Find coordinates to draw arrow head (isoscele triangle) at the end of a line

问题

I am trying to create a function that will return the 3 points coordinates of arrow head (isoscele triangle) that I want to draw at the end of a line.

The challenge is in the orientation (angle) of the line that can vary between 0 and 360 degree in the quadrant.

I have the following values:

//start coordinates of the line
var x0 = 100;
var y0 = 100;

//end coordinates of the line
var x1 = 200;
var y1 = 200;

//height of the triangle
var h = 10;
//width of the base of the triangle
var w = 30 ;

This is my function until now that returns the two point coordinates of the base of the triangle:

var drawHead = function(x0, y0, x1, y1, h, w){
    var L = Math.sqrt(Math.pow((x0 - x1),2)+Math.pow((y0 - y1),2));

    //first base point coordinates
    var base_x0 = x1 + (w/2) * (y1 - y0) / L;
    var base_y0 = y1 + (w/2) * (x0 - x1) / L;

    //second base point coordinates
    var base_x1 = x1 - (w/2) * (y1 - y0) / L;
    var base_y1 = y1 - (w/2) * (x0 - x1) / L;

    //now I have to find the last point coordinates ie the top of the arrow head
}

How can I determine the coordinates of the top of the triangle considering the angle of the line?

回答1:

The head of the arrow will lie along the same line as the body of the arrow. Therefore, the slope of the line segment between (x1, y1) and (head_x, head_y) will be the same as the slope of the line segment between(x0, y0) and (x1, y1). Let's say that dx = head_x - x1 and dy = head_y - y1 and slope = (y1 - y0) / (x1 - x0). Therefore, dy / dx = slope. We also know that dx^2 + dy^2 = h^2. We can solve for dx in terms of slope and h. Then, dy = dx * slope. Once you have dx and dy, you can just add those to x1 and y1 to get the head point. Some pseudocode:

if x1 == x0: #avoid division by 0
    dx = 0
    dy = h
    if y1 < y0:
        dy = -h #make sure arrow head points the right way
    else:
        dy = h
else:
    if x1 < x0: #make sure arrow head points the right way
        h = -h
    slope = (y1 - y0) / (x1 - x0)
    dx = h / sqrt(1 + slope^2)
    dy = dx * slope
head_x = x1 + dx
head_y = y1 + dy

回答2:

I see it like this:

A=(x0,y0) , B=(x1,y1) are the known line endpoints
dir=B-A; dir/=|dir|; is unit vector of direction of line (|| is vector size)

dir.x=B.x-A.x;'
dir.y=B.y-A.y;
dir/=sqrt((dir.x*dir.x)+(dir.y*dir.y));

so you can use it and its 90 degree rotation as as a basis vectors. Let q be the 90 degrees rotated vector, in 2D it is easy to obtain:

q.x=+dir.y
q.y=-dir.x

so now you can compute your wanted points:

C=B-(h*dir)+(w*q/2.0);
D=B-(h*dir)-(w*q/2.0);

it is just translation by h and w/2 along basis vectors

来源：https://stackoverflow.com/questions/31462791/find-coordinates-to-draw-arrow-head-isoscele-triangle-at-the-end-of-a-line