html5 canvas clicking on bezier path shape detection

Question

Hello,

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.

Answer 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.