HTML5 FileApi and Jpeg Meta-data

Wed, 30 Dec 2009 10:03:17 +0000
tech html5 javascript jpeg exif

I’m really impressed with the way the HTML5 spec is going, and the fact that it is quickly going to become the default choice for portable application development.

One of the lastest additions to help support application development is the File API. This API enables a developer to gain access to the contents of files locally. The main new data structure that a developer if provided with is a FileList objects which represents an array of File objects. FileList objects can be obtained from two places; input form elements and from drag & drop DataTransfer objects.

Based on this latest API, I’ve created a simple library, JsJpegMeta for parsing Jpeg meta data.

I’ve hacked together a example that demonstrates the library. Just select a JPEG file from the form, or drag a JPEG file onto the window. For large JPEG files you might need to be a little bit patient, as it can be a little slow. This slowness, suprisingly, doesn’t appear to be the Javascript part, but rather Firefox’s handling of large data: URLs and JPEG display in general.

The rest of this post goes into some of the details. Unfortunately only Firefox 3.6 supports these new APIs right now.

Using the File API

Here is an example of how to get access to a FileList. When the user chooses a file, it calls the Javascript function loadFiles. (Assuming you have already defined that function).

  <form id="form" action="javascript:void(0)">
    <p>Choose file: <input type="file" onchange="loadFiles(this.files)" /></p>

A File object just provides a reference to a file; to actually get some data out of the file you need to use a FileReader object. The FileReader object provides an asynchronous API for reading the file data into memory. Three different methods are provided by the FileReader object; readAsBinaryString, readAsText and readAsDataURL. A callback, onloadend, is executed when the file has been read into memory, the data is then available via the result field.

Here example of what the loadFiles function might look like:

function loadFiles(files) {
    var binary_reader = new FileReader();

    binary_reader.file = files[0];
    binary_reader.onloadend = function() {
        alert("Loaded file: " + + " length: " + this.result.length);


Note the $("form").reset(); clears the input form.

Drag & Drop

Forms are not the only way to get a FileList, you can also get files from drag and drop event. You need to handle three events; dragenter, dragover and drop.

<body ondragenter="dragEnterHandler(event)" ondragover="dragOverHandler(event)" ondrop="dropHandler(event)">

The default handling of these are fairly striaght forward:

function dragEnterHandler(e) { e.preventDefault(); }
function dragOverHandler(e) { e.preventDefault(); }
function dropHandler(e) {

Parsing files

The interesting thing here is the readAsBinaryString, when this method is used result ends up being a binary string. This is pretty new because, as far as I know, there hasn’t really been a good way to access binary data in Javascript before. Each character in the binary string represents a byte, and has a character code in the range [0..255].

This is great, because it means that we can parse binary strings locally, without having to upload files to a server for processing. Unfortunately there isn’t a great deal of support for handling binary data in Javacript; there isn’t anything like Python’s struct module.

Luckily it isn’t too hard to write something close to this. Mostly we wanted to parse unsigned and signed integers of arbitrary length. To be useful, we need to handle both little and big endianess. A very simple implementation of parsing an unsigned integer is:

    function parseNum(endian, data, offset, size) {
	var i;
	var ret;
	var big_endian = (endian === ">");
	if (offset === undefined) offset = 0;
	if (size === undefined) size = data.length - offset;
	for (big_endian ? i = offset : i = offset + size - 1; 
	     big_endian ? i < offset + size : i >= offset; 
	     big_endian ? i++ : i--) {
	    ret <<= 8;
	    ret += data.charCodeAt(i);
	return ret;

endian specifies the endianess; the string literal ">" for big-endian and "<" for little-endian. (Copying the Python struct module). data is the binary data to parse. An offset can be specified to enable parsing from the middle of a binary structure; this defaults to zero. The size of the integer to parse can also be specified; it defaults to the remainder of the string.

Signed integers require a little bit more work. Although there are multiple ways of representing signed numbers, by far the most common is the two’s complement method. A function that has the same inputs as parseNum is:

    function parseSnum(endian, data, offset, size) {
	var i;
	var ret;
	var neg;
	var big_endian = (endian === ">");
	if (offset === undefined) offset = 0;
	if (size === undefined) size = data.length - offset;
	for (big_endian ? i = offset : i = offset + size - 1; 
	     big_endian ? i < offset + size : i >= offset; 
	     big_endian ? i++ : i--) {
	    if (neg === undefined) {
		/* Negative if top bit is set */
		neg = (data.charCodeAt(i) & 0x80) === 0x80;
	    ret <<= 8;
	    /* If it is negative we invert the bits */
	    ret += neg ? ~data.charCodeAt(i) & 0xff: data.charCodeAt(i);
	if (neg) {
	    /* If it is negative we do two's complement */
	    ret += 1;
	    ret *= -1;
	return ret;

JpegMeta API

JpegMeta is a simple, pure Javascript library for parsing Jpeg meta-data. To use it include the jpegmeta.js file. This creates a single, global, module object JpegMeta. The JpegMeta module object has one public interface of use, the JpegFile class. You can use this to construct new JpegFile class instances. The input is a binary string (for example as returned from a FileReader object. An example is:

	var jpeg = new JpegMeta.JpegFile(this.result,;

After creation you can then access various meta-data properties, categorised by meta-data groups. The main groups of meta-data are:

Information extracted from the JPEG SOF segment. In particular the hieght, width and colour depth.
Meta-data from the JFIF APP0 segment. This usually includes resolution, aspect ratio and colour space meta-data.
Generic TIFF meta-data extracted from the Exif meta-data APP1 segment. Includes things such as camera make and model, orientation and date-time.
Exif specific meta-data extracted from the Exif meta-data APP1 segment. Includes camera specific things such as white balance, flash, metering mode, etc.
GPS related information extracted from the Exif meta-data APP1 segment. Includes atitude, longitude etc.

Meta-data groups can be access directly, for example:

 var group = jpeg.gps;

A lookup table is also provided: jpeg.metaGroups. This associative array can be used to determine which meta-groups a particular jpeg file instance actually has.

The MetaGroup object has a name field, a description field and an associative array of properties.

Properties in a given group can be accessed directly. E.g:

 var lat = jpeg.gps.latitude;

Alternatively, the metaProps associative array provides can be used to determine which properties are available.

The metaProp object has a name field, description field, and also a value field.


The File API adds a poweful new capability to native HTML5 applications.

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