Update HTML5 canvas rectangle on hover?

This is a stable code in base of @K3N answer. The basic problem of his code is because when one box is over the another the two may get mouse hover at same time. My answer perfectly solves that adding a 'DESC' to 'ASC' loop.

var canvas = document.getElementById("canvas"),
    ctx = canvas.getContext("2d");

var map = [
    {x: 20, y: 20, w: 60, h: 60},
    {x: 30, y: 50, w: 76, h: 60}
];

var hover = false, id;
var _i, _b;
function renderMap() {
    for(_i = 0; _b = map[_i]; _i ++) {
        ctx.fillStyle = (hover && id === _i) ? "red" : "blue";
        ctx.fillRect(_b.x, _b.y, _b.w, _b.h);
    }
}
// Render everything
renderMap();
canvas.onmousemove = function(e) {
    // Get the current mouse position
    var r = canvas.getBoundingClientRect(),
        x = e.clientX - r.left, y = e.clientY - r.top;
    hover = false;

    ctx.clearRect(0, 0, canvas.width, canvas.height);

    for(var i = map.length - 1, b; b = map[i]; i--) {
        if(x >= b.x && x <= b.x + b.w &&
           y >= b.y && y <= b.y + b.h) {
            // The mouse honestly hits the rect
            hover = true;
            id = i;
            break;
        }
    }
    // Draw the rectangles by Z (ASC)
    renderMap();
}
<canvas id="canvas"></canvas>

You may have to track the mouse on the canvas using JavaScript and see when it is over your rectangle and change the color then. See code below from my blog post

<!DOCTYPE html>
<html>
<body>

<canvas id="myCanvas" width="700" height="500" style="border:1px solid #c3c3c3;">
Your browser does not support the HTML5 canvas tag.
</canvas>

<script>
var myRect={x:150, y:75, w:50, h:50, color:"red"};
var c = document.getElementById("myCanvas");
var ctx = c.getContext("2d");
ctx.fillStyle = myRect.color;
ctx.fillRect(myRect.x, myRect.y, myRect.w, myRect.h);

c.addEventListener("mousemove", function(e){
if ((e.clientX>=myRect.x)&(e.clientX<=myRect.x+myRect.w)&(e.clientY>=myRect.y)&(e.clientY<=myRect.y+myRect.h)){
myRect.color = "green";}
else{
myRect.color = "red";}
updateCanvas();
}, false);


function updateCanvas(){
ctx.fillStyle = myRect.color;
ctx.fillRect(myRect.x, myRect.y, myRect.w, myRect.h);
}
</script>

</body>
</html>

I believe this is a slightly more in-depth answer that would work better for you, especially if you are interested in game design with the canvas element.

The main reason this would work better for you is because it focuses more on an OOP (object orientated programming) approach. This allows for objects to be defined, tracked and altered at a later time via some event or circumstance. It also allows for easy scaling of your code and in my opinion is just more readable and organized.

Essentially what you have here is two shapes colliding. The cursor and the individual point / object it hovers over. With basic squares, rectangles or circles this isn't too bad. But, if you are comparing two more unique shapes, you'll need to read up more on Separating Axis Theorem (SAT) and other collision techniques. At that point optimizing and performance will become a concern, but for now I think this is the optimal approach.

const canvas = document.querySelector('canvas');
const ctx = canvas.getContext('2d');
const width = canvas.width = window.innerWidth;
const height = canvas.height = window.innerHeight;
const cx = width / 2;
const cy = height / 2;
const twoPie = Math.PI * 2;
const points = []; // This will be the array we store our hover points in later

class Point {
  constructor(x, y, r) {
    this.x = x;
    this.y = y;
    this.r = r || 0;
  }
}

class HoverPoint extends Point {
  constructor(x, y, r, color, hoverColor) {
    super(x, y, r);
    this.color = color;
    this.hoverColor = hoverColor;
    this.hovered = false;
    this.path = new Path2D();
  }

  draw() {
    this.hovered ? ctx.fillStyle = this.hoverColor : ctx.fillStyle = this.color;
    this.path.arc(this.x, this.y, this.r, 0, twoPie);
    ctx.fill(this.path);
  }
}

class Cursor extends Point {
  constructor(x, y, r) {
    super(x, y, r);
  }

  collisionCheck(points) {
  // This is the method that will be called during the animate function that 
  // will check the cursors position against each of our objects in the points array.
    document.body.style.cursor = "default";
    points.forEach(point => {
      point.hovered = false;
      if (ctx.isPointInPath(point.path, this.x, this.y)) {
        document.body.style.cursor = "pointer";
        point.hovered = true;
      }
    });
  }
}

function createPoints() {
  // Create your points and add them to the points array.
  points.push(new HoverPoint(cx, cy, 100, 'red', 'coral'));
  points.push(new HoverPoint(cx + 250, cy - 100, 50, 'teal', 'skyBlue'));
  // ....
}

function update() {
  ctx.clearRect(0, 0, width, height);
  points.forEach(point => point.draw());
}

function animate(e) {
  const cursor = new Cursor(e.offsetX, e.offsetY);
  update();
  cursor.collisionCheck(points);
}

createPoints();
update();
canvas.onmousemove = animate;

There is one more thing that I would like to suggest. I haven't done tests on this yet but I suspect that using some simple trigonometry to detect if our circular objects collide would preform better over the ctx.IsPointInPath() method.

However if you are using more complex paths and shapes, then the ctx.IsPointInPath() method would most likely be the way to go. if not some other more extensive form of collision detection as I mentioned earlier.

The resulting change would look like this...

class Cursor extends Point {
  constructor(x, y, r) {
    super(x, y, r);
  }

  collisionCheck(points) {
    document.body.style.cursor = "default";
    points.forEach(point => {
      let dx = point.x - this.x;
      let dy = point.y - this.y;
      let distance = Math.hypot(dx, dy);
      let dr = point.r + this.r;

      point.hovered = false;
      // If the distance between the two objects is less then their combined radius
      // then they must be touching.
      if (distance < dr) {
        document.body.style.cursor = "pointer";
        point.hovered = true;
      }
    });
  }
}

here is a link containing examples an other links related to collision detection

I hope you can see how easily something like this can be modified and used in games and whatever else. Hope this helps.


You can't do this out-of-the-box with canvas. Canvas is just a bitmap, so the hover logic has to be implemented manually.

Here is how:

  • Store all the rectangles you want as simple object
  • For each mouse move on the canvas element:
    • Get mouse position
    • Iterate through the list of objects
    • use isPointInPath() to detect a "hover"
    • Redraw both states

Example

var canvas = document.querySelector("canvas"),
    ctx = canvas.getContext("2d"),
    rects = [
        {x: 10, y: 10, w: 200, h: 50},
        {x: 50, y: 70, w: 150, h: 30}    // etc.
    ], i = 0, r;

// render initial rects.
while(r = rects[i++]) ctx.rect(r.x, r.y, r.w, r.h);
ctx.fillStyle = "blue"; ctx.fill();

canvas.onmousemove = function(e) {

  // important: correct mouse position:
  var rect = this.getBoundingClientRect(),
      x = e.clientX - rect.left,
      y = e.clientY - rect.top,
      i = 0, r;
  
  ctx.clearRect(0, 0, canvas.width, canvas.height); // for demo
   
  while(r = rects[i++]) {
    // add a single rect to path:
    ctx.beginPath();
    ctx.rect(r.x, r.y, r.w, r.h);    
    
    // check if we hover it, fill red, if not fill it blue
    ctx.fillStyle = ctx.isPointInPath(x, y) ? "red" : "blue";
    ctx.fill();
  }

};
<canvas/>

Tags:

Javascript

Html

Canvas

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