x由于您在 ( , y) 位置 ( 100px, 100px) 處繪制了直徑為 的紅色圓圈6px，因此它所占的面積為：
x : 100 - (6 / 2)
y : 100 - (6 / 2)
width : 6
height : 6
您可以使用該方法清除畫布的一部分clearRect。
context.clearRect(97, 97, 6, 6);
如果您的畫布有背景，您將需要清除整個畫布并重新繪制除紅點以外的所有內容，或者您可以調用fillRect… 假設它context.fillStyle已設置為背景色。
context.fillRect(97, 97, 6, 6);
在繪制之前，您必須以某種方式知道紅點的繪制位置（以及它的大?。?。
編輯：在下面的演示之后查看我的 OOP 示例！
演示
const canvas = document.getElementById('canvas');
const context = canvas.getContext('2d');
context.beginPath();
context.arc(100, 100, 3, 0, Math.PI * 2, true); // Outer circle
context.lineWidth = 0;
context.fillStyle = "red";
context.fill();
context.beginPath();
context.arc(36, 100, 3, 0, Math.PI * 2, true); // Outer circle
context.lineWidth = 0;
context.fillStyle = "Orange";
context.fill();
context.beginPath();
context.arc(123, 100, 3, 0, Math.PI * 2, true); // Outer circle
context.lineWidth = 0;
context.fillStyle = "Green";
context.fill();
function removeRedDot() {
context.clearRect(97, 97, 6, 6);
alert('Removed Red Dot');
}
#canvas {
border: 1px solid black;
}
<script src="https://cdnjs.cloudflare.com/ajax/libs/jquery/3.3.1/jquery.min.js"></script>
<h4>Approach the circle with the mouse</h4> <button onclick="removeRedDot()"> Red Remove Dot</button>
<canvas id="canvas" width=300 height=200></canvas>
OOP 來拯救！
更好的方法是了解畫布渲染之外的紅點。您可以將畫布上下文包裝在一個管理圖層和可繪制對象的類中。
const ctx = document.getElementById('canvas').getContext('2d');
const main = () => {
const canvas = new Canvas(ctx);
const layer = canvas.addLayer();
const circles = [
new Circle({ x: 50, y: 50 }, 3, 'red'),
new Circle({ x: 100, y: 100 }, 6, 'green'),
new Circle({ x: 150, y: 150 }, 12, 'blue')
];
layer.add(...circles);
canvas.render();
// After 2 second, remove the red dot and re-render.
setTimeout(() => {
alert('Removing "red" circle, and adding a "cyan" circle...');
layer.remove(circles[0]);
layer.add(new Circle({ x: 150, y: 50 }, 8, 'cyan'));
canvas.render();
}, 2000);
};
class Drawable {
constructor(origin) {
this.origin = origin;
}
draw(ctx) { }
}
class Layer {
constructor(name) {
this.name = name;
this.drawables = [];
}
add(...drawables) {
drawables.forEach(drawable => this.drawables.push(drawable));
}
remove(drawableOrIndex) {
if (isNaN(drawableOrIndex)) {
drawableOrIndex = this.drawables.indexOf(drawableOrIndex);
}
if (drawableOrIndex > -1) {
this.drawables.splice(drawableOrIndex, 1);
}
}
render(ctx) {
this.drawables.forEach(drawable => drawable.render(ctx));
}
}
class Canvas {
constructor(ctx) {
this.ctx = ctx;
this.layers = [];
}
addLayer(name) {
const newLayer = new Layer(name || 'layer-' + this.layers.length);
this.layers.push(newLayer);
return newLayer;
}
getLayer(nameOrIndex) {
return isNaN(nameOrIndex)
? this.layers.find(layer => layer.name === nameOrIndex)
: this.layers[nameOrIndex];
}
render() {
const { width, height } = this.ctx.canvas;
this.ctx.clearRect(0, 0, width, height);
this.layers.forEach(layer => layer.render(this.ctx));
}
}
class Circle extends Drawable {
constructor(origin, radius, color) {
super(origin);
this.radius = radius;
this.color = color;
}
render(ctx) {
const { x, y } = this.origin;
const diameter = this.radius * 2;
ctx.save();
ctx.beginPath();
ctx.arc(x, y, this.radius, 0, Math.PI * 2, true);
ctx.lineWidth = 0;
ctx.fillStyle = this.color;
ctx.fill();
ctx.restore();
}
}
main();
#canvas {
border: 1px solid black;
}
<canvas id="canvas" width=300 height=200></canvas>
