CanvasRenderingContext2D.getImageData() - Web APIs 编辑

The CanvasRenderingContext2D method getImageData() of the Canvas 2D API returns an ImageData object representing the underlying pixel data for a specified portion of the canvas.

This method is not affected by the canvas's transformation matrix. If the specified rectangle extends outside the bounds of the canvas, the pixels outside the canvas are transparent black in the returned ImageData object.

Note: Image data can be painted onto a canvas using the putImageData() method.

You can find more information about getImageData() and general manipulation of canvas contents in Pixel manipulation with canvas.

Syntax

ctx.getImageData(sx, sy, sw, sh);

Parameters

sx

The x-axis coordinate of the top-left corner of the rectangle from which the ImageData will be extracted.

sy

The y-axis coordinate of the top-left corner of the rectangle from which the ImageData will be extracted.

sw

The width of the rectangle from which the ImageData will be extracted. Positive values are to the right, and negative to the left.

sh

The height of the rectangle from which the ImageData will be extracted. Positive values are down, and negative are up.

Return value

An ImageData object containing the image data for the rectangle of the canvas specified. The coordinates of the rectangle's top-left corner are (sx, sy), while the coordinates of the bottom corner are (sx + sw, sy + sh).

Exceptions

IndexSizeError

Thrown if either sw or sh are zero.

SecurityError

The canvas contains or may contain pixels which were loaded from an origin other than the one from which the document itself was loaded. To avoid SecurityError being thrown in this situation, configure CORS to allow the source image to be used in this way. See Allowing cross-origin use of images and canvas.

Example

Getting image data from a canvas

This example draws a rectangle, and then uses getImageData() to grab a portion of the canvas.

HTML

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

JavaScript

The object retrieved by getImageData() has a width of 200 and a height of 100, for a total of 20,000 pixels. Of those pixels, most are either transparent or taken from off the canvas; only 5,000 of them are opaque black (the color of the drawn rectangle).

const canvas = document.getElementById('canvas');
const ctx = canvas.getContext('2d');
ctx.rect(10, 10, 100, 100);
ctx.fill();

let imageData = ctx.getImageData(60, 60, 200, 100);
ctx.putImageData(imageData, 150, 10);

Result

Specifications

Browser compatibility

BCD tables only load in the browser

See also

• The interface defining this method: CanvasRenderingContext2D
• ImageData object
• CanvasRenderingContext2D.putImageData()
• Pixel manipulation with canvas