Reference Talk:Light Source

Suggested looks_like changes

Here is the current wording:

Normally the light source itself has no visible shape. The light simply radiates from an invisible point or area. You may give a light source any shape by adding a looks_like { OBJECT } statement.

There is an implied no_shadow attached to the looks_like object so that light is not blocked by the object. Without the automatic no_shadow the light inside the object would not escape. The object would, in effect, cast a shadow over everything.

If you want the attached object to block light then you should attach it with a union and not a looks_like as follows:

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

Presumably parts of the lamp shade are transparent to let some light out.

Here is my suggested wording:

Normally the light source itself has no visible shape. The light simply radiates from an invisible point or area. You may give a light source any shape by adding a looks_like { OBJECT } statement.

There is an implied no_shadow attached to the looks_like object so that light is not blocked by the object. Without the automatic no_shadow the light inside the object would not escape. The object would, in effect, cast a shadow over everything.

When creating the light source, the looks_like object should be positioned at the origin. For instance:

union {
  light_source { <100, 200, -300> color White }
  object {
    sphere { <0, 0, 0>, My_Lamp_Radius }
    }
  }

Alternately, you could declare the object beforehand and reference the object from within the light source, like this:

#declare My_Lamp_Shape = sphere { <0, 0, 0>, My_Lamp_Radius }

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

For point lights and spotlights you could also place the light source and the looks_like object at the origin and translate them together later. But for parallel lights this will not work.

If you instead want the attached object to block light then you should attach it with a union and not a looks_like as follows:

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

Presumably parts of the lamp shade are transparent to let some light out.

SharkD 22:48, 8 September 2016 (UTC)

Oops, I was wrong. But translation is *dangerous*. As per clipka's response to my post:

And that direction is unchanged: With a parallel light source, the

direction of all light rays depends only on the /relative/ location of two points: The light source's nominal center and the point_at parameter. But transformations act on both these points (I've just checked the code), so a translation operation changes only the absolute location of the points, not their relative location to one another. And

thus the light direction remains unimpressed.

I.e. translating a light source translates the location of the source of the light *and* the location the light is pointing at, which could produce unwanted effects. SharkD 16:59, 12 September 2016 (UTC)