calculate the angle between a line (between two points) and the perpendicular line

if point1 is a(x1,y1) and point 2 is b(x2,y2). I want to draw a vertical from point b in order to calculate the angle between the line ab and the verticle line.

I wrote this code but I think i can't get the verticle line correctly as I got the same value of the angle and i'm sure the angle value is not the same.

 for i=1:size(a,1)
   angle =  atand(b(1,i) - a(1,i))-(b(2,i) - a(2,i)))
 end

any one can help me please?

[Geoff Hayes]

Amneh - I don't understand why you need a for loop especially as you overwrite the angle on each iteration of the loop. Where have you defined the vertical line? Could you use a third point (for this line) as c = (x2, 2*fabs(y2)) (arbitrarily choosing a position on the y-axis) and then use the usual equation for finding the angles of a triangle (your three points)?

[Geoff Hayes]

Amneh's answer moved here

I'm using a loop because points a and b are moving and changing their positions. I want to caluctae the angle for every fram. I assumed that the vericle line is between (point a (x1,y1) and Point (x1,y2))

[Geoff Hayes]

I assumed that the vericle line is between (point a (x1,y1) and Point (x1,y2)) Can you provide a picture? I don't understand how the vertical line is between (x1,y1) and (x1,y2) (or do you mean (x2,y2) for the second one?). Also I thought I want to draw a vertical from point b which suggests that the vertical line is drawn from point b and is not in between the two points. Please clarify.

Answer 1

[Jan]

Maybe you mean:

angle =  atand(b(1,:) - a(1,:)) - (b(2,:) - a(2,:)))
%                                                  ^???

without a loop. But here the number of parentheses does not match. I'm not sure which problem you want to solve.

Answer 2

[Geoff Hayes]

Amneh - if a is (x1,y1) and b is (x2,y2) then let c be (x2,y1). With these three points, you should have a right-angled triangle. We can determine the opposite and adjancent as

opposite = abs(a(1) - c(1));
adjacent = abs(c(2) - b(2));

and so the angle is

myAngleDegrees = atand(opposite / adjacent);

(I think!)

Answer 3

[Amneh Alshawabka]

Thanks alot for your help . I'm a new Matlab user.

Actually i'm not sure how to creat a vertical line either from point a or b!! I want to caculate the angle between line ab and the verticale line from b to the ground as shown in the picture below.

[Jan]

Please post comments in the section for comments. The best location for the picture is the original question, where readers expect the full information (I've inserted it there).