vince83000 vince83000 - 3 months ago 28
Javascript Question

Get pixel color from canvas, on mouseover

Is it possible to get the RGB value pixel under the mouse? Is there a complete example of this? Here's what I have so far:

<script>
function draw() {
var ctx = document.getElementById('canvas').getContext('2d');
var img = new Image();
img.src = 'Your URL';

img.onload = function(){
ctx.drawImage(img,0,0);


};

canvas.onmousemove = function(e) {
var mouseX, mouseY;

if(e.offsetX) {
mouseX = e.offsetX;
mouseY = e.offsetY;
}
else if(e.layerX) {
mouseX = e.layerX;
mouseY = e.layerY;
}
var c = ctx.getImageData(mouseX, mouseY, 1, 1).data;

$('#ttip').css({'left':mouseX+20, 'top':mouseY+20}).html(c[0]+'-'+c[1]+'-'+c[2]);
};
}

</script>

Answer

Here's a complete, self-contained example. First, use the following HTML:

<canvas id="example" width="200" height="60"></canvas>
<div id="status"></div>

The relevant JavaScript:

// set up some sample squares
var example = document.getElementById('example');
var context = example.getContext('2d');
context.fillStyle = "rgb(255,0,0)";
context.fillRect(0, 0, 50, 50);
context.fillStyle = "rgb(0,0,255)";
context.fillRect(55, 0, 50, 50);

$('#example').mousemove(function(e) {
    var pos = findPos(this);
    var x = e.pageX - pos.x;
    var y = e.pageY - pos.y;
    var coord = "x=" + x + ", y=" + y;
    var c = this.getContext('2d');
    var p = c.getImageData(x, y, 1, 1).data; 
    var hex = "#" + ("000000" + rgbToHex(p[0], p[1], p[2])).slice(-6);
    $('#status').html(coord + "<br>" + hex);
});

The code above assumes the presence of jQuery and the following utility functions:

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;
}

function rgbToHex(r, g, b) {
    if (r > 255 || g > 255 || b > 255)
        throw "Invalid color component";
    return ((r << 16) | (g << 8) | b).toString(16);
}

See it in action on JSFIDDLE: