Pixel manipulation with canvas
Until now we haven't looked at the actual pixels of our canvas. With the ImageData
object you can directly read and write a data array to manipulate pixel data. We will also look into how image smoothing (anti-aliasing) can be controlled and how to save images from your canvas.
The ImageData
object
The ImageData
object represents the underlying pixel data of an area of a canvas object. It contains the following read-only attributes:
width
- The width of the image in pixels.
height
- The height of the image in pixels.
data
- A
Uint8ClampedArray
representing a one-dimensional array containing the data in the RGBA order, with integer values between0
and255
(included).
The data
property returns a Uint8ClampedArray
which can be accessed to look at the raw pixel data; each pixel is represented by four one-byte values (red, green, blue, and alpha, in that order; that is, "RGBA" format). Each color component is represented by an integer between 0 and 255. Each component is assigned a consecutive index within the array, with the top left pixel's red component being at index 0 within the array. Pixels then proceed from left to right, then downward, throughout the array.
The Uint8ClampedArray
contains height
× width
× 4 bytes of data, with index values ranging from 0 to (height
×width
×4)-1.
For example, to read the blue component's value from the pixel at column 200, row 50 in the image, you would do the following:
blueComponent = imageData.data[((50*(imageData.width*4)) + (200*4)) + 2];
You may access the size of the pixel array in bytes by reading the Uint8ClampedArray.length
attribute:
var numBytes = imageData.data.length;
Creating an ImageData
object
To create a new, blank ImageData
object, you should use the createImageData()
method. There are two versions of the createImageData()
method:
var myImageData = ctx.createImageData(width, height);
This creates a new ImageData
object with the specified dimensions. All pixels are preset to transparent black.
You can also create a new ImageData
object with the same dimensions as the object specified by anotherImageData
. The new object's pixels are all preset to transparent black. This does not copy the image data!
var myImageData = ctx.createImageData(anotherImageData);
Getting the pixel data for a context
To obtain an ImageData
object containing a copy of the pixel data for a canvas context, you can use the getImageData()
method:
var myImageData = ctx.getImageData(left, top, width, height);
This method returns an ImageData
object representing the pixel data for the area of the canvas whose corners are represented by the points (left
,top
), (left+width
, top
), (left
, top+height
), and (left+width
, top+height
). The coordinates are specified in canvas coordinate space units.
Note: Any pixels outside the canvas are returned as transparent black in the resulting ImageData
object.
This method is also demonstrated in the article Manipulating video using canvas.
A color picker
In this example we are using the getImageData() method to display the color under the mouse cursor. For this, we need the current position of the mouse with layerX
and layerY
, then we look up the pixel data on that position in the pixel array that getImageData() provides us. Finally, we use the array data to set a background color and a text in the <div>
to display the color.
<canvas id="canvas" width="300" height="227" style="float:left"></canvas> <div id="color" style="width:200px;height:50px;float:left"></div>
var img = new Image(); img.src = 'https://mdn.mozillademos.org/files/5397/rhino.jpg'; var canvas = document.getElementById('canvas'); var ctx = canvas.getContext('2d'); img.onload = function() { ctx.drawImage(img, 0, 0); img.style.display = 'none'; }; var color = document.getElementById('color'); function pick(event) { var x = event.layerX; var y = event.layerY; var pixel = ctx.getImageData(x, y, 1, 1); var data = pixel.data; var rgba = 'rgba(' + data[0] + ',' + data[1] + ',' + data[2] + ',' + (data[3] / 255) + ')'; color.style.background = rgba; color.textContent = rgba; } canvas.addEventListener('mousemove', pick);
Painting pixel data into a context
You can use the putImageData() method to paint pixel data into a context:
ctx.putImageData(myImageData, dx, dy);
The dx
and dy
parameters indicate the device coordinates within the context at which to paint the top left corner of the pixel data you wish to draw.
For example, to paint the entire image represented by myImageData
to the top left corner of the context, you can simply do the following:
ctx.putImageData(myImageData, 0, 0);
Grayscaling and inverting colors
In this example we iterate over all pixels to change their values, then we put the modified pixel array back to the canvas using putImageData(). The invert function simply subtracts each color from the max value 255. The grayscale function simply uses the average of red, green and blue. You can also use a weighted average, given by the formula x = 0.299r + 0.587g + 0.114b
, for example. See Grayscale on Wikipedia for more information.
<canvas id="canvas" width="300" height="227"></canvas> <div> <input id="grayscalebtn" value="Grayscale" type="button"> <input id="invertbtn" value="Invert" type="button"> </div>
var img = new Image(); img.src = 'https://mdn.mozillademos.org/files/5397/rhino.jpg'; img.onload = function() { draw(this); }; function draw(img) { var canvas = document.getElementById('canvas'); var ctx = canvas.getContext('2d'); ctx.drawImage(img, 0, 0); img.style.display = 'none'; var imageData = ctx.getImageData(0,0,canvas.width, canvas.height); var data = imageData.data; var invert = function() { for (var i = 0; i < data.length; i += 4) { data[i] = 255 - data[i]; // red data[i + 1] = 255 - data[i + 1]; // green data[i + 2] = 255 - data[i + 2]; // blue } ctx.putImageData(imageData, 0, 0); }; var grayscale = function() { for (var i = 0; i < data.length; i += 4) { var avg = (data[i] + data[i +1] + data[i +2]) / 3; data[i] = avg; // red data[i + 1] = avg; // green data[i + 2] = avg; // blue } ctx.putImageData(imageData, 0, 0); }; var invertbtn = document.getElementById('invertbtn'); invertbtn.addEventListener('click', invert); var grayscalebtn = document.getElementById('grayscalebtn'); grayscalebtn.addEventListener('click', grayscale); }
Zooming and anti-aliasing
With the help of the drawImage()
method, a second canvas and the imageSmoothingEnabled
property, we are able to zoom into our picture and see the details.
We get the position of the mouse and crop an image of 5 pixels left and above to 5 pixels right and below. Then we copy that one over to another canvas and resize the image to the size we want it to. In the zoom canvas we resize a 10×10 pixel crop of the original canvas to 200×200.
zoomctx.drawImage(canvas, Math.abs(x - 5), Math.abs(y - 5), 10, 10, 0, 0, 200, 200);
Because anti-aliasing is enabled by default, we might want to disable the smoothing to see clear pixels. You can toggle the checkbox to see the effect of the imageSmoothingEnabled
property (which needs prefixes for different browsers).
Zoom example
<canvas id="canvas" width="300" height="227"></canvas> <canvas id="zoom" width="300" height="227"></canvas> <div> <label for="smoothbtn"> <input type="checkbox" name="smoothbtn" checked="checked" id="smoothbtn"> Enable image smoothing </label> </div>
var img = new Image(); img.src = 'https://mdn.mozillademos.org/files/5397/rhino.jpg'; img.onload = function() { draw(this); }; function draw(img) { var canvas = document.getElementById('canvas'); var ctx = canvas.getContext('2d'); ctx.drawImage(img, 0, 0); img.style.display = 'none'; var zoomctx = document.getElementById('zoom').getContext('2d'); var smoothbtn = document.getElementById('smoothbtn'); var toggleSmoothing = function(event) { zoomctx.imageSmoothingEnabled = this.checked; zoomctx.mozImageSmoothingEnabled = this.checked; zoomctx.webkitImageSmoothingEnabled = this.checked; zoomctx.msImageSmoothingEnabled = this.checked; }; smoothbtn.addEventListener('change', toggleSmoothing); var zoom = function(event) { var x = event.layerX; var y = event.layerY; zoomctx.drawImage(canvas, Math.abs(x - 5), Math.abs(y - 5), 10, 10, 0, 0, 200, 200); }; canvas.addEventListener('mousemove', zoom); }
Saving images
The HTMLCanvasElement
provides a toDataURL()
method, which is useful when saving images. It returns a data URI containing a representation of the image in the format specified by the type
parameter (defaults to PNG). The returned image is in a resolution of 96 dpi.
canvas.toDataURL('image/png')
- Default setting. Creates a PNG image.
canvas.toDataURL('image/jpeg', quality)
- Creates a JPG image. Optionally, you can provide a quality in the range from 0 to 1, with one being the best quality and with 0 almost not recognizable but small in file size.
Once you have generated a data URI from you canvas, you are able to use it as the source of any <image>
or put it into a hyper link with a download attribute to save it to disc, for example.
You can also create a Blob
from the canvas.
canvas.toBlob(callback, type, encoderOptions)
- Creates a
Blob
object representing the image contained in the canvas.
See also
License
© 2016 Mozilla Contributors
Licensed under the Creative Commons Attribution-ShareAlike License v2.5 or later.
https://developer.mozilla.org/en-us/docs/web/api/canvas_api/tutorial/pixel_manipulation_with_canvas