javascript - 删除之前在 HTML5 Canvas 上绘制的线条

Question

为了尝试使用 HTML5 canvas，我决定制作一个可以绘制模拟表盘的应用程序。一切都很好，除了旧行不会以我期望的方式被删除。我在下面包含了部分代码 - DrawHands() 每秒调用一次:

var hoursPoint = new Object();
var minutesPoint = new Object();
var secondsPoint = new Object();

function drawHands()
{
    var now = new Date();

    drawLine(centerX, centerY, secondsPoint.X, secondsPoint.Y, "white", 1);
    var seconds = now.getSeconds();
    secondsPoint = getOtherEndOfLine(centerX, centerY, 2 * Math.PI / 60 * seconds, 0.75 * radius);
    drawLine(centerX, centerY, secondsPoint.X, secondsPoint.Y, "black", 1);

    drawLine(centerX, centerY, minutesPoint.X, minutesPoint.Y, "white", 3);
    var minutes = now.getMinutes();
    minutesPoint = getOtherEndOfLine(centerX, centerY, 2 * Math.PI / 60 * minutes, 0.75 * radius);
    drawLine(centerX, centerY, minutesPoint.X, minutesPoint.Y, "black", 3);

    drawLine(centerX, centerY, hoursPoint.X, hoursPoint.Y, "white", 3);
    var hours = now.getHours();
    if (hours >= 12) { hours -= 12; } // Hours are 0-11
    hoursPoint = getOtherEndOfLine(centerX, centerY, (2 * Math.PI / 12 * hours) + (2 * Math.PI / 12 / 60 * minutes), 0.6 * radius);
    drawLine(centerX, centerY, hoursPoint.X, hoursPoint.Y, "black", 3);
}

为了理解上面的内容，有两个辅助函数:

• drawLine(x1, y1, x2, y2, color, thickness)
• getOtherEndOfLine(x, y, Angular , 长度)

问题是，虽然所有的手都按预期绘制成黑色，但它们永远不会被删除。我希望因为同一条线是用白色(背景色)绘制的，所以它会有效地删除之前在该点绘制的内容。但事实并非如此。

有什么我想念的吗？

Answer 1

与其删除您不想要的内容，您还可以:

1. 保存 Canvas 的状态
2. 画出你不想要的东西
3. 将 Canvas 恢复到保存状态以“删除”它们

这可以使用 ImageData 很容易地完成:

var canvas = document.querySelector('canvas'),
    context = canvas.getContext('2d');

context.fillStyle = 'blue';
context.fillRect(0,0,200,200);

// save the state of  the canvas here
var imageData = context.getImageData(0,0,canvas.width,canvas.height);

// draw a red rectangle that we'll get rid of in a second
context.fillStyle = 'red';
context.fillRect(50,50,100,100);

setTimeout(function () {
    // return the canvas to the state right after we drew the blue rect
    context.putImageData(imageData, 0, 0);
}, 1000);

<canvas width=200 height=200>