Line to Point Distance in Processing.

line_point_distance.pde

void setup() {
  size(800, 600);
  background(255);
  noLoop();
}

void draw() {
  line(50, 50, 750, 550);
}

void mouseMoved() {
  println(lineDistance(50, 50, 750, 550, mouseX, mouseY));
}

float lineDistance(int x1, int y1, int x2, int y2, int mX, int mY) {
  PVector lineStart, lineEnd, mouse, projection, temp;

  lineStart = new PVector(x1, y1);
  lineEnd = new PVector(x2, y2);
  mouse = new PVector(mX, mY);
  
  temp = PVector.sub(lineEnd, lineStart);

  float lineLength = temp.x * temp.x + temp.y * temp.y; //lineStart.dist(lineEnd);

  if (lineLength == 0F) {
    return mouse.dist(lineStart);
  }

  float t = PVector.dot(PVector.sub(mouse, lineStart), temp) / lineLength;

  if (t < 0F) {
    return mouse.dist(lineStart);
  }

  if (t > lineLength) {
    return mouse.dist(lineEnd);
  }

  projection = PVector.add(lineStart, PVector.mult(temp, t));

  return mouse.dist(projection);
}

t > lineLength should be t > 1, as t is a magnitude of the line vector and is not within the scope of lineLength