I am looking for an algorithm to calculate the angle between two points (defined by x and y valaue) in an image. The angle should be between the horizontal-line of one of these points and the line that connects the two points.
How can I do that?
For points (x1,y1) and (x2,y2) use this:
atan2(y2-y1,x2-x1)
This will return the angle between a horizontal vector from (x1,y1) to (x1+1,y1) and the vector from (x1,y1) to (x2,y2).