function ret = beam_trace(beams, faces, borders)
	%% trace beam and all it reflections and refractions on faces
	% for beams and faces structure description see face_beam_interaction.m
	% border similar to faces cell array which enclose the beam
	% i.e. it does not leave the polygon consisting of border faces

	Nbeams=size(beams)(2);
	if ( Nbeams == 0 )
		% no more beam to trace
		return;
	end

	% trace each beam in beams cell array
	for i=1:Nbeams
		beam=beams{i};
		[is_face_hit, hit_position, hit_distance, new_beams] =  face_beam_interaction(beam, faces);
		if (is_face_hit)
			beam_trace(new_beams, faces, borders );
			continue;
		end

		% beam does not hit face but it should stop and borders
		beam.face=NA;
		[is_face_hit, hit_position, hit_distance, new_beams] =  face_beam_interaction(beam, borders);
		if (!is_face_hit)
			error('borders are badly defined, the beam misses them');
		end
	end
end