Category: "Uncategorized"

Window Recording

As of bug 1536174, I've added a mechanism for Firefox that uses an internal mechanism that can record content frames during composition. Note that this currently only works when on Windows using D3D11 composition. This is still in very early stages and will likely be getting some improvements over the longer term, but right now, basically this is how it works:

  1. Make sure you're on nightly, the keyboard shortcut we'll be using only works there.
  2. Before initiating the action you want to record, press 'Ctrl+Shift+4', this will begin recording
  3. Execute an action in the browser (for example load a page)
  4. Wait for whatever operation you wish to capture to finish
  5. Press 'Ctrl+Shift+4' again, this will end recording and generate output

You will now find a new directory in your working directory called windowrecording-<timestamp> which contains a list of PNGs, this list of PNGs uses the following naming convention:

frame-<framenumber>-<ms since recording start>.png

You will notice only the frames where actual content changes occurred (no scrolling, asynchronous animation, parent process changes, etc.), this will likely become more flexible in the future. The directory where the output is delivered can optionally be selected using the layers.windowrecording.path preference. Use a trailing slash for expected behavior.

There's a couple of caveats when using recording:

  • In order to minimize overhead during recording frames are stored uncompressed in memory, recordings more than a few seconds will consume very large amounts of memory
  • Lossless compression is used for the output, this means the recordings can be relatively large

Pageload Analysis

Recently I've been working on a script that can use the generated recordings to perform pageload analysis. The scripts can be found here and in this post I will attempt to describe the right way to use them. The scripts, as well as this entire description are meant to be used on windows, so these instructions will not be accurate on other platforms (and in any case, recording doesn't work there yet either, nor will most people have an open source PowerShell implementation installed). This is also in very early stages and not particularly user friendly, you probably shouldn't be using it yet and just skip to the demo video :-).


Note that also expects ffmpeg and the imagemagick binary locations to be included in the path.

The next step is to modify the SetPaths.ps1 script to point to the correct binaries for your system, these are the binaries that the AnalyzeWindowRecording script will use.

Recording Pageload

The next step is recording a pageload. The first step would be to go to 'about:blank' to ensure a solid white reference frame to start from. A current weakness is that the timestamp of the video (and therefore the point in time the analysis will consider the 'beginning' of pageload) is the time when the recording begins, rather than navigation start. Therefore, it is best to navigate immediately after beginning the recording, this could be improved by scripting the recording and navigation start to occur at the same time, but for now we'll assume that a small offset may be considered acceptable.

First start the recording and immediately navigate to the desired page, wait for pageload to finish visually and then end the recording.


The final step is to run the analysis script, this is done by executing the script as follows:

.\AnalyzeWindowRecording.ps1 <directory containing recording> <annotated output video>

Note that the script may take a while to execute, particularly on a slower machine. The script will output the recorded FirstVisualChange, LastVisualChange and SpeedIndex to stdout, as well as generate an annotated video that will display information at the stop about the current visual completeness and the different stages of the video.

It is important to note that the script will currently chop off the top 197 pixels of the frames, this was accurate for my recordings but most likely isn't for people recording using different DPI scaling, in the future I will make this parameter configurable or possibly even automatically detected, however for now you will have to manually adjust the $chop variable at the top of the AnalyzeWindowRecording script for your situation.

Finally, I realize these are lengthy instructions and usage of this functionality is currently not for the faint of heart. I wanted to make this information available as quickly as possible though and we expect to be improving on this in the future, let me know in the comments or on IRC if you have any questions or feedback.

So, not having had time to post on here for a long time. I realized there's a problem a bunch of us at Mozilla are facing, we've been using mercurial queues for I don't know how long, but we're increasingly facing a toolchain that isn't compatible with the MQ workflow. I found patches in my queue inadvertently being converted into actual commits and other such things. I'm no expert on versioning systems, and as such mercurial queues provided me with an easy method of just having a bunch of patches, and a file which orders them, and that was easy for me to understand and work with. Seeing an increasing amount of tools not supporting it though, I decided to make the switch, and I'd like to document my experience here, some of my suggestions may not be optimal, please let me know if any of my suggestions are unwise. I also use Windows as my primary OS, mileage on other operating systems may vary, but hopefully not by much.

First, preparation, make sure you've got the latest version of mercurial using ./mach bootstrap, when you get to the mercurial configuration wizard, enable all the history editing and evolve extensions, you will need them for this to work.

Now, to go through the commands, first, the basics:

hg qnew basically just becomes hg ci we're going to assume none of our commits are necessarily permanent, and we're fine having hidden, dead branches in our repository.

hg qqueue is largely replaced by hg bookmark, it allows you to create a new 'bookmarked branch', list the bookmarked branches and which is active. An important difference is that a bookmark describes the tips of the different branches. Making new commits on top of a bookmark will migrate the bookmark along with that commit.

hg up [bookmark name] will activate a different bookmark, hopping to the tip of that branch.

hg qpop once you've created a new commit becomes hg prev an important thing to note is that unlike with qpop, 'tip' will remain the tip of your current bookmark. Note that unlike with qpop, you can 'prev' right past the root of your patch set and through the destination repository, so make sure you're at the right changeset! It's also important to note this deactivates the current bookmark.

Once you've popped all the way to the tree you're working on top of, you can just use hg ci and hg bookmark again to start a new bookmarked branch (or queue, if you will).

hg qpush when you haven't made any changes bascially becomes hg next, it will take you to the next changeset, if there's multiple branches coming off here, it will offer you a prompt to select which one you'd like to continue on.

Making changes inside a queue

Now this is where it gets a little more complicated, there's essentially two ways one could make changes to an existing queue, first, there is the most common action of changing an existing changeset in the queue, this is fairly straightforward:

  • Use `hg prev` to go to the changeset you wish to modify, much like in mq
  • Make your changes and use `hg ci --amend` much like you would `hg qref`, this will orphan its existing children
  • Use `hg next --evolve` as a qpush for your changesets, this will rebase them back on top of your change, and offer a 3-way merging tool if needed.

In short qpop, make change, qref, qpush becomes prev, make change, ci --amend, next --evolve.

The second method to make changes inside a queue is to add a changeset inbetween two changesets already in the queue. In the past this was straightforward, you qpopped, made changes, qnewed, and just happily qpushed the rest of your queue on top of it, the new method is this:

  • Use `hg prev` to go to the changeset you wish to modify, much like in mq
  • Make your changes and use `hg ci ` much like you would `hg qnew`, this will create a new branching point
  • Use `hg rebase -b [bookmark name/revision]`, this will rebase your queue back on top of your change, and offer a 3-way merging tool if needed.
  • Use `hg next` to go back down your 'queue'


Basically hg qfin is no longer needed, you go to the changeset where you want to push and you can push up until that changeset directly to, for example, central. ./mach try also seems to work as expected and submits the changeset you're currently at.

Some additional tips

The hg absorb extension I've found to be quite powerful in combination with the new system, particularly when processing review comments. Essentially you can make a bunch of changes from the tip of your patch queue, execute the command, and based on where the changes are it will attempt to figure out which commits they belong to, and essentially amend these commits with the changes, without you ever having to leave the tip of your branch. This not only takes away a bunch of work, it also means you don't retouch all of the files affected by your queue, greatly reducing rebuild times.

I've found that being able to create additional branching points, or queues, if you will, off some existing work on occasion is a helpful addition to the abilities I had with mercurial queues.

Final Thoughts

In the end I like my versioning system not to get in the way of my work, I'm not necessarily convinced that the benefits outweigh the cost of learning a new system or the slightly more complex actions required for what to me are the more common operations. But with the extensions now available I can keep my workflow mostly the same, with the added benefit of hg absorb I hope this guide will make the transition easy enough that in the end most of us can be satisfied with the switch.

If I missed something, am in error somewhere, or if a better guide exists out there somewhere (I wasn't able to find one or I wouldn't have written this :-)), do let me know!

Disclaimer: The opinions I will be expressing will be solely my own, they will in no way be related to Mozilla or the work I do there.

This is the second part of a series of post I will be doing on society's current issues, you can find my previous post here: When silence just isn't an option

So, I've heard a lot of people say we now live in a 'post-fact' society. I've found this a very interesting topic recently, since information is so important when making decisions, if 'facts' are no longer important. It seems like that might be a bit of a problem? I will attempt to divulge my thoughts here on what the deal is with facts, what they are, and how we can go (back) to being a 'factual' society.

It should be pretty simple to establish facts, right?

Let me start with a little about what makes a fact, whenever you talk about something like this anyone can come up with quasi-philosophical arguments about how there are no facts. Maybe they're right, our eyes can easily be tricked, maybe the moon isn't there when I'm not looking at it, maybe the sun does have an army of invisible tiny little midgets orbiting it and maybe gravity will suddenly disappear tomorrow. But that is not how we generally live our lives, in general in our day to day lives we take the things for which we have directly observable evidence and say they're facts. We do this for observations (the wall is white, the book is made of paper) and predictions (when I press the light-switch the light will turn on, when I heat up this chicken it will be cooked). Those types of deductions based on our senses and inductions based on past experiences are essential to being able to live our lives, and I really don't want to get into those type of 'fact or no fact' arguments.

Now next to facts we have beliefs. Often a belief is something we have been told many times during our lives, something that we would like to be true, that makes us feel justified or better about ourselves or something we were told by someone we believe in and we can experience them with various degrees of certainty. As humans we act upon our beliefs as much as we act upon facts, you might believe in a deity, you might believe everyone in the world should be kind to one another or you might be an existential nihilists that believes there's no point to life either way. The point is that we all have a large collection of beliefs, and the vast majority of us is naturally uncomfortable changing those beliefs. In order not to be in this position we subject apparent evidence in favor of those beliefs to far less scrutiny before we admit it into our lives, and we reject apparent evidence against those beliefs much more easily than we would reject evidence confirming them.

But the line between those two can become somewhat blurry, essentially we have a spectrum where anything we can say with near certainty is a fact, a large range between those things we are 'certain' are true, and things we are 'certain' not to be true we call beliefs that we do or do not ascribe to, and finally there are things we are certain not to be true. When an individual holds a belief with an extremely high degree of certainty, to them that belief is as much a fact as anything else that most of us might agree on is a fact. It would take a lot of observable evidence to dissuade them from that idea. If you've lived your life believing the universe revolves around the Earth, the notion of that not being true would be extremely hard to accept. Superficial observation does seem to confirm that that is the case, and after all that does seem to be the most likely explanation. It will take a lot of convincing for you to understand that the Earth, in fact rotates around the Sun.

Let's stick with that example for a second. In the modern world, for some of us, the Earth orbiting the sun is a fact, we have looked through a telescope, traced the motions of the sun, the stars and the planets in our sky, and we have observed directly that the universe revolving around the Earth cannot explain our observations, subsequently we have continued to make observations which show the only possible explanation (without going into absurdity) to be the Earth revolving around the Sun. Now the people who have done the observations of this fact, can now produce a set of predictions and descriptions of observations, and as experts, through the system of education, we can share these with as many people as possible. Now if we're in a position where people consider us credible, they will acknowledge our observations and predictions, they won't try to reproduce them themselves, but in the absence of a credible source to the contrary they will believe us. As such most of the western world holds a strong belief in heliocentrism, strong enough for most, to accept this belief as fact.

You end up believing in individuals or institutions to tell you what is or isn't fact. That is essentially a good thing, since none of us would have time in our lives to observe all the evidence for all the things we are told are facts and that are relevant to our modern-day lives. Therefore belief in various forms plays an extremely important and often positive role in our lives. But there is always a danger in believing in ideas, people or institutions. When rapper B.o.B. sent out a number of tweets supposedly proving the Earth was flat [1] the vast majority of people disagreed. They pointed out that although some of his own predictions were right, his observations were lacking. However there were some who saw their own beliefs confirmed in his observations, and rushed to join them, there is actually a society that collects evidence for a flat earth [2], partially grounded in geocentric religious beliefs.

So what's the point? The point is that in the case of any fact for which we do not take the time to observe the evidence ourselves (of which there will always be a lot), whether we accept it as a fact or not depends on the strength of our belief in the source that has collected the evidence for that fact.

Now how do we get to the facts?

Scientists and engineers probably have an above average ability and tendency to try to look at claims from different angles and seek unbiased evidence for those claims. But it's important to remember that even for them, it turns out to be extremely difficult to tune out beliefs and biases, and there exists a long history in science of conducting research focused on confirming views for which there really wasn't that much evidence in the first place. Often, like with normal human beliefs, that path involved taking evidence at face value that we liked, and extensive attempts to debunk evidence that we didn't like.

This continues to the present day, and we all do it! After all how many liberals have really investigated the claims about the crowd size at the inauguration of Donald Trump? Didn't most of those people see the pretty picture on the news and go 'Well see, there's all we need to know!', whereas that picture would not have been particularly hard to manipulate? (SPOILER: The news gave a reasonably realistic idea of crowd size difference, but don't take my word for it![3][4]) How many really went to look at research about crime in Sweden rather than read the article that said 'nah, it isn't true' and then went: 'Ah, I'm sure CNN did their homework!'? (SPOILER: They did [5]) So we can argue that once you've done research into this, it seems certain media outlets turn out to be more reliable than others, but most of us don't really do that. Our environment has told us certain media outlets are reliable, and we take what they say at face value. That really isn't that different from what people are doing that put their faith into media outlets that in practice really do manipulate facts on a large scale, so it would be silly to think that you can just point people at the media outlet you believe to be factual and expect them to accept that information as such.

So how do we convince people the Earth orbits the Sun?

Here is where I believe the internet is actually helping us, and should make the job of autocratic dictators and media outlets with strong political agenda's a lot harder. Unlike as recent as two decades ago, everyone with an internet connection now has direct access to a wealth of open source information. Images, tweets and many other types of data directly from their sources. A single source might be unreliable, but by combining a wide array of sources from facebook, instagram and twitter to public satellite imagery. Several parties (one of the better known being bellingcat [6]) have shown that it's possible to collect information in such a way that stories from media outlets and the government can be corroborated (or debunked!) in an independent way, citing sources that anyone can verify directly from their articles. This wealth of data has made it far easier for anyone to verify the reliability of the media outlets they consume.

In the end, the issue of how much someone can change your beliefs is all about trust. If there's one thing that is almost certainly true, it's that trust in the traditional media outlets is at a historical low point [7], however, a source of optimism is that research indicates that citations, expert opinions, etc. all those things do matter. They do increase the amount of trust people experience towards a source of information and, perhaps more importantly, once people discover a source of information gets their facts wrong, their trust in that source does decrease[8]. I believe there's a lesson in there for those of us, be it journalists or scientists, who are attempting to convene information to the masses, or even those of us simply trying to convene a 'fact' over dinner. Don't simply tell people their wrong, don't simply tell people you have facts that are better than theirs and try to get away with one questionable image to back up your story.

When you want to convince people of your views, dig deep, look for a myriad of different sources that show evidence for your point of view (the 'fact' you are trying to convene, if you will) and never forget to really look for evidence to the contrary. In the worst case you'll discover that you were wrong and you'll have to give up a belief, which although uncomfortable is worthwhile. In the best case you will be much better prepared when someone tries to confront you with their conflicting ideas. Whenever possible communicate with sources that are accessible and understandable for whomever you are trying to reach with your story. Never argue from authority, information has no authorities, only experts. But most importantly never be deceptive when trying to convince someone of your point of view. There is no way you will be able to instantly change someone's beliefs, but a consistent, steady stream of truthful information can erode it.

Nicolaus Copernicus' ideas went against everything people believed in, the government and the church tried to do everything in their power to give people 'alternative facts'. Copernicus made a wealth of observations and predictions, consistently showing the superiority of his ideas[9]. Eventually he would die under house arrest and it would take until 1758 for books describing the heliocentric model to be allowed by the Catholic church. Governments and other institutions holding beliefs which hold back the flow of facts is nothing new in this world, but in the end, no amount of misinformation can stand the test of time when confronted with real facts.

And now we have the internet.

I realize Social Media provide an interesting perspective on the spread of information, I will treat those in a separate article.


Disclaimer: The opinions I will be expressing will be solely my own, they will in no way be related to Mozilla or the work I do there.

Today, I will be doing something I didn’t think I would ever be doing. I will start to post publicly on topics that may be considered somewhat political. My personal opinions and thoughts, if you will. This is something I have always said I would never do, once something is written, it is unlikely to ever go away, and may always be associated with you and the work we do. Having said that, I do believe the climate we currently live in has reached a point where I don't think I have any other choice.

Why wouldn't I speak up?

As engineers, scientists, and other types more concerned with what we're building and discovering about the world around us, it seems to only make sense that we wouldn't publicly take a stance that could be considered political. Above all our task is to follow the data, wherever it leads and use that data in various ways to benefit human kind. In this process our personal feelings about topics only pose a threat to our scientific integrity, every one of us is susceptible to biases, and all we can do is try as much as possible to eliminate them from our work. Throwing out a bit of data because it doesn't seem to support the hypothesis you're trying to prove, building something that inherently puts one group at a disadvantage compared to another for personal gain, all those things are fundamental crimes against our professional integrity. And not only that, they will always backfire eventually, but more on this later.

And this is where it all started. If I speak openly about my political opinions, won't the data I present, the things I built, inevitably be viewed as colored by them? I have genuinely seen someone comment on a crash once along the lines of 'Firefox always crashes when I go to conservative websites, it never does when I go to your liberal websites. If you don't stop promoting your liberal agenda I will switch browsers.'. Whether that is true or not (it is not), it was exactly that phenomenon, an organization perceived as liberal produced a product, and people believed that product was inherently designed to put them and their ideas at a disadvantage. Considering the amount of effort I put into letting the data I collect and the things that I produce not be colored by my personal positions, this is something I want to avoid at all costs.

After all, as long as scientists collect a wealth of data, ensure that experiments are reproducible by any group of people, and the knowledge we gather is then used to build things that obviously and visibly work, we don't need to make a public political stance, right? People will look at the data we collect and the things we build, realize that they're good, and be able to make well-informed opinions for themselves, that fit within their ideological views. Since I believe only a tiny percentage of the population is inherently evil, things will then work themselves out just fine, so we're good!

So what changed?

With the tools we're building, giving people the ability to communicate with people from all over the world, people with different ideas and cultural backgrounds, I always believed an atmosphere of compassion, understanding and a desire to help others, whoever they are, would automatically arise. I thought that with the knowledge we were collecting and spreading - about our place in the universe and how small and vulnerable we are on a cosmic scale - we could automatically foster an appreciation of life on this planet, we would cherish it going forward.

I thought we were at a point where history wouldn't repeat itself, where things would only get better from here.

I have now come at a point where I can no longer deny that it seems that I was wrong. Both new and old problems are festering among our species, and we have to find new ways of dealing with them, because the old ones aren't working. As humans we appear to be inherently complacent, particularly when our own livelihood isn't directly threatened, but the time for that is past, we have to change course now, the risk of 'sitting this one out' is simply too great. As engineers and scientists we have given humanity the means to do a tremendous amount of damage, and now we have a role to play in making sure it doesn't.

What are you going to do about it?

It is likely not many people will ever see what I write here, and most likely much less of those people will read anything they didn't already know. However it has occurred to me that if I can say just one sensible thing, give just a couple of people a nudge in the right direction, that may have a trickle down effect that makes something of a difference in the world, and that, I have to try. Perhaps more importantly for me directly it will help me structure my thoughts, give me a place to point to rather than explaining standpoints over and over again, and possibly even get some useful feedback to improve on my own understanding of the world I live in.

And so, I have decided that over the next couple of weeks I will write a couple of posts in which I outline my thoughts on a number of topics that seem to apply to the troubles of the world today. Feel free to disagree with me, but please do so respectfully, and if you want to have a debate, support your argument with (conventional) facts. On most of the topics I will be writing about I will not be an expert, sometimes I will presumably be wrong and say things which aren't true, although I will do my best to include reliable references whenever I can. I'm okay with all of that, because even in the cases where I am, somewhere I might spark a debate, create some more understanding and ever so slightly nudge someone towards my utopian fantasy of a world where we all live together in peace on a planet (or multiple planets) that we care for.



To those that have not given up on reading my blog, I will be giving a talk at FOSDEM 2014 this year about utilizing GPUs to accelerate 2D graphics. It will be at 16:30 in the Mozilla Developer room and it will be fairly technical! For those of you that have enjoyed my other posts on here you might find this interesting as well.

My shortest post ever! Not even any pictures!

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Certain events have made me realise it's probably a good idea to have a 'blog' to share ideas and get feedback...


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