Could someone explain me the difference between -i and -I in rtrace command?
I want to sample irradiance on a few points at the center of polygons.
With -i option, I placed the “sensor” slightly on top of the polygon and the normal is pointing down to the polygon.
With -I option, I placed the sensor on the polygon plane and the normal pointing upward.
I was expecting to get the same but it’s not the case. Is it because when I use -i the ray energy is affected by the polygon material when with -I option it’s not the case?
Then what is the best to do in my case? Basically I just want to get the irradiance value arriving to those “sensors”.
They should return the same result, as you expected. Were the rays you sent with the -i option in the correct direction to intersect the same surface points as those placed manually with the -I option? If so, then you may see random sampling differences, which are always present at some level.
How far above do you move the starting point with the -i rays? A distance of at least 1e-4 is recommended, since Radiance refuses to accept an intersection within 10^-6 of the ray origin. Also, why does your plot say “Upward with i”?
Just to check – what material is your polygon made of? The -i option will bypass “void” polygons or any surface made with a transparent (e.g., glass, trans) material.