Find Angle Between Two Line

How i want to find the angle between two line.. Example my code:

%%%Angle Between Two Line%%%
% Line 1: point(0,3) to (4,3)
% Line 2: point(0,2) to (3,0)

v1=[4,3]-[0,3]; 
v2=[0,2]-[3,0];

angle = mod( atan2( det([v1,v2]) , dot(v1,v2) ) , 2*pi );

what i get was

Error using ==> det
Matrix must be square..

Why I cannot get the angle.???

Answer 1

[Andrei Bobrov]

v1=[4,3]-[0,3]; 
v2=[0,2]-[3,0];
angle = mod( atan2( det([v1;v2]) , dot(v1,v2) ) , 2*pi );

EDIT

a1 = mod(atan2( det([v1;v2;]) , dot(v1,v2) ), 2*pi );
angleout = abs((a1>pi/2)*pi-a1)

[mnhafizuddin]

thank for your answer..

but why i calculate manually the angle and not get the exactly answer like matlab..???

It is my coding is wrong.???

[Jan]

@mnhafizuddin: I don't understand the question. The problem of your code is the comma in "[v1,v2]", bcause it creates a [1 x 4] vector, but DET needs a [2x2] matrix. See some of the most popular thread in CSSM: http://www.mathworks.com/matlabcentral/newsreader/view_thread/151925#870338

[mnhafizuddin]

thanks for your suggestion website..

It's really helps me alot..

thanks!!!

Answer 2

[Geoff]

When I want to find the angle between two vectors, I take the dot product of the unit vectors:

clamp = @(val, low, high) min(max(val,low), high);
angle = acos( clamp(dot(v1,v2) / norm(v1) / norm(v2), -1, 1);

If you want to know which direction that's in, you can do a trick with the cross-product. Since it's 2D, you do this:

vc = cross([v1,0], [v2,0]);
anticlock = vc(3) > 0;

[mnhafizuddin]

what is the function clamp for.???

[Jan]

ACOS(DOT(u,v)) is instable if u and v are near to (anti)-parallel. ASIN(CROSS()) is not sufficient near to perpendicular vectors. Therefore the ATAN2 approach is recommended.

[Geoff]

Oh righto. I never had issues with stability. But that's what the clamp is for. It prevents the value from exceeding 1 (or -1) due to floating-point precision errors.