function [WorldCoord, residual] = BackProject2UVsBy2Projs( xa, xb, Pa, Pb )

if( size( xa, 1) == 3 )
    xa = xa(1:2) / xa(3);
end;

if( size( xb, 1) == 3 )
    xb = xb(1:2) / xb(3);
end;

% Rename all image a and b params to be like "Left and Right":
uL = xa(1);
vL = xa(2);
uR = xb(1);
vR = xb(2);

aL11 = Pa(1,1);
aL12 = Pa(1,2);
aL13 = Pa(1,3);
aL14 = Pa(1,4);
aL21 = Pa(2,1);
aL22 = Pa(2,2);
aL23 = Pa(2,3);
aL24 = Pa(2,4);
aL31 = Pa(3,1);
aL32 = Pa(3,2);
aL33 = Pa(3,3);
aL34 = Pa(3,4);

aR11 = Pb(1,1);
aR12 = Pb(1,2);
aR13 = Pb(1,3);
aR14 = Pb(1,4);
aR21 = Pb(2,1);
aR22 = Pb(2,2);
aR23 = Pb(2,3);
aR24 = Pb(2,4);
aR31 = Pb(3,1);
aR32 = Pb(3,2);
aR33 = Pb(3,3);
aR34 = Pb(3,4);

B = zeros( 4, 3 );
B(1,1) = uR * aR31 - aR11;
B(1,2) = uR * aR32 - aR12;
B(1,3) = uR * aR33 - aR13;

B(2,1) = vR * aR31 - aR21;
B(2,2) = vR * aR32 - aR22;
B(2,3) = vR * aR33 - aR23;

B(3,1) = uL * aL31 - aL11;
B(3,2) = uL * aL32 - aL12;
B(3,3) = uL * aL33 - aL13;

B(4,1) = vL * aL31 - aL21;
B(4,2) = vL * aL32 - aL22;
B(4,3) = vL * aL33 - aL23;

q = zeros(4,1);
q(1) = aR14 - uR * aR34;
q(2) = aR24 - vR * aR34;
q(3) = aL14 - uL * aL34;
q(4) = aL24 - vL * aL34;

WorldCoord = B \ q;
residual = B * WorldCoord - q;