javascript - Help with canvas drawing mosaic problem, color selection is always inaccurate

When drawing with the mouse, take the color value of the current area of the mouse, and then set the color of the brush. However, the color selection is always very different. I don’t know if there is something wrong with my calculation. Please help!
<!DOCTYPE html>
<html>
<meta http-equiv="Content-Type" content="text/html; charset=utf-8" />
<meta name="viewport" content="width=device-width,user-scalable=no,initial-scale=1,maximum-scale=1" />
<title></title>
<head>
    <style>
        *{
            margin: 0;
            padding: 0;
        }
        .box {
            position: fixed;
            top: 300px;
        }
        img {
            vertical-align: middle;
        }
    </style>
</head>
<body>
    <p class="box">
        <button id="create">生成</button>
        <button id="clear">清除</button>
    </p>

    <canvas id="canvas"></canvas>
    <script src="js/jquery-1.9.1.js"></script>
    <script>
        //    本地图片路径
        var imgSrc = 'img/aaa.png';
        var height = 300;
        var width = 480;
        
        //将图片分成100份
        var xW = width / 100;
        var yH = height / 100;
        var clip = new mosaic(height, width, imgSrc);

        function mosaic(height, width, src) {
            var img = new Image();
            var canvas = $('#canvas')[0];
            var ctx = canvas.getContext('2d');
            
            img.addEventListener('load', function (e) {
                var mousedown = false,
                    offsetX = canvas.offsetLeft,
                    offsetY = canvas.offsetTop;

                canvas.width = width;
                canvas.height = height;

                ctx.drawImage(img, 0, 0, width, height, 0, 0, width, height);

                $('#clear').on('click', function () {
                    ctx.drawImage(img, 0, 0, width, height, 0, 0, width, height);
                });

                $('#create').on('click', function () {
                    var images = new Image();
                    images.onload = function () {
                        $('body').append(images);
                    }
                    images.src = canvas.toDataURL()
                });

                // 计算当前鼠标坐标值在这个100份格子中的位置
                function getPos(x, y){
                    var px, py;
                    var result = {x, y};
                    var posArr = createPosArr();
          
                    for(var i = 0; i < posArr.length; i++){
                        px = posArr[i].x;
                        py = posArr[i].y;
                        if(x >= px){
                            result.x = posArr[i].x;
                            continue;
                        }
                    }

                    for(var i = 0; i < posArr.length; i++){
                        px = posArr[i].x;
                        py = posArr[i].y;
                        if(y >= py){
                            result.y = posArr[i].y;
                            continue;
                        }
                    }

                    return result;
                }

                function createPosArr(){
                    var arr = [];
                    for(var i = 0; i < 100; i++){
                        arr.push({
                            x: i * xW,
                            y: i * yH
                        });
                    }
                    return arr;
                }

                function down(e) {
                    e.preventDefault();
                    mousedown = true;
                }

                function up(e) {
                    e.preventDefault();
                    mousedown = false;
                }

                function move(e) {
                    if(!mousedown){return;}
                    e.preventDefault();
                    
                    // 如果鼠标按下
                    if (mousedown) {
                        var pos = getPos(e.clientX, e.clientY);
                        console.log(pos);
                        var imgData = ctx.getImageData(pos.x, pos.y, 5, 5);
                        var red   = imgData.data[0];
                        var green = imgData.data[1];
                        var blue  = imgData.data[2];
                        var alpha = imgData.data[3] / 255;

                        ctx.fillStyle = 'rgba(' + red + ',' + green + ',' + blue + ',' + alpha / 2 + ')'
                        ctx.fillRect(pos.x, pos.y, 10, 10);
                    }
                }

                // 在源图像外显示目标图像。只有源图像外的目标图像部分会被显示，源图像是透明的。
                // ctx.globalCompositeOperation = 'destination-out';
                canvas.addEventListener('mousedown', down);
                canvas.addEventListener('mousemove', move);
                canvas.addEventListener('mouseup', up);
            });

            img.src = src;
        }
    </script>
</body>

</html>
三叔2722 days ago888
javascript - Help with canvas drawing mosaic problem, color selection is always inaccurate

reply all(2)I'll reply
