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function [is_face_hit, hit_position, hit_distance, new_beams] = face_beam_interaction(beam, faces)
%% calculates refracted and reflected beam after interaction with a face
% beam - structure defining the light beam
% beam.origin - array [x,y] origin/soutce of the light beam
% beam.k - k vector i.e. direction [kx,ky]
% beam.intensity - intensity of the beam
% beam.face - if beam starts from face then its index is here
% beam.polarization - can be either 1 for 's' or 2 for 'p'
% faces cell array of face structures
% face - structure definiong the beam
% face.vertex1 - [x,y] of the 1st point/vertex of the face
% face.vertex2 - [x,y] of the 2nd point/vertex of the face
% face.n_left - indexes of refraction on the left hand side
% with respect to 1st -> 2nd vertex direction
% [ns,np] - for s and p polarizations
% face.n_right - indexes of refraction on the right hand side
% [ns,np] - for s and p polarizations
k=beam.k;
polarization=beam.polarization;
%% we go over all faces to find the closest which beam hits
Nfaces=size(faces)(2);
hit_distance=Inf;
is_face_hit = false;
hit_position = [NA, NA];
closest_face_index=NA;
for i=1:Nfaces
if ( beam.face == i) continue; end
face=faces{i};
[hit_distance_tmp, hit_position_tmp, is_face_hit_tmp] = beam2face_distance(beam,face);
if ( hit_distance_tmp < hit_distance )
% this is the closer face
is_face_hit=is_face_hit_tmp;
hit_position=hit_position_tmp;
hit_distance=hit_distance_tmp;
closest_face_index=i;
end
end
if (!is_face_hit)
new_beams={};
return;
end
%% closest face
face=faces{closest_face_index};
kf=face.vertex2 - face.vertex1; % not a unit vector
hold on;
% draw face
line([face.vertex1(1), face.vertex2(1)], [face.vertex1(2), face.vertex2(2)] , 'linestyle', '-', 'color', [0,0,0] );
% draw beam
Ncolors=256;
linewidth=2;
if ( polarization == 1)
% s-polarization
line_base_color=[1,0,0]; % RGB - red
else
% p-polarization
line_base_color=[0,0,1]; % RGB - blue
end
%plot(x,y,line_str);
line_color = color_gradient( beam.intensity*Ncolors, line_base_color, Ncolors);
line([beam.origin(1), hit_position(1)], [beam.origin(2), hit_position(2)], 'color', line_color, 'linewidth', 3, 'linestyle', '-');
% find is beam arriving from left or right. I will use vectors cross product property.
% if z component of the product 'k x kf' is positive then beam arrives from the left
if ( ( k(1)*kf(2)-k(2)*kf(1) ) > 0 )
% beam coming from the left
n1=face.n_left(polarization);
n2=face.n_right(polarization);
% this means that notmal and beam k vector point to the same half plane
% relative to the face
are_nf_and_k_in_the_same_plane=true;
else
% beam coming from the right
n1=face.n_right(polarization);
n2=face.n_left(polarization);
% this means that notmal and beam k vector point to the different half plane
% relative to the face
are_nf_and_k_in_the_same_plane=false;
end
% normal vector to the face, looks to the left of it
nf=[ kf(2), -kf(1) ] / norm(kf);
% incidence angle calculation
cos_theta_i = dot(k, nf) / (norm(k)*norm(nf));
sin_theta_i = - ( k(1)*nf(2)-k(2)*nf(1) ) / (norm(k)*norm(nf));
% positive angle to the right from normal before incidence to the face
theta_i = atan2(sin_theta_i, cos_theta_i);
% we need to make sure that angle of incidence belong to [-pi/2,pi/2] interval
if( theta_i > pi/2)
theta_i=pi-theta_i;
end
if( theta_i < -pi/2)
theta_i=-pi+theta_i;
end
% angle of the normal with respect to horizon
theta_normal = atan2(nf(2), nf(1));
%% reflected beam direction
if (are_nf_and_k_in_the_same_plane)
theta_reflected = theta_normal - theta_i + pi;
else
theta_reflected = theta_normal + theta_i;
end
%% coefficients of reflection and transmission for given polarization
[R,theta_refracted_rel2normal]=fresnel_reflection(n1, n2, theta_i);
reflectivity = R(polarization);
transmission = 1 - reflectivity;
beam_reflected.origin = hit_position;
beam_reflected.k = [cos(theta_reflected), sin(theta_reflected)];
beam_reflected.face=closest_face_index;
beam_reflected.polarization = polarization;
beam_reflected.intensity = beam.intensity * reflectivity;
new_beams{1} = beam_reflected;
%% refracted beam direction
% refracted angle with respect to normal
if ( transmission == 0 )
% total internal reflection
else
% beam refracts
if (are_nf_and_k_in_the_same_plane)
theta_refracted = theta_normal + theta_refracted_rel2normal;
else
theta_refracted = theta_normal - theta_refracted_rel2normal + pi;
end
beam_refracted.origin = hit_position;
beam_refracted.k = [cos(theta_refracted), sin(theta_refracted)];
beam_refracted.face=closest_face_index;
beam_refracted.polarization = polarization;
beam_refracted.intensity = beam.intensity * transmission;
new_beams{2} = beam_refracted;
end
end
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