Check if user is clicking on non-transparent part of a png?-CodePudding

Like the title suggests, I'd like to know if a user is clicking on the non-transparent part of a png.

Further - imagine there are several layers of pngs that are layered ontop of each other. Same size, but for each layer, the non-transparent part gets smaller and smaller. I'd like to be able to identify which of the pngs are being clicked by identifying which non-transparent part the user clicks on.

CodePudding user response：

Let's try using a canvas. Thanks to @Yogi from the comments.

var url = "https://upload.wikimedia.org/wikipedia/commons/thumb/4/47/PNG_transparency_demonstration_1.png/420px-PNG_transparency_demonstration_1.png";

var image = new Image();
image.onload = function() {
  var canvas = document.createElement("canvas");
  document.body.appendChild(canvas);
  canvas.width = image.width;
  canvas.height = image.height;
  var context = canvas.getContext("2d");
  context.drawImage(image, 0, 0);

  canvas.addEventListener('mousedown', function(ev) {
    var cx = ev.offsetX
    var cy = ev.offsetY
    var data = context.getImageData(cx, cy, 1, 1).data;
    console.log("" data)

  })
}
image.crossOrigin = "anonymous"
image.src = url;

<body>
  click and get color in console.<br>

</body>

CodePudding user response：

Mixing and matching the answers and comments above:

Attach a click handler for each png you've rendered. On click, the event will propagate from the top-most png of the stack (the first png the click "meets"), all the way to the bottom-most (the first one rendered).

The handler will execute the same logic for each png (you may reuse the same function):

Get the relative coordinates of the click (relative to each image).
Determine if the pixel is transparent or not (this is the key the answer).
If it meets your criteria, record which png was clicked using event.currentTarget. Then, stop propagating the event, to prevent deeper pngs from recording an event as well.

If the png didn't meet your criteria, allow the event to keep propagating.