Shortest distance from CGPoint to segment

Question

I've been trying to implement Douglas-Peucker algorithm into my code and I'm able to translate pseudocode into Swift, except for the shortestDistanceToSegment function. Only Swift version I could find was answered here but I don't understand what that actually does.

I need a function that gets three points as arguments (point and both ends of line) and returns the shortest distance between a CGPoint and a line segment. Some explanation about what (and why) the code does would great but not necessary.

The code you refer from another SO answer is pretty well-documented; what about it don't you understand? — Scott Hunter, Feb 13 '15 at 17:36
so you're expecting someone to do it for you? Try something for yourself — martskins, Feb 13 '15 at 17:36
@ScottHunter I find variable names such as A, B, C, D, xx and yy quite difficult to understand, so I must admit that even though it would work, I wouldn't understand the code. — Aleksi Sjöberg, Feb 13 '15 at 18:09
@lascort I would like to see how it's done in Swift, yes. And explained also. — Aleksi Sjöberg, Feb 13 '15 at 18:11

score 15 · Accepted Answer · edited May 23 '17 at 10:29

Answer from https://stackoverflow.com/a/27737081/535275 w/ variables renamed & some comments added:

/* Distance from a point (p1) to line l1 l2 */
func distanceFromPoint(p: CGPoint, toLineSegment v: CGPoint, and w: CGPoint) -> CGFloat {
    let pv_dx = p.x - v.x
    let pv_dy = p.y - v.y
    let wv_dx = w.x - v.x
    let wv_dy = w.y - v.y

    let dot = pv_dx * wv_dx + pv_dy * wv_dy
    let len_sq = wv_dx * wv_dx + wv_dy * wv_dy
    let param = dot / len_sq

    var int_x, int_y: CGFloat /* intersection of normal to vw that goes through p */

    if param < 0 || (v.x == w.x && v.y == w.y) {
        int_x = v.x
        int_y = v.y
    } else if param > 1 {
        int_x = w.x
        int_y = w.y
    } else {
        int_x = v.x + param * wv_dx
        int_y = v.y + param * wv_dy
    }

    /* Components of normal */
    let dx = p.x - int_x
    let dy = p.y - int_y

    return sqrt(dx * dx + dy * dy)
}

Shortest distance from CGPoint to segment

1 Answers1