(This page wouldn't be there without the help and explanations of  Eric Huchet alias Huric, who is the forum master of news://news. newz.net/nzn.fr.3D, and who has produced the pictures accompanying this page as well as the example file. I hope I haven't betrayed his tought. )

When you're constructing the lighting of a 3D scene, unless you are using a radiosity module, you do not have to think like in the real life, it's a mistake.

We can  -  we must  -  place  a light source hidden by a wall or any else object and use the appropriate distance  to obtain the desired effects. The process may appear strange. However this is a particularity of the omnidirectional lighting in scan line  rendering type. For these sources no shadows are cast on the other objects of the scene (we will see later on how the shadow can and must be simulated). Moreover, nothing stop the luminous ray, only the opposite faces of the polygons are shaded.
 

At the end, we finally must conclude that the lighting of an interior scene through the exterior is the best system that we have to simulate a radiosity effect and we can even extend this principal to any kind of 3D lighting.  In lighting the opposite wall through the exterior we can precisely control the spreading of light on the surface. Explanation ...	oooooooooooooooooooooooo        o                                                 o        o                                                 o        o                          X--->              o        o                                                 o        o                                                 o        oooooooooooooooooooooooo
If we locate the luminous source symbolized through "X" in the interior of the scene, we can be sure to obtain "stains" of light and a "bad distribution" of the light like the one here after :
Here, of course, the effect is deliberately emphasized. This is due to the fact that the light source is too close of the wall and the way the luminous intensity is calculated  : depending on the angle with the lighted surface. When the source is too close, the fluctuation of the angle is very hard, which produces a not very regular lighting. Moving the source away from the lighted surface, and even moving it out of the scene ... we obtain a less important angle fluctuation, and thus a more regular spreading of light on the lighted surface.	oooooooooooooooooooooooo                o                                                 o                o                                                 o       X-----o---------------------->                   o                o                                                 o                o                                                 o                oooooooooooooooooooooooo
Radiance of the wall
Red exterior light source parameters
Light reflected by objects Putting light sources in the interior of an object is also a manner to approach as close as possible of the realistic rendering. In this case a light of the "Sphere" type allows to control the intensity since this one is inversely proportional to the distance of the centre .
Radiance of the blue sphere
	The Blue sphere light source parameters
The dropped shadows  Obviously it is necessary to add a projector (Spot) to obtain dropped shadows. However there is always a more dense shadow area. To obtain this effect we add a negative light that, as it's name indicates it, produces darkness.
contact shadow
Negative light parameters
Dropped Shadows can be entirely simulated with negative light. You can test this effect in the here attached file.

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