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问题:

I have a canvas with some irregular shape drawings in it, and I would like to have a feedback when someone clicks on a specific one?

I've been looking everywhere for this and have only found solutions for rectangle.

I think it may have to do with isPointInPath(), but I've yet to find a concise explanation on how to use it.

Any help welcome.

回答1:

I made a tutorial that uses a second invisible canvas to do object picking/hit testing. Draw all your shapes, one by one, onto the second canvas until one of them has a black pixel where the mouse location is. Then you've found your object!

Here's a bit from the tutorial I wrote on selecting objects with canvas:

  // gctx is ghost context, made from the second canvas
  // clear(gctx)

  // ...

  // run through all the boxes
  var l = boxes.length;
  for (var i = l-1; i >= 0; i--) {
    // draw shape onto ghost context
    drawshape(gctx, boxes[i], 'black', 'black');

    // get image data at the mouse x,y pixel
    var imageData = gctx.getImageData(mx, my, 1, 1);
    var index = (mx + my * imageData.width) * 4;

    // if the mouse pixel exists, select and break
    if (imageData.data[3] > 0) {
      mySel = boxes[i];
      offsetx = mx - mySel.x;
      offsety = my - mySel.y;
      mySel.x = mx - offsetx;
      mySel.y = my - offsety;
      isDrag = true;
      canvas.onmousemove = myMove;
      invalidate();
      clear(gctx);
      return;
    }

  }

My full demo only uses rectangles but in a later version I will use circles/paths/text.

If you want to see the demo and my full code it is here.

回答2:

You can use a pathiterator, that turns all shapes into approximated polygons. Then use a 'point in polygon' algorithm to check if the point is in the shape.

回答3:

This can be achived by using Path2D.

var div = document.getElementById("result");
var canvas = document.getElementById("canvas");

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

var path1 = new Path2D();
path1.rect(10, 10, 100, 100);
path1.closePath();
ctx.stroke(path1);

var path2 = new Path2D();
path2.moveTo(220, 60);
path2.arc(170, 60, 50, 0, 2 * Math.PI);
path2.closePath();
ctx.stroke(path2);

var path3 = new Path2D("M230 10 h 80 v 80 h -80 Z");
ctx.fill(path3);
path3.closePath();

$('canvas').click( function(event) 
{
  div.innerHTML = "";
  
  var x = event.pageX;
  var y = event.pageY;

  if (ctx.isPointInPath(path1, x, y))
    div.innerHTML = "Path1 clicked";

  if (ctx.isPointInPath(path2, x, y))
    div.innerHTML = "Path2 clicked";
  
  if (ctx.isPointInPath(path3, x, y))
    div.innerHTML = "Path3 clicked";
});

<script src="https://ajax.googleapis.com/ajax/libs/jquery/2.1.1/jquery.min.js"></script>
<body>
  <canvas id="canvas"></canvas>
  <div id="result"></div>
</body>