Signature Pad is a JavaScript library for drawing smooth signatures. It's HTML5 canvas based and uses variable width Bézier curve interpolation based on Smoother Signatures post by Square. It works in all modern desktop and mobile browsers and doesn't depend on any external libraries.

Demo works in desktop and mobile browsers. You can check out its source code for some tips on how to handle window resize and high DPI screens. You can also find more about the latter in HTML5 Rocks tutorial.

var canvas = document.querySelector("canvas");

var signaturePad = new SignaturePad(canvas);

// Returns signature image as data URL
signaturePad.toDataURL();

// Draws signature image from data URL
signaturePad.fromDataURL("data:image/png;base64,iVBORw0K...");

// Clears the canvas
signaturePad.clear();

// Returns true if canvas is empty, otherwise returns false
signaturePad.isEmpty();

If you are not familiar with data URI scheme, you can read more about it on Wikipedia.

There are 2 ways you can handle data URI encoded images.

You could simply store it in your database as a string and display it in HTML like this:

<img src="data:image/png;base64,iVBORw0K..." />

but this way has many disadvantages - it's not easy to get image dimensions, you can't manipulate it e.g. to create a thumbnail and it also has some performance issues on mobile devices.

Thus, more common way is to decode it and store as a file. Here's an example in Ruby:

require "base64"

data_uri = "data:image/png;base64,iVBORw0K..."
encoded_image = data_uri.split(",")[1]
decoded_image = Base64.decode64(encoded_image)
File.open("signature.png", "wb") { |f| f.write(decoded_image) }

• Fix bug where stroke becomes very thin. mvirkkunen

• Fix SignaturePad#fromDataURL on Firefox. Fr3nzzy

• Make SignaturePad#isEmpty return false after loading an image using SignaturePad#fromDataURL. krisivanov

• Fixed SignaturePad#clear().

• Add backgroundColor option to set custom color of the background on SignaturePad#clear().
• Rename color option to penColor.
• Fix passing arguments to canvas element on SignaturePad#toDataURL().

Released under the MIT License.