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@georgebyte

georgebyte/background-color-picker.js

Last active September 13, 2020 20:13
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jQuery: Background color picker (pick color from image by clicking on it)
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background-color-picker.js
var image = new Image();
image.src = "sample.jpg";
$(image).load(function() {
ctx.drawImage(image, 0, 0);
});
$(canvas).click(function(e) {
var canvasOffset = $(canvas).offset();
var canvasX = Math.floor(e.pageX-canvasOffset.left);
var canvasY = Math.floor(e.pageY-canvasOffset.top);
var imageData = ctx.getImageData(0, 0, canvas.width, canvas.height);
var pixels = imageData.data;
var pixelRedIndex = ((canvasY - 1) * (imageData.width * 4)) + ((canvasX - 1) * 4);
var pixelcolor = "rgba("+pixels[pixelRedIndex]+", "+pixels[pixelRedIndex+1]+", "+pixels[pixelRedIndex+2]+", "+pixels[pixelRedIndex+3]+")";
$("body").css("backgroundColor", pixelcolor);
});
@georgebyte
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Author

Sorry for such a late answer. I didn't see your comment.

Here is a sample html page, that uses this script.

<!DOCTYPE html>
<html>
<head>
    <script src="http://ajax.googleapis.com/ajax/libs/jquery/1.10.2/jquery.min.js"></script>
    <script>
        $(document).ready(function() {
            var canvas = $("#myCanvas").get(0);
            var ctx = canvas.getContext("2d");

            var image = new Image();
            image.src = "sample.jpg";
            $(image).load(function() {
                ctx.drawImage(image, 0, 0);
            });

            $(canvas).click(function(e) {
                var canvasOffset = $(canvas).offset();
                var canvasX = Math.floor(e.pageX-canvasOffset.left);
                var canvasY = Math.floor(e.pageY-canvasOffset.top);

                var imageData = ctx.getImageData(0, 0, canvas.width, canvas.height);
                var pixels = imageData.data;
                var pixelRedIndex = ((canvasY - 1) * (imageData.width * 4)) + ((canvasX - 1) * 4);
                var pixelcolor = "rgba("+pixels[pixelRedIndex]+", "+pixels[pixelRedIndex+1]+", "+pixels[pixelRedIndex+2]+", "+pixels[pixelRedIndex+3]+")";

                $("body").css("backgroundColor", pixelcolor);
            });
        });
    </script>
</head>
<body>
    <canvas id="myCanvas" width="500" height="500">
        Your user agent does not support the HTML5 Canvas element.
    </canvas>
</body>
</html>

@duykhoa
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duykhoa commented May 11, 2017

What is pixelRedIndex mean, and why we need to multiply with number 4.

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