how to measure angle between the lines?

I have plotted the lines with the following code.

x = [3 0];

y = [0 9];

line(x,y);

a = [1 2];

b = [0 3];

line(a,b);

axis([0 4 0 10]);

Now, how can I measure the angle between these lines?

[Jan]

The mathematical angle, the angle of the created lines in screen coordinates or the physical angle on the screen (while the last two are almost identical on modern flat screens, while they can differ substantially on arched CRT monitors)?

Do you want to "measure" the angle or to calculate it?

[BV]

i wanted a way to calculate the angle.

Answer 1

[Iain]

 diff = (atan((y(2)-y(1))/(x(2)-x(1))) - atan((b(2)-b(1))/(a(2)-a(1)))) * 180/pi;

[BV]

thank u) so the difference what i get out of this('diff' code) is the angle?

[Iain]

Yup

 Angle1 = atan((y(2)-y(1))/(x(2)-x(1))) 
 Angle2 = atan((b(2)-b(1))/(a(2)-a(1)))

Are the angles each line makes to the x-axis. The difference is the angle between them in radians. *180/pi is the conversion to degrees.

[Jan]

diff() is an important Matlab command. Shadowing this name by using it as variable can lead to unexpected problems.

[BV]

thank u very much.

[MFB]

difference = (atan((y(2)-x(2))/(y(1)-x(1))) - atan((b(2)-a(2))/(b(1)-a(1)))) * 180/pi;

actually this will be the correct formula arrangement if you consider x as vector points and y as vector points making a line together and a and b making a line together. Algebraically, Jan gave correct formula to calculate angle difference. Thanks.

Answer 2

[Paul Kelly]

You can fit a straight line using polyval:

p = polyval(x, y, 1)

p(1) is the gradient and you can calculate the angle:

a = atan(p(1))

If you do this for each line you have two angles and can calculate the difference

Whether you mean measure or calculate (as per Jan's question) you will have to decide the coordinate system

[BV]

is there a way to plot these lines in 3D?

[Iain]

"plot3" is a function that lets you plot lines in 3D.

If you are trying to find the angle between two lines, in a 3D space, then my solution is NOT the one you want. Mine only works for coplanar lines and an axis set that matches that plane.

[benedikta siboro]

I want to measure angle of human body using GUI.how i can measure?