Le 2021-01-03 à 05:36, kurtz le pirate a écrit :
> Hello,
> 
> Little brain teaser : the attached image is part of an animation that 
> simulates a simplified sundial. In order not to overload the image, the 
> shadow of the sundial has been removed with the "no_shadow" attribute. 
> But not for the gnomon to get the "time". As you can see, its shadow is 
> also projected on the ground "through" the wall.
> 
> So, how to remove this shadow on the floor but keep the shadow on the 
> wall ?
> 

First a little question : Does the illumination of the floor need to 
vary according to the Sun's position ?

If no, then, you can do the following :
Remove that no_shadow as you'll no longer need it.
Change the finish of the floor for this :
finish{ diffuse 0 emission 0.6 }

That way, the floor no longer receive any illumination from the light, 
and the emission make it clearly visible.

If the illumination of the floor must reflect the Sun's location, use a 
light_group :

Place the floor in a light_group.
In that, have a light that mirrors the main light.
Have the diffuse set as, keeping the rest of the finish unchanged :
finish { diffuse 0, 0.6 }
Add an inside_texture with : pigment {rgbt 1}

The front diffuse don't diffuse any light. The backside part reproduce 
the normal diffuse.
The inside_texture ensure that the light coming from bellow won't get 
blocked by the bottom part of the floor.

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