how to measure angle between the lines?

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BV 2013-6-6

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编辑： MFB 2019-4-15

采纳的回答： Iain

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?

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Jan 2013-6-6

编辑：Jan 2013-6-6

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 2013-6-6

i wanted a way to calculate the angle.

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Iain 2013-6-6

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在 MATLAB Online 中打开

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

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BV 2013-6-6

thank u very much.

MFB 2019-4-15

编辑：MFB 2019-4-15

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.

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Paul Kelly 2013-6-6

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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

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Iain 2013-6-6

"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.