Monthly Archives: May 2008

English Phonetic CAPTCHA

Jeff Atwood writes of automated spamming recently in Designing For Evil. The ensuing discussion.presented plenty of technical anti-spam brainstorms as well as the usual violent anti-spammer fantasies. However, one interesting insight I gained from the comment thread concerned the automated nature of Wikipedia’s anti-spam measures:

There is an IRC channel that receives every edit done to Wikiepdia, a bot then check the page for known bad URLs and string and reverts if necessary.

Aha! So it isn’t just a global network of diligent and vigilant volunteer Wikipedians keeping the content clean. That always struck me as largely intractable and learning this punctures the starry-eyed ethos behind the Wiki concept. But I did a little research and it seems to be a real thing.

I suppose something like that would be vast overkill for the MultimediaWiki. As the discussion also details, not all public discussion forums are created equally in terms of attractiveness to spammers, and the MultimediaWiki would probably be pretty far down the list. Some kind of registration CAPTCHA would probably be adequate. And now that I understand a little more about PHP programming thanks to FATE, I may have enough knowledge to try my hand at such a system.

Hey, here’s a CAPTCHA idea that I have entertained: Call it a phonetic CAPTCHA and challenge the user to type in the proper English word with a certain phonetic pronunciation; for example: KAH MEW NIK AY SHUNZ (communications). I was inspired by Infocom’s old Planetfall interactive fiction game where things were labeled phonetically. Perhaps it discriminates against non-native English speakers (and the less educated among the native set) as well as the spambots, but I guess every measure has its pros and cons.

QT And Duck FATE Tests

More new test specs tonight:

I meant to add the QT SMC and RLE tests some months ago. But in a fit of programming purity, I opted instead to log some bugs for myself to fix before I added test specs. However, I remembered that I should get as much basic functionality covered by the FATE suite first. SMC works, except in some odd resolution cases; same for QT RLE. Most of those 2 decoders’ code paths work and I want to be able to catch regressions sooner than later.

I also been tightening up some older video decoder tests. I used to have a problem when using “-f framecrc” for output where FFmpeg was outputting a huge number of identical frames at very short periods. Michael tipped me off somewhere along the line that I should use “-vsync 0” in such cases to effectively eliminate the duplicates. I still don’t understand exactly what that option does but it works splendidly.

Pickled FATE

Some people have been checking out the new test client described in the previous post. So far, most of the questions I have received concern the format of the fate-test-cache.bz2 file downloaded from the FATE server. I admire that people are taking an interest in file format particulars — as you know, I encourage that. It’s nothing too special, though. I simply have a Python script called that queries FATE’s test_spec table into an array of dictionary data structures. Then it serializes/marshals/flattens the data using Python’s built-in pickle module. It’s trivial to de-pickle on the client-side. Of course, Python’s bzip2 module helps with size concerns.

What’s the pickle format? Darned if I know, but it works famously, so I don’t really care about reinventing that wheel. Especially when the code for decompressing and deserializing boils down to these 3 lines of Python:

Also, about that rsync command I mentioned in the last post:

rsync -aL rsync:// samples

Does that actually work for anyone? Occasionally, it works for me. Most of the time, it tells me:

rsync: failed to connect to Can't assign requested address (49)
rsync error: error in socket IO (code 10) at /SourceCache/rsync/rsync-30/rsync/clientserver.c(94)

which, according to my searches, is a fairly generic network error (at least the bit about assigning the requested address). Since I am usually populating the sample repository manually anyway, this hasn’t been a big problem. But I am trying to be more diligent about making sure the rsync repository is up to date since I expect more people will be using it.

Anyway, FATE growth plods on with 2 new tests tonight: nsv-demux and real-14_4 (weird, I just realized that the db assigned that one ID 144 completely by coincidence).

FATE Testers Wanted

This evening, I finally got my script minimally ready for general consumption. is the unimaginatively named program I threw together some time ago to allow me to validate test specs before I activate them so that FATE will automatically test them. It works like this:

  • download the script (
  • rsync the FATE suite of samples that live on mphq: ‘rsync -aL rsync:// samples’ (without the quotes, of course) — this presently amounts to ~150 MB
  • build FFmpeg as normal
  • ‘./ -f </path/to/ffmpeg-binary> -s </path/to/fate-suite/samples>’

That’s it. The script will ask the FATE server for a set of test specifications and run through them. You may also need to specify -l/–libpath= if you built and installed FFmpeg with shared libraries. Naturally, ‘./ -h’ will spell out all the options.

You would do well to make sure that all the options are valid or else suffer Python bailout exceptions. I just added the command line options tonight and have not made them very resilient. I have been promising this utility for a long time and I wanted to get something out there sooner than later.

Remember that I’m still a rank amateur at Python, so don’t be afraid to call me out if I’m doing anything in the worst Pythonic way imaginable.

Ideas for future improvement:

  • Better logging– Instead of dumping to stdout, maybe dump all the results to a CSV file (for spreadsheet analysis) and/or an HTML file for easy viewing
  • Proper versioning– I track the script via a local git repository, but how do I communicate the current version? Would this be version dd394ef8f3dad056c39ab4e1c76951190621cf8b?
  • Robust error handling
  • Range testing (run all tests up to ID n, or run all tests after ID n, or from IDs m to n)
  • Skip a list of tests (for example, it would be useful to skip test #128 — the internal FFmpeg regression test — since it’s not that helpful in this particular scenario)
  • [Your idea here]

It’s open source, GPL v2, so patches welcome. Moreover, I would love to hear if this script works at all for anyone else. Then, I would like to hear how it works on platforms outside of the 3 that FATE now rigorously tests– I speak of Mac OS X, *BSD, Win32 with either MSVC or MinGW, Open/Solaris on all its various platforms, even PlayStation 3 and whatever else. I actually did get that OpenSolaris VMware session to boot after I waited long enough but I had no idea how to do anything useful with it. That’s when I decided to get down to it and get this script out there so that hopefully someone else will test those platforms.

Extra credit: Figure out why, when bailing out of the test sequence early with Ctrl-C, terminal character echo is off. I.e., the terminal refuses to print keystrokes.