Category Archives: code

Best economy flight evva!

Over the years, I have flown a lot – mainly between Sydney and Frankfurt or Sydney and San Francisco. Today, for the first time in a long time, I had a flight with Qantas from Sydney to San Francisco. And I must say: it was the most productive and most comfortable economy flight I had in a long time.

This is gonna feel awkward, since it’s not one of my usual technical posts. But I just have to say “Thank you” to Qantas. When I fly to the US, I tend to catch a US airline because they usually turn up as the cheapest. This time, Qantas was the second cheapest, so I decided to spend the extra hundred bucks on getting a modern airline. Yes, get that US airlines: no matter which of you I take, I always feel like I am thrown back into the last century. Legspace is rare, seats are uncomfortable, food is crap, service is poor, oh … and have you ever heard of personal entertainment screens? Yes, I know, your planes are from the last century. But honestly: I had a personal entertainment screen on my Singapore Airlines flight when coming to Australia for the first time in 1998! Couldn’t you at least upgrade the inside of your planes?

Anyway, back to this flight. It all started with the question: would you like to sit in the centre isle in front of the baby bassinet? Oh, I usually take a window seat to get some peace and quiet – but hey, I’m not going to say “no” to space! And, man did I use it!

I settled in with a good book and a little nap until the first meal and after that felt strengthened and awake enough to start hacking. With my new MacBook Pro, I was bound to get a few hours in before the battery would die on me. Not the 7 hours, that Apple claims, but that’s because I was going to do lots of compiles of Firefox. Anyway – without a seat in front of me, without the personal entertainment screen pulled out, and with the nice thick cushion that Qantas supply on my lap, protecting me from the laptop heat, I almost felt like I was back home in my living room.

On top of that – and unfortunately for Qantas, but fortunately for me – the plane was only two thirds full, so I had the middle seat on my left empty, which I immediately used to extend my table space. I had continuing catering service for the next 4-5 hours of compiling, applying OggK patches to the new Chris Double Firefox codebase, and fixing compile errors (all configuration based – I have yet to get to writing actual code). Ongoing catering service, no need to cook for myself, uninterrupted coding time, good music from the inflight entertainment service – I think I’ll move my office into a Qantas plane! Not been this productive in ages!

Everywhere around me the lights were out, people were watching movies, but I was working and really enjoying it. And then, the battery was empty, half way into the flight. Bummer! But I didn’t give up this easily. Thought it’d be worth asking if there was a way to recharge without occupying a toilet for two hours. And as with everything else, Qantas inflight personnel made an extra effort to please: they found me a empty seat in business class and hooked up the laptop for an hour to recharge. Totally, utterly awesome! I got it back after another nice reading break – cannot start watching movies, since that makes the brain go mash. I got another few hours of compiling in before my body forced me to catch a few hours of sleep.

Now, I’m about an hour away from San Fran and the laptop claims 40min of power left. Funnily, that number seems to go up rather than down, so I’m sure it will last until arrival (uh! It’s now at 1:24min – oh, compilation just finished!). Hopefully I will be able to find out, why some of the Ogg Theora/Vorbis/Kate videos that I created using kateenc and oggz-merge don’t play in the patched Firefox. After all, it would be awesome to be able to show it off in the upcoming HTML5 Video Accessibility workshop!

New proposal for captions and other timed text for HTML5

The first specification for how to include captions, subtitles, lyrics, and similar time-aligned text with HTML5 media elements has received a lot of feedback – probably because there are several demos available.

The feedback has encouraged me to develop a new specification that includes the concerns and makes it easier to associate out-of-band time-aligned text (i.e. subtitles stored in separate files to the video/audio file). A simple example of the new specification using srt files is this:

<video src="video.ogv" controls>
   <itextlist category="CC">
     <itext src="" lang="en"/>
     <itext src="" lang="de"/>
     <itext src="" lang="fr"/>
     <itext src="" lang="jp"/>

By default, the charset of the itext file is UTF-8, and the default format is text/srt (incidentally a mime type the still needs to be registered). Also by default the browser is expected to select for display the track that matches the set default language of the browser. This has been proven to work well in the previous experiments.

Check out the new itext specification, read on to get an introduction to what has changed, and leave me your feedback if you can!

The itextlist element
You will have noticed that in comparison to the previous specification, this specification contains a grouping element called “itextlist”. This is necessary because we have to distinguish between alternative time-aligned text tracks and ones that can be additional, i.e. displayed at the same time. In the first specification this was done by inspecting each itext element’s category and grouping them together, but that resulted in much repetition and unreadable specifications.

Also, it was not clear which itext elements were to be displayed in the same region and which in different ones. Now, their styling can be controlled uniformly.

The final advantage is that association of callbacks for entering and leaving text segments as extracted from the itext elements can now be controlled from the itextlist element in a uniform manner.

This change also makes it simple for a parser to determine the structure of the menu that is created and included in the controls element of the audio or video element.

Incidentally, a patch for Firefox already exists that makes this part of the browser. It does not yet support this new itext specification, but here is a screenshot that Felipe Corr

HTML5 audio element accessibility

As part of my experiments in video accessibility I am also looking at the audio element. I have just finished a proof of concept for parsing Lyrics files for music in lrc format.

The demo uses Tay Zonday’s “Chocolate Rain” song both as a video with subtitles and as an audio file with lyrics. Fortunately, he published these all under a creative commons license, so I was able to use this music file. BTW: I found it really difficult to find a openly licensed music file with lyrics.

While I was at it, I also cleaned up all the old demos and now have a nice list of all demos in a central file.

Updated video accessibility demo

Just a brief note to share that I have updated the video accessibility demo at

It should now support ARIA and tab access to the menu, which I have simply put next to the video. I implemented the menu by learning from YUI. My Firefox 3.5.3 actually doesn’t tab through it, but then it also doesn’t tab through the YUI example, which I think is correct. Go figure.

Also, the textual audio descriptions are improved and should now work better with screenreaders.

I have also just prepared a recorded audio description of “Elephants Dreams” (German accent warning).

You can also download the multitrack Ogg Theora video file that contains the original audio and video track plus the audio description as an extra track, created using oggz-merge.

As soon as some kind soul donates a sign language track for “Elephants Dream”, I will have a pretty complete set of video accessibility tracks for that video. This will certainly become the basis for more video a11y work!

URI fragments vs URI queries for media fragment addressing

In the W3C Media Fragment Working Group (MFWG) we have had long discussions about the use of the URI query (“?”) or the URI fragment (“#”) addressing approach for addressing directly into media fragments, and the diverse new HTTP headers required to serve such URI requests, considering such side conditions as the stripping-off of fragment parameters from a URI by Web browsers, or the existence of caching Web proxies.

As explained earlier, URI queries request (primary) resources, while URI fragments address secondary resources, which have a relationship to their primary resource. So, in the strictest sense of their specifications, to address segments in media resources without losing the context of the primary resource, we can only use URI fragments.

Browser-supported Media Fragment URIs

For this reason, URI fragments are also the way in which my last media fragment addressing demo has been implemented. For example, I would address

Demo of deep hyperlinking into HTML5 video

In an effort to give a demo of some of the W3C Media Fragment WG specification capabilities, I implemented a HTML5 page with a video element that reacts to fragment offset changes to the URL bar and the <video> element.

Demo Features

The demo can be found on the Annodex Web server. It has the following features:

If you simply load that Web page, you will see the video jump to an offset because it is referred to as “elephants_dream/elephant.ogv#t=20”.

If you change or add a temporal fragment in the URL bar, the video jumps to this time offset and overrules the video’s fragment addressing. (This only works in Firefox 3.6, see below – in older Firefoxes you actually have to reload the page for this to happen.) This functionality is similar to a time linking functionality that YouTube also provides.

When you hit the “play” button on the video and let it play a bit before hitting “pause” again – the second at which you hit “pause” is displayed in the page’s URL bar . In Firefox, this even leads to an addition to the browser’s history, so you can jump back to the previous pause position.

Three input boxes allow for experimentation with different functionality.

  • The first one contains a link to the current Web page with the media fragment for the current video playback position. This text is displayed for cut-and-paste purposes, e.g. to send it in an email to friends.
  • The second one is an entry box which accepts float values as time offsets. Once entered, the video will jump to the given time offset. The URL of the video and the page URL will be updated.
  • The third one is an entry box which accepts a video URL that replaces the <video> element’s @src attribute value. It is meant for experimentation with different temporal media fragment URLs as they get loaded into the <video> element.

Javascript Hacks

You can look at the source code of the page – all the javascript in use is actually at the bottom of the page. Here are some of the juicy bits of what I’ve done:

Since Web browsers do not support the parsing and reaction to media fragment URIs, I implemented this in javascript. Once the video is loaded, i.e. the “loadedmetadata” event is called on the video, I parse the video’s @currentSrc attribute and jump to a time offset if given. I use the @currentSrc, because it will be the URL that the video element is using after having parsed the @src attribute and all the containing <source> elements (if they exist). This function is also called when the video’s @src attribute is changed through javascript.

This is the only bit from the demo that the browsers should do natively. The remaining functionality hooks up the temporal addressing for the video with the browser’s URL bar.

To display a URL in the URL bar that people can cut and paste to send to their friends, I hooked up the video’s “pause” event with an update to the URL bar. If you are jumping around through javascript calls to video.currentTime, you will also have to make these changes to the URL bar.

Finally, I am capturing the window’s “hashchange” event, which is new in HTML5 and only implemented in Firefox 3.6. This means that if you change the temporal offset on the page’s URL, the browser will parse it and jump the video to the offset time.


Doing these kinds of jumps around on video can be very slow when the seeking is happening on the remote server. Firefox actually implements seeking over the network, which in the case of Ogg can require multiple jumps back and forth on the remote video file with byte range requests to locate the correct offset location.

To reduce as much as possible the effort that Firefox has to make with seeking, I referred to Mozilla’s very useful help page to speed up video. It is recommended to deliver the X-Content-Duration HTTP header from your Web server. For Ogg media, this can be provided through the oggz-chop CGI. Since I didn’t want to install it on my Apache server, I hard coded X-Content-Duration in a .htaccess file in the directory that serves the media file. The .htaccess file looks as follows:

<Files "elephant.ogv">
Header set X-Content-Duration "653.791"

This should now help Firefox to avoid the extra seek necessary to determine the video’s duration and display the transport bar faster.

I also added the @autobuffer attribute to the <video> element, which should make the complete video file available to the browser and thus speed up seeking enormously since it will not need to do any network requests and can just do it on the local file.


This is only a first and very simple demo of media fragments and video. I have not made an effort to capture any errors or to parse a URL that is more complicated than simply containing “#t=”. Feel free to report any bugs to me in the comments or send me patches.

Also, I have not made an effort to use time ranges, which is part of the W3C Media Fragment spec. This should be simple to add, since it just requires to stop the video playback at the given end time.

Also, I have only implemented parsing of the most simple default time spec in seconds and fragments. None of the more complicated npt, smpte, or clock specifications have been implemented yet.

The possibilities for deeper access to video and for improved video accessibility with these URLs are vast. Just imagine hooking up the caption elements of e.g. an srt file with temporal hyperlinks and you can provide deep interaction between the video content and the captions. You could even drive this to the extreme and jump between single words if you mark up each with its time relationship. Happy experimenting!

UPDATE: I forgot to mention that it is really annoying that the video has to be re-loaded when the @src attribute is changed, even if only the hash changes. As support for media fragments is implemented in <video> and <audio> elements, it would be advantageous if the “load()” function checked whether only the hash changed and does not re-load the full resource in these cases.

Thanks go to Chris Double and Chris Pearce from Mozilla for their feedback and suggestions for improvement on an early version of this.

Media Fragment addressing into a live stream

A few months back, Thomas reported on a cool flumotion experiment that he hacked together which allows jumping back in time on a live video stream.

Thomas used a URI scheme with a negative offset to do the jumping back on the http stream:

John left a comment pointing to current work being done in the W3C on Media Fragment addressing, but had to notice that despite Annodex’s temporal URIs having a live stream addressing feature, the new W3C draft didn’t accommodate such a use case.

We got to work in the working group and I am very happy to announce that as of today there is now a draft specification for addressing time offsets by wall-clock time.

Say, you are watching Thomas’ live stream from above at http://localhost:8800 and you want to jump back by 2 min. Your player would grab the current streaming time, e.g. 2009-08-26T12:34:04Z and subtract the two minutes, giving 2009-08-26T12:32:04Z. Then the player would use this to tell your streaming server to jump back by two minutes using this URL:

Or another example would be: you had a stream running all day from a conference and you want to go back to a particular session. You know that it was between 10am and 11am German time (UTC+2 right now). Then your URL would be as follows:

Now if only there was an implementation… 🙂

Jumping to time offsets in HTML5 video

For many years now I have been progressing a deeper view of video on the Web than just as a binary blob. We need direct access to time offsets and sections of videos.

Such direct access can be achieved either by providing a javascript interface through which a video’s playback position can be controlled, or by using URLs that directly communicate with the Web server about controlling the playback position. I will explain the approaches that can be applied on the HTML5 <video> tag for such deep video interaction.

Controlling a video’s playback with javascript


Right now, you can use the video element’s “currentTime” property to read and set the current playback position of a video resource. This is very useful to directly jump between different sections in the video, such as exemplified in the BBC’s recent R&D TV demo. To jump to a time offset in a video, all you have to do in javascript is:

var video = document.getElementsByTagName("video")[0];
video.currentTime = starttimeoffset;


Further, if you want to stop playback at a certain time point, you can use another functionality of the HTML5 <video> tag: the “timeupdate” event:

video.addEventListener("timeupdate", function() {
if (video.currentTime >= endtimeoffset) {
}}, false);

When the “timeupdate” event fires, which is supposed to happen at a min resolution of 250ms, you can catch the end of your desired interval fairly accurately.

setTimeout / setInterval

Alternatively to using the “timeupdate” event that is provided by the <video> tag, there is always the possibility of using the javascript “setTimeout” or “setInterval” functions:

setTimeout(video.pause(), (endtimeoffset - starttimeoffset)*1000);

The “setTimeout” function is used to call a function or evaluate an expression after a specified number of milliseconds. So, you’d have to call this straight after starting the playback at the given starttimeoffset.

If instead you wanted something to happen at a frequent rate in parallel to the video playback (such as check if you need to display a new ad or a new subtitle), you could use the javascript setInterval function:

setInterval( function() {displaySubtitle(video.currentTime);}, 100);

The “setInterval” function is used to call a function or evaluate an expression at the specified intervall. So, in the given example, every 100ms it is tested whether a new subtitle needs to be displayed for the video current playback time.

Note that for subtitles it makes a lot more sense to use the existing “timeupdate” event of the video rather than creating a frequenty setInterval interrupt, since this will continue calling the function until clearInterval() is called or the window is closed. Also, the BBC found in experiments with Firefox that “timeupdate” is more accurate than polling the “currentTime” regularly.

Controlling a video’s playback through a URL

There are some existing example implementations that control a video’s playback time through a URL.

In 2001, in the Annodex project we proposed temporal URIs and implemented the spec for Ogg content. This is now successfully in use at, where it is very useful since Metavid handles very long videos where direct access to subsections is critical. A URL such as work well to directly view that segment.

More recently, YouTube rolled out a URI scheme to directly jump to an offset in a YouTube video, e.g. While most YouTube content is short form, and such direct access may not make much sense for a video of less than 2 min duration, some YouTube content is long enough to make this a very useful feature.

You may have noticed that the YouTube use of URIs for jumping to offsets is slightly different to the one used by Metavid. The YouTube video will be displayed as always, but the playback position in the video player changes based on the time offset. The Metavid video in contrast will not display a transport bar for the full video, but instead only present the requested part of the video with an appropriate localised keyframe.

Having realised the need for such URLs, the W3C created a Media Fragments working group.

Proposed Time schemes

For temporal addressing, it currently proposes the following schemes:


If there is no time scheme given, it defaults to “npt”, which stands for “normal playback time”. It is basically a time offset given in seconds, but can be provided in a few different formats.

If a “smpte” scheme is given, the time code is provided in the way in which DVRs display time codes, namely according to the SMPTE timecode standard.

Finally, a “clock” time scheme can be given. This is relevant in particular to live streaming applications, which would like to provide a URL under which a live video is provided, but also allow the user to jump back in time to previously streamed data.

Fragments and Queries

Further, the W3C Media Fragment Working Group is discussing the use of both URI addressing schemes for time offsets: fragments (“#”) and queries (“?”).

The important difference is that queries produce a new resource, while fragments provide a sub-resource.

This means that if you load a URI such as,100 , the resulting resource is a video of duration 40s. Since relates to the full resource, it is possible to expect from the user agent (i.e. web browser) to display a timeline of 60-100 rather than 0-40 – after all, the browser could just get this out of the URL. However, it is essentially a new resource and could therefore just be regarded as a different video.

If instead you load a URI such as,100, the user agent recognizes as the resource and knows that it is supposed to display the 40s extract of that resource. Using no special server support, the browser could just implement this using the currentTime and timeUpdate javascript functionality.

An optimisation should, however, be made on this latter fragment delivery such that a user does not have to wait until the full beginning of the resource is downloaded before playback starts: Web servers should be expected to implement a server extension that can deal with such offsets and then deliver from the time offset rather than the beginning of the file.

How this is communicated to the server – what extra headers or http communication mechanisms should be used – is currently under discussion at the W3C Media Fragments working group.

The different aspects of video accessibility

In the last week, I have received many emails replying to my request for feedback on the video accessibility demo. Thanks very much to everyone who took the time.

Interestingly, I got very little feedback on the subtitles and textual audio annotation aspects of my demo, actually, even though that was the key aspect of my analysis. It’s my own fault, however, because I chose a good looking video player skin over an accessible one.

This is where I need to take a step back and explain about the status of HTML5 video and its general accessibility aspects. Some of this is a repetition of an email that I sent to the W3C WAI-XTECH mailing list.

Browser support of HTML5 video

The HTML5 video tag is still a rather new tag that has not been implemented in all browsers yet – and not all browsers support the Ogg Theora/Video codec that my demo uses. Only the latest Firefox 3.5 release will support my demo out of the box. For Chrome and Opera you will have to use the latest nightly build (which I am not even sure are publicly available). IE does not support it at all. For Safari/Webkit you will need the latest release and install the XiphQT quicktime component to provide support for the codec.

My recommendation is clearly to use Firefox 3.5 to try this demo.

Standardisation status of HTML5 video

The standardisation of the HTML5 video tag is still in process. Some of the attributes have not been validated through implementations, some of the use cases have not been turned into specifications, and most importantly to the topic of interest here, there have been very little experiments with accessibility around the HTML5 video tag.

Accessibility of video controls

Most of the comments that I received on my demo were concerned with the accessibility of the video controls.

In HTML5 video, there is a attribute called @controls. If it is available, the browser is expected to display default controls on top of the video. Here is what the current specification says:

“This user interface should include features to begin playback, pause playback, seek to an arbitrary position in the content (if the content supports arbitrary seeking), change the volume, and show the media content in manners more suitable to the user (e.g. full-screen video or in an independent resizable window).”

In Firefox 3.5, the controls attribute currently creates the following controls:

  • play/pause button (toggles between the two)
  • slider for current playback position and seeking (also displays how much of the video has currently been downloaded)
  • duration display
  • roll-over button for volume on/off and to display slider for volume
  • FAIK fullscreen is not currently implemented

Further, the HTML5 specification prescribes that if the @controls attribute is not available, “user agents may provide controls to affect playback of the media resource (e.g. play, pause, seeking, and volume controls), but such features should not interfere with the page’s normal rendering. For example, such features could be exposed in the media element’s context menu.”

In Firefox 3.5, this has been implemented with a right-click context menu, which contains:

  • play/pause toggle
  • mute/unmute toggle
  • show/hide controls toggle

When the controls are being displayed, there are keyboard shortcuts to control them:

  • space bar toggles between play and pause
  • left/right arrow winds video forward/back by 5 sec
  • CTRL+left/right arrow winds video forward/back by 60sec
  • HOME+left/right jumps to beginning/end of video
  • when focused on the volume button, up/down arrow increases/decreases volume

As for exposure of these controls to screen readers, Mozilla implemented this in June, see Marco Zehe’s blog post on it. It implies having to use focus mode for now, so if you haven’t been able to use keyboard for controlling the video element yet, that may be the reason.

New video accessibility work

My work is actually meant to take video accessibility a step further and explore how to deal with what I call time-aligned text files for video and audio. For the purposes of accessibility, I am mainly concerned with subtitles, captions, and audio descriptions that come in textual form and should be read out by a screen reader or made available to braille devices.

I am exploring both, time-aligned text that comes within a video file, but also those that are available as external Web resources and are just associated to the video through HTML. It is this latter use case that my demo explored.

To create a nice looking demo, I used a skin for the video player that was developed by somebody else. Now, I didn’t pay attention to whether that skin was actually accessible and this is the source of most of the problems that have been mentioned to me thus far.

A new, simpler demo

I have now developed a new demo that uses the default player controls which should be accessible as described above. I
hope that the extra button that I implemented for the menu with all the text tracks is now accessible through a screen reader, too.

UPDATE: Note that there is currently a bug in Firefox that prevents tabbing to the video element from working. This will be possible in future.

First experiments with itext

My accessibility work for Mozilla is showing first results.

I have now implemented a demo for the previously proposed <itext> element. During the development process, the specification became more concrete.

I’m sure you’re keen to check out the demo.

Please note the following features of the demo:

  • It experiments with four different types of time-aligned text: subtitles, captions, chapters, and textual audio annotations.
  • It extends the video controls by a menu button for the time-aligned text tracks. This enables the user to switch between different languages for the different tracks.
  • The textual audio annotations are mapped into an aria-live activated div element, such that they are indeed read out by screen-readers; this div sits behind the video, invisible to everyone else.
  • The chapters are displayed as text on top of the video.
  • The subtitles and captions are displayed as overlays at the bottom of the video.
  • The display styles and positions are supposed to be default display mechanisms for these kinds of tracks, that could be overwritten by the stylesheet of a Web developer, who intends to place the text elsewhere on screen.

In order to “hear” the textual audio annotations work, you will need to install a screen reader such as JAWS, NVDA, or the firevox plugin on the Mac.

As far as I am aware, this is the first demo of HTML5 video accessibility that includes support for the vision-impaired, hearing-impaired, and also for foreign language speakers.

There have been initial discussions about this proposal, the results of which are captured in the wiki page. I expect a lot more heated discussion will happen on the WHATWG mailing list when I post it soon. I am well aware that probably most of the javascript API will need to be changed, and also some of the HTML.

Also please note that there are some bugs still left on the software, which should not inhibit the discussion at this stage. We will definitely develop a newer and better version.

I am particularly proud that I was able to make this work in the experimental builds of Opera and Chrome, as well as in Safari with XiphQT installed, and of course in Firefox 3.5.

Screenshot of first itext video player
Screenshot of first itext video player experiment