Detect if Mouse is over an object inside canvas

Question

I have created a line inside a canvas element. I am looking for the easiest way to detect if the position of the mouse is inside the line, which is inside the canvas.

I have used this function to see the position of the mouse inside the canvas, but I am very confused on how I should proceed.

function getMousePos(c, evt) {
            var rect = c.getBoundingClientRect();
            return {
                x: evt.clientX - rect.left,
                y: evt.clientY - rect.top
            };
        }

I have also looked at this topic Fabricjs detect mouse over object path , but it detects if the mouse is inside the canvas, not inside the object.

The line that I create is a part of smaller lines, connected to each other.

 for (var i = 0; i < 140 ; i++) {

                ctx.beginPath();

                ctx.moveTo(x[i],y[i]);
                ctx.quadraticCurveTo(x[i],50,x[i+1],y[i+1]);
                ctx.lineWidth = 40;

                ctx.strokeStyle = 'white';
                ctx.lineCap = 'round';
                ctx.stroke();

            }

where x[i] and y[i] are the arrays with the coordinates that I want.

I hope my question is clear, although I am not very familiar with javascript.

Thanks Dimitra

Answer 1

A Demo: http://jsfiddle.net/m1erickson/Cw4ZN/

You need these concepts to check if the mouse is inside a line:

• Define the starting & ending points of a line

• Listen for mouse events

• On mousemove, check if the mouse is within a specified distance of the line

Here's annotated example code for you to learn from.

$(function() {

  // canvas related variables
  var canvas = document.getElementById("canvas");
  var ctx = canvas.getContext("2d");
  var $canvas = $("#canvas");
  var canvasOffset = $canvas.offset();
  var offsetX = canvasOffset.left;
  var offsetY = canvasOffset.top;

  // dom element to indicate if mouse is inside/outside line
  var $hit = $("#hit");

  // determine how close the mouse must be to the line
  // for the mouse to be inside the line
  var tolerance = 5;

  // define the starting & ending points of the line
  var line = {
    x0: 50,
    y0: 50,
    x1: 100,
    y1: 100
  };

  // set the fillstyle of the canvas
  ctx.fillStyle = "red";

  // draw the line for the first time
  draw(line);

  // function to draw the line
  // and optionally draw a dot when the mouse is inside
  function draw(line, mouseX, mouseY, lineX, lineY) {
    ctx.clearRect(0, 0, canvas.width, canvas.height);
    ctx.beginPath();
    ctx.moveTo(line.x0, line.y0);
    ctx.lineTo(line.x1, line.y1);
    ctx.stroke();
    if (mouseX && lineX) {
      ctx.beginPath();
      ctx.arc(lineX, lineY, tolerance, 0, Math.PI * 2);
      ctx.closePath();
      ctx.fill();
    }
  }

  // calculate the point on the line that's 
  // nearest to the mouse position
  function linepointNearestMouse(line, x, y) {
    //
    lerp = function(a, b, x) {
      return (a + x * (b - a));
    };
    var dx = line.x1 - line.x0;
    var dy = line.y1 - line.y0;
    var t = ((x - line.x0) * dx + (y - line.y0) * dy) / (dx * dx + dy * dy);
    var lineX = lerp(line.x0, line.x1, t);
    var lineY = lerp(line.y0, line.y1, t);
    return ({
      x: lineX,
      y: lineY
    });
  };

  // handle mousemove events
  // calculate how close the mouse is to the line
  // if that distance is less than tolerance then
  // display a dot on the line
  function handleMousemove(e) {
    e.preventDefault();
    e.stopPropagation();
    mouseX = parseInt(e.clientX - offsetX);
    mouseY = parseInt(e.clientY - offsetY);
    if (mouseX < line.x0 || mouseX > line.x1) {
      $hit.text("Outside");
      draw(line);
      return;
    }
    var linepoint = linepointNearestMouse(line, mouseX, mouseY);
    var dx = mouseX - linepoint.x;
    var dy = mouseY - linepoint.y;
    var distance = Math.abs(Math.sqrt(dx * dx + dy * dy));
    if (distance < tolerance) {
      $hit.text("Inside the line");
      draw(line, mouseX, mouseY, linepoint.x, linepoint.y);
    } else {
      $hit.text("Outside");
      draw(line);
    }
  }

  // tell the browser to call handleMousedown
  // whenever the mouse moves
  $("#canvas").mousemove(function(e) {
    handleMousemove(e);
  });

}); // end $(function(){});

body {
  background-color: ivory;
}

canvas {
  border: 1px solid red;
}

<!doctype html>
<html>

<head>
  <link rel="stylesheet" type="text/css" media="all" href="css/reset.css" />
  <!-- reset css -->
  <script type="text/javascript" src="http://code.jquery.com/jquery.min.js"></script>

</head>

<body>
  <h2 id="hit">Move mouse near line</h2>
  <canvas id="canvas" width=300 height=300></canvas>
</body>

</html>

About hit-testing Paths:

If you create Paths using path commands you can use context.isPointInPath(mouseX,mouseY) to check if the mouse is inside a path. context.isPointInPath does not work well with lines however because lines theoretically have zero width to "hit".