How to find angle between two straight lines (paths) on a SVG in Javascript?

Question

I have two straight lines as <path> in SVG canvas. Using pixel coordinates of LineA (A1x, A1y) (A2x, A2y) and LineB (B1x, B1y) (B2x, B2y) how can I calculate the angle between these lines.

I have below code which works for THREE points (it works for green cases in below image). It does not work when (A2x, A2y) != (B1x, B1y).

How can I modify this formula to work even when lines are not joined.

function find_angle(p0,p1,c) {
var p0c = Math.sqrt(Math.pow(c.x-p0.x,2)+
                    Math.pow(c.y-p0.y,2));  
var p1c = Math.sqrt(Math.pow(c.x-p1.x,2)+
                    Math.pow(c.y-p1.y,2));
var p0p1 = Math.sqrt(Math.pow(p1.x-p0.x,2)+
                     Math.pow(p1.y-p0.y,2));
var angle = Math.acos((p1c*p1c+p0c*p0c-p0p1*p0p1)/(2*p1c*p0c));
return angle * (180 / Math.PI);
}

Answer 1

You can exploit Math.atan2 function with cross product and dot product of direction vectors for these segments. Note the atan2 returns signed angle in range -Pi...Pi

//find vector components
var dAx = A2x - A1x;
var dAy = A2y - A1y;
var dBx = B2x - B1x;
var dBy = B2y - B1y;
var angle = Math.atan2(dAx * dBy - dAy * dBx, dAx * dBx + dAy * dBy);
if(angle < 0) {angle = angle * -1;}
var degree_angle = angle * (180 / Math.PI);

Answer 2

You want P0-C, P1-D instead of P0-C, P1-C: just translate one of the segments to let D and C coincide: P1' = P1 - D + C (then D' = C).