Ecliptic Ecliptic - 7 months ago 17
Javascript Question

How to make it so that Hovering Over Transparency in a PNG doesn't count as hovering?

When I hover over the transparent part of a PNG, it still acts as though I'm hovering over the actual image. Is there a way that I can prevent that from happening? So that it only takes action when I hover over the visible part of the image, and not the transparent part?

I tried to crop out the transparency, but I couldn't find a way how.

Answer

Can be done by converting png to canvas element

This works by loading a png into an HTML-5 canvas element, and then querying the canvas for the alpha value of the clicked pixel.

Working demo: http://jsfiddle.net/x9ScK/3/

HTML as follows...

<!-- create a canvas element to hold the PNG image -->
<canvas id="canvas1" width="500" height="500"></canvas>

Javascript like this...

// select the canvas element with jQuery, and set up
// a click handler for the whole canvas
$('#canvas1').on('click', function(e) {
    // utility function for finding the position of the clicked pixel
    function findPos(obj) {
        var curleft = 0, curtop = 0;
        if (obj.offsetParent) {
            do {
                curleft += obj.offsetLeft;
                curtop += obj.offsetTop;
            } while (obj = obj.offsetParent);
            return { x: curleft, y: curtop };
        }
        return undefined;
    }
    // get the position of clicked pixel
    var pos = findPos(this);
    var x = e.pageX - pos.x;
    var y = e.pageY - pos.y;
    // get reference to canvas element clicked on
    var canvas = this.getContext('2d');
    // return array of [RED,GREEN,BLUE,ALPHA] as 0-255 of clicked pixel
    var pixel = canvas.getImageData(x, y, 1, 1).data;
    // if the alpha is not 0, we clicked a non-transparent pixel
    // could be easily adjusted to detect other features of the clicked pixel
    if(pixel[3] != 0){
        // do something when clicking on image...
        alert("Clicked the dice!");
    }
});

// get reference to canvas DOM element
var canvas = $('#canvas1')[0];
// get reference to canvas context
var context = canvas.getContext('2d');

// create an empty image
var img = new Image();
// after loading...
img.onload = function() {
    // draw the image onto the canvas
    context.drawImage(img, 0, 0);
}

// set the image source (can be any url - I used data URI to keep demo self contained)
img.src = "data:image/png;base64,iVBORw0KGgoAAAANS ... more image data ...TkSuQmCC"; // PNG with transparency
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