CanvasRenderingContext2D.clearRect()

Canvas 2D API 的 CanvasRenderingContext2D.clearRect() 方法可以設定指定矩形（經由坐標 (x, y) 及大小 (width, height)）範圍內的所有像素為透明，清除所有先前繪製的內容。

語法

void ctx.clearRect(x, y, width, height);

參數

x: The x axis of the coordinate for the rectangle starting point.
y: The y axis of the coordinate for the rectangle starting point.
width: The rectangle's width.
height: The rectangle's height.

A common problem with clearRect is that it may appear it does not work when not using paths properly. Don't forget to call beginPath() (en-US) before starting to draw the new frame after calling clearRect.

範例

Using the `clearRect` method

This is just a simple code snippet which uses the clearRect method.

HTML

<canvas id="canvas"></canvas>

JavaScript

var canvas = document.getElementById('canvas');
var ctx = canvas.getContext('2d');

ctx.beginPath();
ctx.moveTo(20, 20);
ctx.lineTo(200, 20);
ctx.lineTo(120, 120);
ctx.closePath(); // draws last line of the triangle
ctx.stroke();

ctx.clearRect(10, 10, 100, 100);

// clear the whole canvas
// ctx.clearRect(0, 0, canvas.width, canvas.height);

Edit the code below and see your changes update live in the canvas:

<canvas id="canvas" width="400" height="200" class="playable-canvas"></canvas>
<div class="playable-buttons">
  <input id="edit" type="button" value="Edit" />
  <input id="reset" type="button" value="Reset" />
</div>
<textarea id="code" class="playable-code" style="height:140px;">
ctx.beginPath();
ctx.moveTo(20,20);
ctx.lineTo(200,20);
ctx.lineTo(120,120);
ctx.closePath(); // draws last line of the triangle
ctx.stroke();

ctx.clearRect(10, 10, 100, 100);</textarea>

var canvas = document.getElementById("canvas");
var ctx = canvas.getContext("2d");
var textarea = document.getElementById("code");
var reset = document.getElementById("reset");
var edit = document.getElementById("edit");
var code = textarea.value;

function drawCanvas() {
  ctx.clearRect(0, 0, canvas.width, canvas.height);
  eval(textarea.value);
}

reset.addEventListener("click", function() {
  textarea.value = code;
  drawCanvas();
});

edit.addEventListener("click", function() {
  textarea.focus();
})

textarea.addEventListener("input", drawCanvas);
window.addEventListener("load", drawCanvas);

規範

Specification
HTML Standard # dom-context-2d-clearrect-dev

瀏覽器相容性

BCD tables only load in the browser

參見

The interface defining it, CanvasRenderingContext2D
CanvasRenderingContext2D.fillRect() (en-US)
CanvasRenderingContext2D.strokeRect() (en-US)