getImageData() Method - Javascript Canvas Reference

Javascript examples for Canvas Reference:getImageData

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Description

The getImageData() method returns an ImageData object returns the pixel data from the rectangle on a canvas.

Pixel in ImageData object have 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 stored in an array, which is the data property of the ImageData object.

JavaScript syntax 

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

Parameter Values

Parameter Description
x The x coordinate in pixels of the upper-left corner
y The y coordinate in pixels of the upper-left corner
width The width of the rectangular area you will copy
heightThe height of the rectangular area you will copy

Example:

The following code shows how to get color/alpha information of the first pixel in the returned ImageData object:

Demo Code

ResultView the demo in separate window 

<!DOCTYPE html>
<html>
<body>

<canvas id="myCanvas" width="300" height="250" style="border:1px solid #d3d3d3;">
Your browser does not support the HTML5 canvas tag.</canvas>

<script>

var c = document.getElementById("myCanvas");
var ctx = c.getContext("2d");
ctx.fillStyle = "red";
ctx.fillRect(10, 10, 50, 50);//  w  w  w  .j  a v  a 2s.c o  m

var imgData = ctx.getImageData(30, 30, 50, 50);
red = imgData.data[0];
green = imgData.data[1];
blue = imgData.data[2];
alpha = imgData.data[3];
console.log(red + " " + green + " " + blue + " " + alpha);

</script>

</body>
</html>

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