Le 2021-01-08 à 23:06, Subclick a écrit :
> "Kenneth" <kdw### [at] gmailcom> writes:
>> At first, I thought that the negative light_source would 'invert' the color of
>> the wall object; but that's not the case.
> 
> If it were the only light shining on the wall, those pixels would get
> overall negative color components (each—negative—component of the light
> multiplied by the corresponding—positive—component of the wall’s pigment
> and by its—positive—diffuse value, plus the highlights, which are
> similarly negative, plus the ambient illumination, which is positive,
> but too small to overcome the two previous summands), which would in
> turn be clipped to zero—pure black—in the final image; but since there
> is also a global light with the opposite—positive—intensity, it just
> cancels it out.
> 
> Another option would be to leave out “no_shadow” for the gnomon, and
> instead to fill in its shadow on the ground with another positive light
> projected through the gnomon, in a light group with the ground.
> However, I think it’d be easy to notice rounding errors around the edge
> of the shadow, where it’s not exactly cancelled out, especially in an
> animation.  With the negative light projected through the gnomon onto
> the wall, there’s no such problem, since the eye has no way to tell our
> makeshift shadow is not exactly the same as the one it’d see if we let
> the gnomon cast one normally.  Only if there were no ambient light might
> it notice the shadow is not perfectly black.
> 
>> (Note that the light_group requires global_lights ON).
> 
> Certainly.
> 

The faked shadow using projected_through can match exactly. The only 
thing needed is that the location of the light with projected_through 
exactly match the general light : Same location, and eventually, 
area_light parameters.


light_source{1000 rgb <0,1,1> area_light z x 17 17 adaptive 0 circular 
orient} // A cyan light

light_group{
  light_source{1000 rgb <1,-1,-1> // make the shadow red
   area_light z x 17 17 adaptive 0 circular orient
   projected_through object{Gnomon}}
  object{Gnomon texture{Gnomon_Texture}}
  global_lights on
}

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