thanks
I made some test and change material settings
the object with lowest nested dielectric priority :
Hidden from GI
the material settings :
ND = 1
Attenuation = 999 m
Transmittance : any color
Reflectance 0 : RGB = 0,0,0
Reflectance 90 : any color
Roughness = 100
the material give the same result as material with opacity / mask = 0 or matte material
Hidden from GI
the material settings :
ND = 1
Attenuation = 999 m
Transmittance : any color
Reflectance 0 : any color
Reflectance 90 : any color
Roughness = 0
The boolean worked fine
Not Hidden from GI
the material settings :
ND = 1
Attenuation = 999 m
Transmittance : any color
Reflectance 0 : any color
Reflectance 90 : any color
Roughness = 0
The boolean worked fine
Not Hidden from GI
the material settings :
ND = 1
Attenuation = 999 m
Transmittance : any color
Reflectance 0 : RGB = 0,0,0
Reflectance 90 : any color
Roughness = 100
The boolean worked fine
Hidden or Not Hidden from GI
the material settings :
ND = 1
Attenuation = 1 m
Transmittance : R or G or B = 254.9 and the two other values : 255
Reflectance 0 : any color
Reflectance 90 : any color
Roughness = 0
The boolean worked fine
if the object with lowest
nested dielectrics was hidden from Gi ; is better to avoid its shadow ; but the intersection between the two objects don't receive lights from emitters or shadows from other objects ; and always black when the scene iluminated with emitters only