光标未使用 HTML canvas 对齐

Question

我有以下 Javascript、HTML 和 CSS 代码

function distanceBetween(point1, point2) {
    return Math.sqrt(Math.pow(point2.x - point1.x, 2) + Math.pow(point2.y - point1.y, 2));
}

function angleBetween(point1, point2) {
    return Math.atan2(point2.x - point1.x, point2.y - point1.y);
}

function draw() {
    var img = new Image();
    img.src = 'http://www.tricedesigns.com/wp-content/uploads/2012/01/brush2.png';

    var el = document.getElementById('DrawingPattern');
    var ctx = el.getContext('2d');
    ctx.lineJoin = ctx.lineCap = 'round';

    var isDrawing, lastPoint;

    el.onmousedown = function (e) {
        isDrawing = true;
        lastPoint = {x: e.clientX, y: e.clientY};
    };

    el.onmousemove = function (e) {
        if (!isDrawing) return;

        var currentPoint = {x: e.clientX, y: e.clientY};
        var dist = distanceBetween(lastPoint, currentPoint);
        var angle = angleBetween(lastPoint, currentPoint);

        for (var i = 0; i < dist; i++) {
            x = lastPoint.x + (Math.sin(angle) * i) - 25;
            y = lastPoint.y + (Math.cos(angle) * i) - 25;
            ctx.drawImage(img, x, y);
        }

        lastPoint = currentPoint;
    };

    el.onmouseup = function () {
        isDrawing = false;
    };
}

html, body {
    color: #fff;
    background: #000;
    font-family: "Lucida Grande";
}

canvas{
    position: static;
    cursor: move;
    margin: auto;
    width: 70%;
    border: 3px solid #73AD21;
    background: white;
    overflow: visible;
}

<body onload=draw()>
<canvas width="1024" height="649" id="DrawingPattern">
<!-- add fallback content-->
</canvas>
</body>

整个代码有效，我遇到的唯一问题是当我开始在 canvas 上绘图时，光标与光标的有效位置不对齐。因此，当我绘制它时，线条会位于另一个位置，高于或低于光标的实际位置。我尝试了不同的方法，但我猜是某种缩放问题，但我无法解决它。

Answer 1

这是一个缩放问题。即使您通过按百分比设置 CSS 宽度来缩放 canvas 元素，绘图区域的大小也不会改变。你在屏幕上得到了鼠标坐标space，但是在绘图的时候需要转换成canvasspace

为此，请通过考虑 el.width（canvas 宽度）和 el.offsetWidth（元素宽度）之间的差异来计算比例。由于 canvas 通过保持纵横比按比例缩放，因此可以对 X 和 Y 坐标使用相同的缩放比例。

我添加了一个 elementScale() 函数并更新了代码以将 clientX 和 clientY 与 return 值相乘。

function distanceBetween(point1, point2) {
    return Math.sqrt(Math.pow(point2.x - point1.x, 2) + Math.pow(point2.y - point1.y, 2));
}

function angleBetween(point1, point2) {
    return Math.atan2(point2.x - point1.x, point2.y - point1.y);
}

function elementScale(el) {
    return el.offsetWidth === 0 ? 0 : (el.width / el.offsetWidth);
}

function draw() {
    var img = new Image();
    img.src = 'http://www.tricedesigns.com/wp-content/uploads/2012/01/brush2.png';

    var el = document.getElementById('DrawingPattern');
    var ctx = el.getContext('2d');
    ctx.lineJoin = ctx.lineCap = 'round';

    var isDrawing, lastPoint;

    el.onmousedown = function (e) {
        var scale = elementScale(el);
        isDrawing = true;
        lastPoint = {x: e.clientX * scale, y: e.clientY * scale};
    };

    el.onmousemove = function (e) {
        if (!isDrawing) return;

        var scale = elementScale(el);
        var currentPoint = {x: e.clientX * scale, y: e.clientY * scale};
        var dist = distanceBetween(lastPoint, currentPoint);
        var angle = angleBetween(lastPoint, currentPoint);

        for (var i = 0; i < dist; i++) {
            x = lastPoint.x + (Math.sin(angle) * i) - 25;
            y = lastPoint.y + (Math.cos(angle) * i) - 25;
            ctx.drawImage(img, x, y);
        }

        lastPoint = currentPoint;
    };

    el.onmouseup = function () {
        isDrawing = false;
    };
}

html, body {
    color: #fff;
    background: #000;
    font-family: "Lucida Grande";
}

canvas{
    position: static;
    cursor: move;
    margin: auto;
    width: 70%;
    border: 3px solid #73AD21;
    background: white;
    overflow: visible;
}

<body onload=draw()>
<canvas width="1024" height="649" id="DrawingPattern">
<!-- add fallback content-->
</canvas>
</body>

Answer 2

正如 DarthJDG 所说，这是您 CSS

的缩放问题

您可以删除 canvas 上的 CSS width:70%，然后像往常一样尝试调整

CSS 代码

    html, body {
        color: #fff;
        background: #000;
        font-family: "Lucida Grande";
    }

    canvas{
        position: static;
        cursor: move;
        margin: auto;
        //width:70%; //REMOVED
        border: 3px solid #73AD21;
        background: white;
        overflow: visible;
    }

JAVASCRIPT 代码

for (var i = 0; i < dist; i++) {
    x = lastPoint.x + (Math.sin(angle) * i) - 25; //CHANGED
    y = lastPoint.y + (Math.cos(angle) * i) - 25; //CHANGED
    ctx.drawImage(img, x, y);
}

只需删除 CSS 宽度，缩放就会恢复正常。

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