Click through PNG image - only if clicked coordinate is transparent

Question

I have an image like this:

I want it so that when I click the transparent parts, the click should go through to the underlying element, but when I click a non-transparent part, this image element should get the click.

Is this even possible?

Answer 1

1. Use a temporary canvas element.
2. Get the overlay image mousedown coordinates.
3. Populate your canvas with that image data.
4. Read from canvas the 1px image data (from the same coordinates).
   If the retrieved Alpha channel data is 0 means it's transparent.
5. If pixel is transparent set pointerEvents to "none" and retrieve the underneath element using Document.elementFromPoint
   1 Trigger "click" event on the underneath element
6. Reset pointerEvents back to "auto" for all overlays

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

$('#logo').on("mousedown", function(event) {
  
  // Get click coordinates
  var x = event.pageX - this.offsetLeft,
      y = event.pageY - this.offsetTop,
      w = ctx.canvas.width = this.width,
      h = ctx.canvas.height = this.height,
      alpha;

  // Draw image to canvas
  // and read Alpha channel value
  ctx.drawImage(this, 0, 0, w, h);
  alpha = ctx.getImageData(x, y, 1, 1).data[3]; // [0]R [1]G [2]B [3]A

  // If pixel is transparent,
  // retrieve the element underneath and trigger it's click event
  if( alpha===0 ) {
    this.style.pointerEvents = "none";
    $(document.elementFromPoint(event.clientX, event.clientY)).trigger("click");
    this.style.pointerEvents = "auto";
  } else {
    console.log("LOGO clicked!");
  }
});


$("#green").on("click", function(){
  console.log("Green image clicked!");
});

img{border:1px solid #000;}
img + img{position:absolute;top:0; left:0;}

<script src="https://ajax.googleapis.com/ajax/libs/jquery/2.1.1/jquery.min.js"></script>
<img id="green" src="//placehold.it/200x140/cf5">
<img id="logo"  src="data:image/png;base64,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
">

Answer 2

For your scenario, would it be sufficient to capture click events on the non-transparent area of the image?

This can be done using a map and some JavaScript:

<img src="https://i.sstatic.net/Ab6aW.jpg" alt="" usemap="#Map" />
<map name="Map" id="Map">
<area
  href="javascript:alert('You clicked on the image!')"
  shape="poly" 
  coords="193,148,50,193,254,292,461,191,302,141,332,99,285,27,220,16,170,89"
/>
</map>

(The coordinates were derived using this site.)