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.

Usage notes

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() 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:

Playable code
<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

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