画布中矩形大小错误

Question

我正在实现一个颜色选择器。渲染过程中出现了问题。当我调用c.fillRect(0, 0, 100, 80);时，该矩形的大小为103x42像素而不是100x80像素。这里出了什么问题？

此外，矩形是抗锯齿的。我需要通过（0.5，0.5）偏移位置以避免反锯齿吗？我没有使用任何坐标系统转换。

colorSlider = function($e, color) {
    this._$canvas = $('<canvas></canvas>');
    this._c = this._$canvas[0].getContext('2d');
    this._color = color || { r: 0, g: 0, b: 0 };
    this._$canvas.width('310px');
    this._$canvas.height('80px');
    $e.append(this._$canvas);
    this._render();
    var me = this;
    this._$canvas.mousedown(function(e) { me._mouseDown.call(me, e) });
    this._$canvas.mouseup(function(e) { me._mouseUp.call(me, e) });
    this._$canvas.mousemove(function(e) { me._mouseMove.call(me, e) });
    this._dragChannel = 0;
}

colorSlider.prototype._pointInRect = function(x, y, rect) {
    return x >= rect.x && x <= rect.x + rect.w && y >= rect.y && y <= rect.y + rect.h;
}

colorSlider.prototype._findTarget = function(event) {
    var x = event.offsetX;
    var y = event.offsetY;
    console.log(x, y, this._rectR);
    if (this._pointInRect(x, y, this._rectRThumb)) {
        return { target: 1, value: x - this._rectR.x };
    }
    if (this._pointInRect(x, y, this._rectGThumb)) {
        return { target: 2, value: x - this._rectG.x };
    }
    if (this._pointInRect(x, y, this._rectBThumb)) {
        return { target: 3, value: x - this._rectB.x };
    }
    if (this._pointInRect(x, y, this._rectR)) {
        return { target: 4, value: x - this._rectR.x };
    }
    if (this._pointInRect(x, y, this._rectG)) {
        return { target: 5, value: x - this._rectG.x };
    }
    if (this._pointInRect(x, y, this._rectB)) {
        return { target: 6, value: x - this._rectB.x };
    }
    return null;
}

colorSlider.prototype._mouseDown = function(event) {
    this._dragChannel = 0;
    var target = this._findTarget(event);
    if (target) {
        switch (target.target) {
            case 1:
                this._dragChannel = 1;
                break;
            case 2:
                this._dragChannel = 2;
                break;
            case 3:
                this._dragChannel = 3;
                break;
            case 4:
                this._color.r = target.value;
                break;
            case 5:
                this._color.g = target.value;
                break;
            case 6:
                this._color.b = target.value;
                break;
        }
        this._render();
    }
};

colorSlider.prototype._mouseUp = function(event) {
    //console.log('mouseUp');
};

colorSlider.prototype._mouseMove = function(event) {
    //console.log('mouseMove', event);
};

colorSlider.prototype.padding = 4;

colorSlider.prototype._render = function() {
    var padding = this.padding;
    var thickness = 16;
    var c = this._c;
    var w = 255;
    var h = this._$canvas.height();

    c.clearRect(0, 0, this._$canvas.width(), this._$canvas.height());

    var gradient = c.createLinearGradient(padding, 0, w, 0);
    c.fillStyle = gradient;

    gradient.addColorStop(0, this.colorToHex({ r: 0, g: this._color.g, b: this._color.b }));
    gradient.addColorStop(1, this.colorToHex({ r: 255, g: this._color.g, b: this._color.b }));
    c.fillRect(padding, padding, w, thickness);
    c.lineWidth = 0;
    c.fillRect(0, 0, 100, 80);
    this._rectR = { x: padding, y: padding, w: w, h: thickness };

    gradient = c.createLinearGradient(padding, 0, w, 0);
    c.fillStyle = gradient;
    gradient.addColorStop(0, this.colorToHex({ r: this._color.r, g: 0, b: this._color.b }));
    gradient.addColorStop(1, this.colorToHex({ r: this._color.r, g: 255, b: this._color.b }));
    c.fillRect(padding, padding + thickness + 2 * padding, w, thickness);
    this._rectG = { x: padding, y: padding + thickness + 2 * padding, w: w, h: thickness };

    gradient = c.createLinearGradient(padding, 0, w, 0);
    c.fillStyle = gradient;
    gradient.addColorStop(0, this.colorToHex({ r: this._color.r, g: this._color.g, b: 0 }));
    gradient.addColorStop(1, this.colorToHex({ r: this._color.r, g: this._color.g, b: 255 }));
    c.fillRect(padding, padding + 2 * (thickness + 2 * padding), w, thickness);
    this._rectB = { x: padding, y: padding + 2 * (thickness + 2 * padding), w: w, h: thickness };

    c.lineWidth = 2;
    c.fillStyle = "white";
    c.strokeStyle = "#888888";

    this._rectRThumb = { x: padding + this._color.r - 2, y: padding / 2, w: 8, h: 20, r: 2 };
    this.drawRoundedRectangle(c, this._rectRThumb);

    this._rectGThumb = { x: padding + this._color.g - 2, y: padding / 2 + 2 * padding + thickness, w: 8, h: 20, r: 2 };
    this.drawRoundedRectangle(c, this._rectGThumb);

    this._rectBThumb = { x: padding + this._color.b - 2, y: padding / 2 + 2 * (2 * padding + thickness), w: 8, h: 20, r: 2 };
    this.drawRoundedRectangle(c, this._rectBThumb);
};

colorSlider.prototype.colorToHex = function(color) {
    var c = '#'
    + (color.r + 256).toString(16).substr(1, 2)
    + (color.g + 256).toString(16).substr(1, 2)
    + (color.b + 256).toString(16).substr(1, 2);
    console.log(c);
    return c;
};

// https://dev59.com/EXM_5IYBdhLWcg3wt1vD
colorSlider.prototype.drawRoundedRectangle = function(c, rect) {
    var x = rect.x;
    var y = rect.y;
    var width = rect.w;
    var height = rect.h;
    var radius = rect.r;
    c.beginPath();
    c.moveTo(x + radius, y);
    c.lineTo(x + width - radius, y);
    c.quadraticCurveTo(x + width, y, x + width, y + radius);
    c.lineTo(x + width, y + height - radius);
    c.quadraticCurveTo(x + width, y + height, x + width - radius, y + height);
    c.lineTo(x + radius, y + height);
    c.quadraticCurveTo(x, y + height, x, y + height - radius);
    c.lineTo(x, y + radius);
    c.quadraticCurveTo(x, y, x + radius, y);
    c.closePath();
    c.stroke();
    c.fill();
};

index.html

<script>
$(function() {
    $("#directionalLight,#ambientLight").each(function() {
        new colorSlider($(this));
    });

});
</script>

<body>
<div>Directional light</div>
<div id="directionalLight"></div>
<div>Ambient light</div>
<div id="ambientLight"></div>
</body>

- DraganS

Answer 1

首先要知道的是，一个canvas标签有内部坐标空间中的像素数量(由width和height属性和属性设置)。它还有外部维度(style.width和style.height)，即图像在网页中占用的像素数。内部像素被缩放以适应外部空间。

这很令人困惑，因为img标签也有内部和外部尺寸，但是属性名称与canvas完全不同。如果您在图像上设置width和height，基本上就相当于设置style.width或style.height；它们都设置了外部尺寸，以便在页面内缩放图像。与此同时，您只能使用新的naturalWidth和naturalHeight(仅限HTML5浏览器)属性来获取img的内在尺寸。

如果未在img和canvas上设置外部尺寸，则图像将按照内在尺寸的大小进行布局(即缩放因子为1)。

现在，当您使用jQuery时，$(canvas).width('310px')与$(canvas).css('310px')相同，它们都设置外部尺寸。您必须调用$(canvas).prop('width', 310)或只需设置canvas.width = 310来设置内在宽度。

- yonran