get rgb color code from a selected portion of 2D canvas

StackOverflow https://stackoverflow.com/questions/23657656

  •  22-07-2023
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문제

I have made a small red rectangle inside an orange canvas. I introduced a click event which will occur when I only click on the red portion. It works fine and gives me (255 0 0). But at the lower right corner it gives me (255 165 0) which is orange and clearly not what I wanted. Why is this happening? If there is any smarter way to do this, please advise. Thanks!

<html>
<head>
    <style>

    </style>
</head>

<body>
    <script>
        function makeit() {
            var canvas = document.getElementById("mycanvas");
            canvas.width = 200;
            canvas.height = 50;
            canvas.style.position = "absolute";
            canvas.style.top = "30px";
            canvas.style.left = "100px";
            canvas.style.border = "1px solid black";

            var ctx = canvas.getContext('2d');
            ctx.fillStyle = "orange";
            ctx.fillRect(0, 0, canvas.width, canvas.height);
            ctx.fill();

            ctx.fillStyle = "red";
            ctx.fillRect(0, 0, 20, 20);
            ctx.fill();
            canvas.addEventListener('click', myfunc, false);

            function myfunc(e) {
                if (e.layerX <= 20 && e.layerY <= 20) {
                    var imageData = ctx.getImageData(e.layerX, e.layerY, 1, 1);
                    alert(imageData.data[0] + " " + imageData.data[1] + " " + imageData.data[2]);
                }
            }
        }

        window.onload = makeit;
    </script>

    <canvas id="mycanvas"></canvas>
</body>
</html>
도움이 되었습니까?

해결책

You should change to:

if (e.layerX < 20 && e.layerY < 20)

Here's why: context2D.getImageData samples a rectangle. If you sample a rectangle:

(20, any_value, 1, 1)

or

(any_value, 20, 1, 1)

your sample is outside of your red rectangle (0, 0, 20, 20). In other words, pixels at positions (x, 20) and (20, y) are orange.

다른 팁

The CSS border is nudging the whole thing off by 1px. If you want a border, draw it on the canvas :)

var canvas = document.getElementById("mycanvas");
canvas.width = 200;
canvas.height = 50;
canvas.style.position = "absolute";
canvas.style.top = "30px";
canvas.style.left = "100px";

var ctx = canvas.getContext('2d');
ctx.fillStyle = "orange";
ctx.fillRect(0, 0, canvas.width, canvas.height);
ctx.fill();

ctx.fillStyle = "red";
ctx.fillRect(0, 0, 20, 20);
ctx.fill();
canvas.addEventListener('click', myfunc, false);

function myfunc(e) {
    if (e.layerX <= 20 && e.layerY <= 20) {
        var imageData = ctx.getImageData(e.layerX, e.layerY, 1, 1);
        alert(imageData.data[0] + " " + imageData.data[1] + " " + imageData.data[2]);
    }
}

http://jsfiddle.net/UQM9Q/

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