Change cursor depending on section of canvas

Question

I have a canvas and I want to change the users cursor (like style cursor pointer crosshair move etc).

Is it possible to change the users cursor while its over a certain area of my canvas without introducing "hit boxes" with style of cursor on these hit boxes?

Answer 1

Shapes drawn on the canvas do not individually receive mouse events, so individual shapes cannot receive hover events.

A shape drawn on the canvas can be represented as a set of path commands

A Shape == A set of path commands.

// Example: A set of path commands drawing a triangle
context.beginPath();
context.moveTo(50,50);
context.lineTo(75,100);
context.lineTo(25,100);
context.closePath();

To change cursors when hovering over individual shapes you must do mouse hit-testing versus each shape (versus each path).

You can hit-test a shape (a path) using the isPointInPath method.

To use isPointInPath you must re-issue the path command for a shape (but no need to stroke or fill) and then call isPointInPath with the current mouse coordinates:

// first re-issue the path commands for the shape being tested
// and then test if the mouse is inside the shape using isPointInPath
if( context.isPointInPath(mouseX,mouseY) ){
    alert('The mouse is inside this shape');
}

Here's example code and a Demo:

var canvas=document.getElementById("canvas");
var ctx=canvas.getContext("2d");
var cw=canvas.width;
var ch=canvas.height;
function reOffset(){
  var BB=canvas.getBoundingClientRect();
  offsetX=BB.left;
  offsetY=BB.top;        
}
var offsetX,offsetY;
reOffset();
window.onscroll=function(e){ reOffset(); }
window.onresize=function(e){ reOffset(); }

var isDown=false;
var startX,startY;

var cursors=['default','w-resize','n-resize'];
var currentCursor=0;

var shapes=[];
shapes.push({
  points:[{x:20,y:50},{x:100,y:10},{x:180,y:50},{x:100,y:90}],
  cursor:1,
});
shapes.push({
  points:[{x:200,y:50},{x:250,y:150},{x:200,y:250},{x:150,y:150}],
  cursor:2,
});

for(var i=0;i<shapes.length;i++){
  var s=shapes[i];
  definePath(s.points);
  ctx.stroke();
}


$("#canvas").mousemove(function(e){handleMouseMove(e);});


function definePath(p){
  ctx.beginPath();
  ctx.moveTo(p[0].x,p[0].y);
  for(var i=1;i<p.length;i++){
    ctx.lineTo(p[i].x,p[i].y);
  }
  ctx.closePath();
}

function handleMouseMove(e){
  // tell the browser we're handling this event
  e.preventDefault();
  e.stopPropagation();

  mouseX=parseInt(e.clientX-offsetX);
  mouseY=parseInt(e.clientY-offsetY);

  // Put your mousemove stuff here
  var newCursor;
  for(var i=0;i<shapes.length;i++){
    var s=shapes[i];
    definePath(s.points);
    if(ctx.isPointInPath(mouseX,mouseY)){
      newCursor=s.cursor;
      break;
    }
  }
  if(!newCursor){
    if(currentCursor>0){
      currentCursor=0;
      canvas.style.cursor=cursors[currentCursor];              
    }
  }else if(!newCursor==currentCursor){
    currentCursor=newCursor;
    canvas.style.cursor=cursors[currentCursor];              
  }
}

body{ background-color: ivory; }
#canvas{border:1px solid red; margin:0 auto; }

<script src="https://ajax.googleapis.com/ajax/libs/jquery/1.9.1/jquery.min.js"></script>
<h4>Move the mouse over the shapes and the cursor will change.</h4>
<canvas id="canvas" width=300 height=300></canvas>