Synchronizing a git mirror with hg-git

Posting this since it might save someone else the time it took me to figure out.

There are several tutorials for creating a git repo from an hg one using hg-git. But none of them go much into how to keep the git repo up to date when the hg one changes (assuming work is done in the hg repo). And, just doing a push with hg-git to github fails... silently :( (But it succeeded the first time, to create the repo.) Even adding -v --debug in hopes of getting some useful information is no help.

The solution is to push to a local git repo first, then push from there to github. But even pushing to a local git repo won't just work - you will get

error: refusing to update checked out branch: refs/heads/master
error: By default, updating the current branch in a non-bare repository
error: is denied, because it will make the index and work tree inconsistent
error: with what you pushed,

To get around that, create another branch in the local git repo (git branch side), switch to it (git checkout side), then push to it using hg-git. Then switch back to master (git checkout master) and finally push that to github.

I think it might also work to make the local git repo into a bare repo, but the above worked for me so I stopped there.

Git Mirror for Emscripten

We now have a git mirror on GitHub! Getting the Emscripten code is now as easy as

git clone git://github.com/kripken/emscripten.git

Code will be mirrored there, so you can either use git with that GitHub repo, or hg with the Google Code repo, and the result will be the same. (However for project stuff - issues, wiki, etc. - we will continue to use the Google Code project page.)

FreeType Demo

A simple demo of FreeType in JavaScript can be seen here.

Emscripten 0.8!

The main highlights of this release are:

• Tests for FreeType and zlib, two important real-world codebases. Aside from all the fixes and improvements necessary to get them to work, the test infrastructure now runs the entire build procedure (using emmaken) for those two tests, giving even more complete test coverage.
• File emulation. Just enough to let compiled C/C++ code think it is accessing a filesystem. For example, the FreeType test loads a TrueType font from a file (but really it's a virtual filesystem, set up in JavaScript).
  
• Additional compilation options for overflows and signedness (CHECK_OVERFLOWS, CORRECT_OVERFLOWS, CHECK_SIGNS). These allow even more C/C++ code to be compiled and run properly, but are switchable, so code that doesn't need them can run fast.
  

I'll post a web demo soon.

Emscripten Overview Writeup

If you're interested in how Emscripten works, then this writeup I am working on may interest you. It is currently the best explanation of the underlying techniques Emscripten uses to compile LLVM to JavaScript, including the memory model, the Relooper algorithm, etc.

It isn't meant to be a manual or a practical guide. For that, as always see the wiki.

LIL browser demo

I just saw this demo of LIL (a Little Interpreted Language) running in the browser, compiled from C to JavaScript using Emscripten. Cool stuff!

A Completely Speculative History & Future of H.264 and WebM

1. Several years ago, Google decides something needs to be done about web video, because (1) H.264 requires royalties, which means that parts of the web are proprietary (even if it is a standard), and Google believes an open web is in its best interest, and (2) for similar reasons, H.264 is incompatible with the W3C, Mozilla's Firefox, and Opera, so it will never become universal anyhow. Google is willing to go to great lengths to solve this issue, including large sums of money and developer time.
2. Google approaches MPEG-LA (or major entities that are members), and quietly floats the idea of 'freeing' H.264, by way of a large one-time payment from Google, after which H.264 will be royalty-free, and can then be blessed by the W3C.
3. Negotiations fail. Google offers large amounts of cash, but it isn't enough for MPEG-LA, which believes it is close to having a complete lock on the market, which it can leverage for even more cash later on.
4. Google threatens to support a competing format with all its resources, thereby threatening the future profitability of H.264.
5. MPEG-LA decides to call Google's bluff.
6. Google makes good on its threat, buying On2 and freeing its VP8 video codec as part of WebM, a royalty-free format for web video. (Note: I'll use 'WebM' to refer to 'VP8', a lot of the time.)
   
7. Mozilla, Opera, etc. naturally support this move, as it is good for the open web. Apple and Microsoft, whose motivations are otherwise, do not support this move - they are both already heavily invested in H.264, and for them life would be simplest if WebM never existed.
8. As a reaction to WebM, MPEG-LA makes H.264's licensing less expensive, and for a longer period of time.
   
9. Google makes good on another part of its threat to MPEG-LA, removing H.264 support from Chrome. MPEG-LA is surprised Google is willing to hobble its own browser in order to get a leg up in this fight.
   
   (This brings us to the present time.)
   
   
   
10. Nothing much changes, at first. Most web video is seen through Flash anyhow. However, the block of WebM supporters, which is now Firefox, Chrome and Opera - whose share in the market is large, and growing - gets video providers on the web to pay close attention to WebM.
11. Flash introduces WebM support. Most video encoded for desktop viewing can now be encoded in WebM, and viewable through Flash or an HTML5 video element in Firefox, Chrome and Opera. Even video shown with DRM can be encoded in WebM, but must be shown in Flash. On the other hand, in the mobile space, a complete stack of hardware&software support is still really just present for H.264, and Apple doesn't support anything else, so video encoded for mobile viewing is primarily done in H.264.
12. WebM's video quality improves, in part benefiting from the fact that while open source and royalty-free, WebM is not a formal specification or standard, so rapid development and changes are possible. WebM becomes equivalent or superior to H.264.
    
13. Google switches YouTube to primarily use WebM for encoding video meant for desktop use. There is hardly any impact on users, due to most video being shown in Flash anyhow (which now supports WebM). However, video for mobile viewing remains encoded in H.264.
    
14. Hardware support for WebM begins to ship in a great deal of new mobile devices, and eventually in a majority of new mobile devices.
15. As a reaction to WebM's rise, MPEG-LA once more lessens the royalties for H.264, in an attempt to make it more competitive.
    
16. A new version of Google's Android ships, on a new flagship phone from Google, that has complete hardware and software support for WebM. The device primarily views YouTube video in WebM format.
    
17. Google, stating WebM's superior quality, begins to 'favor' WebM over H.264 on YouTube, for mobile content. More specifically, while both WebM and H.264 are supported, WebM content is encoded at higher quality levels (this is accomplished not by decreasing H.264 quality, but by adding a higher level of quality exclusively for WebM). The result is that mobile devices viewing YouTube give a better user experience if the device does so using WebM.
18. Apple makes the rational decision and supports WebM on new iOS devices - hardware support is already there, and Apple cannot compromise on user experience. Whatever monetary benefit Apple gains from MPEG-LA from H.264 is completely eclipsed by Apple's iOS business, so this is a no-brainer.
19. With the majority of new mobile devices shipping with WebM support (Android and iOS), smaller players (Blackberry, WebOS, Windows Phone) are forced to support it as well.
20. The online video market has its anti-DRM moment, just like online audio already had. Video is shown without DRM, which simplifies delivery and cuts costs, and piracy remains at the same levels as before (just as with audio).
    
21. Once it is clear H.264 has lost in the mobile space, and that DRM is no longer needed, there is no reason for Microsoft and Apple not to support WebM in the HTML5 video element, on Windows and OS X respectively, in order to ensure their users the best experience.
    
22. With DRM no longer an issue and widespread support for WebM in the HTML5 video element, WebM becomes the universal standard for video on the web, on both desktop and mobile. Content producers have little reason to even support a fallback to Flash - some do, but many do not, at little detriment to them or their users.
23. Google wins the fight, and the open web greatly benefits.
    

Some things that can change this future history:

• MPEG-LA deciding to make H.264 100% royalty-free. This will kill MPEG-LA's profits, but may still be worthwhile for MPEG-LA members, since if done properly - and promptly - it can ensure H.264 becomes the standard for web video. Whether there remains enough profit from H.264 (from hardware, services, etc.) for this move to make sense, is not clear. But if this does happen, WebM loses, but really Google wins, since it got what it set out to get.
  
• A new video format can appear, or a newer version of an existing format, which requires new hardware support but has benefits to justify the switch. Given the battle between H.264 and WebM, I would expect the new format's backers to learn the lessons of the past and make it free on the web (or, if they are not willing to do that - then to not even bother to create a new format). If such a new format appears, and becomes the universal standard for web video, the result is that Google wins in this case as well.
• The fight gets taken to the courts. I doubt a simple injunction will be granted to either side, as both are powerful, influential, and have many patents to back up their claims - so there is no quick victory. Instead there is a lengthy court battle. To justify the cost, there must be a significant chance of large future profits, and if H.264 is already in decline, that might not be the case. However, it might still make sense for MPEG-LA to take Google to court, just to get it to settle for some amount of money, in which case Google wins overall, but MPEG-LA gets a little more money than otherwise. However, if the goal isn't a settlement, but an actual attempt to kill WebM, then things can get interesting. I don't think anyone can say for sure how that fight would turn out - does WebM infringe on H.264 patents? Does H.264 infringe on WebM patents (VP8 patents, granted to On2, and now owned by Google, which would countersue)? Perhaps both? Such a 'fight to the death' in the courts seems unlikely, in part due to that unpredictability, so all we can say for sure in this case is that several law firms will greatly benefit.
  

DISCLAIMER: I have no inside knowledge about any of this.