Detecting taps on transparent section of images

Question

I am currently working on a project where users tap on an irregularly shaped image in order to place a dot (Think of a dartboard or pin the tail on the donkey). I have no proble

Answer 1

• Solution 1 : You have a complicated image with transparent border and don't want the transparent part to be clickable :

  Draw the image into the canvas and use getImageData(x,y,width,height) to check if the pixel under the click is transparent.

var pointMap = [], canvas, ctx, img;

$(document).ready(function () {
  img = $('#bodyImageFront');
  img[0].onload = init;
});


$(window).resize(function () {
    canvas.width = img[0].width;
    canvas.height = img[0].height;
    drawCanvas();
    var tempArray = pointMap;
    pointMap = [];
    for (var i = 0; i < tempArray.length; i++) {
        addPointToMap(tempArray[i]);
    }
});

function init(){
   canvas = document.createElement("canvas");
   canvas.id="bodyMapFrontCanvas";
   canvas.width = img[0].width;
   canvas.height = img[0].height;
   $(canvas).css('position:relative');
   $(canvas).click(addPointToMap);
   ctx = canvas.getContext('2d');
   img.parent().append(canvas);
   img.css('opacity:0');
   drawCanvas();
  }
function drawCanvas() {
  if(!canvas) return;
  ctx.drawImage(img[0], 0,0,canvas.width,canvas.height);
}

function addPointToMap(evt) {
    var target = evt.target,
    x = evt.offsetX ? evt.offsetX : evt.pageX - target.offsetLeft,
    y = evt.offsetY ? evt.offsetY : evt.pageY - target.offsetTop;
    // get the image data of our clicked point  
    var pointData = ctx.getImageData(x,y,1,1).data;
    // if its alpha channel is 0, don't go farther
    if(pointData[3]===0) return;

    ctx.fillRect(x-5, y-5, 10, 10);
    if(evt.type){
      pointMap.push({target: target, offsetX: x, offsetY: y});
      }
}

#bodyImageFront {
    display:block;
    max-height: 75vh;
    max-width: 90%;
    height:auto;
    width: auto;
    margin-bottom: 10px;
    z-index:-1;
    position:absolute;
}

canvas {
    z-index:20;
    background:#AAFFAA;
}

<script src="https://ajax.googleapis.com/ajax/libs/jquery/2.1.1/jquery.min.js"></script>

<form id="mainForm">
    <div id="canvasContainer"></div>
    <div id="bodyMapFrontContainer">
        <img src="https://dl.dropboxusercontent.com/s/nqtih1j7vj850ff/c6RvK.png" id="bodyImageFront" crossorigin="Anonymous" />
    </div>
</form>

• Solution 2 : You really only want to draw a simple shape like in the example you gave :

  Draw this shape directly onto the canvas and use isPointInPath() to detect whether it should activate the click or not.

var canvas = document.querySelector('canvas'),
    ctx = canvas.getContext('2d');
function draw(){
  
  var w = canvas.width, h = canvas.height;
  ctx.fillStyle = "#5b9bd5";
  ctx.beginPath();
  var radius = w/3,
      x= canvas.width/2,
      y = radius, a= 11;
  for ( var i = 0; i <= 4 * Math.PI; i += ( 4 * Math.PI ) / 5 ) {
    ctx.lineTo( x + radius * Math.cos(i + a), y + radius * Math.sin(i + a));
  }
  ctx.closePath();
  ctx.fill();
  }

function clickHandler(e){
  var target = e.target,
    x = e.clientX,
    y = e.clientY;
   if(ctx.isPointInPath(e.clientX, e.clientY)){
     ctx.fillStyle="#000";
     ctx.fillRect(x-5,y-5,10,10);
     }
  }
canvas.addEventListener('click', clickHandler, false);
draw();

canvas{background:#AAFFAA;}
*{margin:0; overflow: hidden;}

<canvas height=500 width=500/>

• Solution 3: Once again you only want a simple shape, and you don't really need <canvas>:

  Draw it with svg and add the event listener only to the svg element you want to listen to.

document.getElementById('star').addEventListener('click', clickHandler, false);
var svg = document.querySelector('svg');

function clickHandler(e){
   var vB = svg.getBBox(),
       bB = svg.getBoundingClientRect(),
       ratio = {w:(vB.width/bB.width), h:(vB.height/bB.height)};

   var x = e.clientX*ratio.w,
       y = e.clientY*ratio.h;
  
    var rect = document.createElementNS('http://www.w3.org/2000/svg', 'rect');
    rect.setAttribute('x', x);
    rect.setAttribute('y', y);
    rect.setAttribute('width', 10*ratio.w);
    rect.setAttribute('height', 10*ratio.w);
    rect.setAttribute('fill', "#000");
    var parent = e.target.parentNode;
    parent.appendChild(rect);
    
  }

*{margin: 0; overflow: hidden;}
svg{
  height: 100vh;
}

<svg version="1.1" id="Layer_1" xmlns="http://www.w3.org/2000/svg" x="0px" y="0px"
	 viewBox="0 0 100 100" enable-background="new 0 0 100 100" xml:space="preserve">
<rect fill="#AAFFAA" width="100" height="100"/>
<polygon id="star" fill="#29ABE2" points="50,16.5 60.9,40.6 85.2,42.1 65.6,59.3 71.8,83.5 50,70.1 28.2,83.5 34.4,59.3 14.8,42.1 39.1,41.6 
	"/>
</svg>