No, I do not understand the question.
Do you want to project the two vectors into a specified plane an determine the angle between the projected vectors? This would equal the view through a 2D X-ray image.
Such "projection angles" can be calculated by the following code:
function Angle = ProjAngleV(L1, L2, LP)
% Angle between 2 vectors projected into plane
% Angle = ProjAngleV(L1, L2, LP)
% INPUT:
% L1, L2, LP: Vectors of dimension [T x 3] or [1 x 3].
% They need not be normalized.
% OUTPUT:
% Angle: Angle between L1 and L2 projected into the plane orthogonal to LP.
% In other words: the angle between L1 and L2 when looking along LP.
% The sign of the angle is positive, if [L1, L2, LP] is a right handed
% system, so the sign changes if L1 and L2 are interchanged.
% Range of output: -180 < Angle <= 180:
% L1 = L2 => Angle = 0
% L1 ~= L2, L1 <-> L2 => Angle <-> -Angle
% Discontinuity at antiparallel lines:
% L1 = -L2 => Angle = 180
% L1 = -L2 + [0,eps,0] => Angle = -180
% Author: Jan Simon, Heidelberg, (C) 2005-2013 matlab.2010@n-simon.de
% License: Use, copy, modify as you like, don't blame me for troubles
% General algorithm:
% A = L1 x LP;
% B = L2 x LP;
% Angle = ATAN2(Norm(A x B), Dot(A, B)) * sign(Dot(L1 x L2, LP))
% Project L1 and L2 into plane perpendicular to LP:
A = CrossTx3(L1, LP);
B = CrossTx3(L2, LP);
% Dot product between projected lines:
AdotB = A(:,1).*B(:,1) + A(:,2).*B(:,2) + A(:,3).*B(:,3);
AxB = [A(:, 2) .* B(:, 3) - A(:, 3) .* B(:, 2), ...
A(:, 3) .* B(:, 1) - A(:, 1) .* B(:, 3), ...
A(:, 1) .* B(:, 2) - A(:, 2) .* B(:, 1)];
Angle = atan2(NormTx3(AxB), AdotB);
% Sign(LX) is zero for LX==0, but this would destroy Inf values.
negS = (DotTx3(AxB, LP) < 0);
Angle(negS) = -Angle(negS);
% Care for infinite values:
% Angle(isinf(AdotB)) = Inf; % -Inf -> Inf
Angle(~isfinite(AdotB)) = Inf; % NaN and -Inf -> Inf
% No valid angle for to0 short input vectors::
Small = 1.490116119384766e-008; % SQRT(EPS)
Angle(sum(L1 .* L1, 2) < Small) = Inf;
Angle(sum(L2 .* L2, 2) < Small) = Inf;
Angle(sum(LP .* LP, 2) < Small) = Inf;
function R = CrossTx3(X, Y)
R = [ X(:,2) .* Y(:,3) - X(:,3) .* Y(:,2), ...
X(:,3) .* Y(:,1) - X(:,1) .* Y(:,3), ...
X(:,1) .* Y(:,2) - X(:,2) .* Y(:,1)];
function R = NormTx3(X)
R = sqrt(X(:, 1) .* X(:, 1) + X(:, 2) .* X(:, 2) + X(:, 3) .* X(:, 3));
function R = DotTx3(X, Y)
R = X(:, 1) .* Y(:, 1) + X(:, 2) .* Y(:, 2) + X(:, 3) .* Y(:, 3);
