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Finding an angle for a line to point at.

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IronFist    122
What I am trying to do is draw a line from a central point out to the x and y coordinates of the mouse, and the line has to be a certain length. In other words, I want the line to be pointing at the mouse all the time from a central point, but it cannot be longer than a set length (So I can't just have it draw a line from the central point to the mouse coords).
C - Central pivot point (beginning of the line)
E - end of line 
/ - line (the length of the line is the distance between # and @)
. - imaginary line
M - Mouse location
So far, to draw a line that is a certain length, I would do something like this:
xfinalpos = xbeginposition + sin(20*(PI/180))*linelength;
yfinalpos = ybeginposition + cos(20*(PI/180))*linelength;
That works because I already know the angle -> 20. But when I move the mouse around the central point, I don't know the angle. I think I need to use arcsin, arccos, or arctan, but I am not sure which one, or how to use them. Can anyone help me do this? Edited by - IronFist on July 28, 2001 2:25:42 AM

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mossmoss    326
Note that this sample code is C++ and certainly not the best setup, nor functionally complete, but should suffice to show how to get things done without need for cos/sin/tan/arc-whatever. It does involve a square root (the length of a vector is the square root of the dot product of the vector with itself), but otherwise it''s pretty simple.

Also note that this doesn''t check to see if the mouse is closer to the center point than the specified distance. It shouldn''t be difficult to add that, though.

struct Point {
float x, y;
float len() const { return fsqrt(x*x + y*y); }
void normalize() {
float oo_len = 1.0f / len(); //one over length

x *= oo_len;
y *= oo_len;
Point& operator-=(const Point& rhs) {
x -= rhs.x;
y -= rhs.y;
return *this;
friend Point operator-(const Point& a, const Point& b) {
Point r = a;
r -= b;
return r;
Point& operator*=(float k) {
x *= k;
y *= k;
return *this;

Point get_radial_point(const Point& source, const Point& target, float dist)
//get the direction

Point aim = target - source;

//normalize it (ie, make its length == 1)


//scale it by the desired distance

aim *= dist;

return aim;

//somewhere in your code

Point mouse = read_mouse();
Point center = calculate_center();
Point endpoint = get_radial_point(center, mouse, distance);
draw_line(center, endpoint);

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