Am 03.04.2016 um 13:14 schrieb Stephen:
> My brain has gone dead.
> How do you set up a scene so that a box with an image map is visible
> when it is only lit from behind.


To _light_ an object from its _inside_, there are two alternatives:

(A) double_illuminate

Using "double_illuminate" in an object will cause incident light from
the back side of a surface to cause the exact same effect as light from
the front side (as far as diffuse illumination is concerned; it does not
cause highlights).

(B) two-parameter diffuse

Using "diffuse FRONT,BACK" in a finish does virtually the same thing,
except that this property is tied to the finish rather than the object,
and allows different brightness levels depening on what side of the
surface the light is coming from.


To _light_ a solid object from its _back_, there are two alternatives as
well (presuming you want a diffuse effect):

(A) dense scattering media

(B) subsurface light transport


> It is visible when the background is
> not black.

Not sure what to make of this sentence.

Maybe I misunderstood your request, and what you're really after is just
coloured transparency? In that case, there's only one "alternative":

(A) filtering pigment

Using "colour rgb SOMETHING filter 1.0" will cause the background to be
visible, but tinted with the specified colour.

To achieve the same effect with an image-based pigment, use "image_map {
... filter all 1.0 }"

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