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Canvas getImageData() Method

❮ Canvas Reference

Example

Copy the pixel data for a specified rectangle on the canvas and then put the image data back onto the canvas:

const canvas = document.getElementById("myCanvas");
const ctx = canvas.getContext("2d");
ctx.fillStyle = "red";
ctx.fillRect(10, 10, 50, 50);

function copy() {
  const imgData = ctx.getImageData(10, 10, 50, 50);
  ctx.putImageData(imgData, 10, 70);
}
Try it Yourself »

More examples below.


Description

The getImageData() method returns an ImageData object that copies the pixel data for the specified rectangle on a canvas.

Note: The ImageData object is not a picture, it specifies a part (rectangle) on the canvas, and holds information of every pixel inside that rectangle.

For every pixel in an ImageData object there are four pieces of information, the RGBA values:

R - The color red (from 0-255)
G - The color green (from 0-255)
B - The color blue (from 0-255)
A - The alpha channel (from 0-255; 0 is transparent and 255 is fully visible)

The color/alpha information is held in an array, and is stored in the data property of the ImageData object.

Tip: After you have manipulated the color/alpha information in the array, you can copy the image data back onto the canvas with the putImageData() method.

Example:

The code for getting color/alpha information of the first pixel in the returned ImageData object:

red = imgData.data[0];
green = imgData.data[1];
blue = imgData.data[2];
alpha = imgData.data[3];

Try it Yourself

Tip: You can also use the getImageData() method to invert the color of every pixels of an image on the canvas.

Loop through all the pixels and change the color values using this formula:

red = 255-old_red;
green = 255-old_green;
blue = 255-old_blue;

See below for a "Try it Yourself" example!


Syntax

context.getImageData(x, y, width, height)

Parameter Values

Param Description
x The x coordinate (in pixels) of the upper-left corner to copy from
y The y coordinate (in pixels) of the upper-left corner to copy from
width The width of the rectangular area to copy
height The height of the rectangular area to copy

Return Value

Image Data object


More Examples

Image to use:

The Scream

Example

Use getImageData() to invert the color of every pixels of an image on the canvas:

YourbrowserdoesnotsupporttheHTML5canvastag.

JavaScript:

const canvas = document.getElementById("myCanvas");
const ctx = canvas.getContext("2d");
const img = document.getElementById("scream");
ctx.drawImage(img, 0, 0);
const imgData = ctx.getImageData(0, 0, c.width, c.height);

// invert colors
for (let i = 0; i < imgData.data.length; i += 4) {
  imgData.data[i] = 255-imgData.data[i];
  imgData.data[i+1] = 255-imgData.data[i+1];
  imgData.data[i+2] = 255-imgData.data[i+2];
  imgData.data[i+3] = 255;
}
ctx.putImageData(imgData, 0, 0);
Try it Yourself »

Browser Support

The <canvas> element is an HTML5 standard (2014).

getImageData() is supported in all modern browsers:

Chrome Edge Firefox Safari Opera IE
Yes Yes Yes Yes Yes 9-11

❮ Canvas Reference