HTML canvas getImageData() Method
Example
The code below copies the pixel data for a specified rectangle on the canvas with getImageData(), and then put the image data back onto the canvas with putImageData():
var c=document.getElementById("myCanvas");
var ctx=c.getContext("2d");
ctx.fillStyle="red";
ctx.fillRect(10,10,50,50);
function copy()
{
var imgData=ctx.getImageData(10,10,50,50);
ctx.putImageData(imgData,10,70);
}
Try it yourself »
Browser Support
Internet Explorer 9, Firefox, Opera, Chrome, and Safari support the getImageData() method.
Note: Internet Explorer 8 and earlier versions, do not support the <canvas> element.
Definition and Usage
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];
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!
JavaScript Syntax
| JavaScript syntax: | context.getImageData(x,y,width,height); |
|---|
Parameter Values
| Parameter | Description |
|---|---|
| x | The x coordinate (in pixels) of the upper-left corner to start copy from |
| y | The y coordinate (in pixels) of the upper-left corner to start copy from |
| width | The width of the rectangular area you will copy |
| height | The height of the rectangular area you will copy |
More Examples
Image to use:
Example
Use getImageData() to invert the color of every pixels of an image on the canvas:
JavaScript:
var c=document.getElementById("myCanvas");
var ctx=c.getContext("2d");
var img=document.getElementById("scream");
ctx.drawImage(img,0,0);
var imgData=ctx.getImageData(0,0,c.width,c.height);
// invert colors
for (var 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 »
HTML Canvas Reference
