angle between two points
snippet in c

angle between two points

user3405

double angle = atan2(y2 - y1, x2 - x1) * 180 / PI;".

processing angle between two points

user3774

PVector v1 = new PVector(10, 20);
PVector v2 = new PVector(60, 80); 
float a = PVector.angleBetween(v1, v2);
println(degrees(a));  // Prints "10.304827"