0, 0, 0>, Some_Radius }

// using looks_like
light_source {
  <100, 200, -300> color White
  looks_like { My_Lamp_Shape }
  }

// using union
union {
  light_source { <100, 200, -300> color White }
  object { My_Lamp_Shape translate <100, 200, -300> }
  }

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