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• The New Samples Regime

  1er décembre 2011, par Multimedia Mike — General

  A little while ago, I got a big head over the fact that I owned and controlled the feared and revered MPlayer samples archive. This is the repository that retains more than a decade of multimedia samples.

  Conflict
  Where once there was one multimedia project (FFmpeg), there are now 2 (also Libav). There were various political and technical snafus regarding the previous infrastructure. I volunteered to take over hosting the vast samples archive (53 GB at the time) at samples.mplayerhq.hu (s.mphq for this post).

  However, a brand new server is online at samples.libav.org (s.libav for this post).

  Policies
  The server at s.libav will be the authoritative samples repository going forward. Why does s.libav receive the honor ? Mostly by virtue of having more advanced features. My simple (yet bandwidth-rich) web hosting plan does not provide for rsync or anonymous FTP services, both of which have traditionally been essential for the samples server. In the course of hosting s.mphq for the past few months, a few more discrepancies have come to light– apparently, the symlinks weren’t properly replicated. And perhaps most unusual is that if a directory contains a README file, it won’t be displayed in the directory listing (which frustrated me greatly when I couldn’t find this README file that I carefully and lovingly crafted years ago).

  The s.mphq archive will continue to exist — nay, must exist — going forward since there are years’ worth of web links pointing into it. I’ll likely set up a mirroring script that periodically (daily) rsyncs from s.libav to my local machine and then uses lftp (the best facility I have available) to mirror the files up to s.mphq.

  Also, since we’re starting fresh with a new upload directory, I think we need to be far more ruthless about policing its content. This means making sure that anything that is uploaded has an accompanying file which explains why it’s there and ideally links the sample to a bug report somewhere. No explanation = sample terminated.

  RSS
  I think it would be nifty to have an RSS feed that shows the latest samples to appear in the repository. I figure that I can use the Unix ‘find’ command on my local repository in concert with something like PyRSS2Gen to accomplish this goal.

  Monetization
  In the few months that I have been managing the repository, I have had numerous requests for permission to leech the entire collection in one recursive web-suck. These requests often from commercial organizations who wish to test their multimedia product on a large corpus of diverse samples. Personally, I believe the archive makes a rather poor corpus for such an endeavor, but so be it. Go ahead ; hosting this archive barely makes a dent in my fairly low-end web hosting plan. However, at least one person indicated that it might be easier to mail a hard drive to me, have me copy it, and send it back.

  This got me thinking about monetization opportunities. Perhaps, I should provide a service to send HDs filled with samples for the cost of the HD, shipping, and a small donation to the multimedia projects. I immediately realized that that is precisely the point at which the vast multimedia samples archive — with all of its media of questionable fair use status — would officially run afoul of copyright laws.

  Which brings me to…

  Clean Up
  I think we need to clean up some samples, starting with the ones that were marked not-readable in the old repository. Apparently, some ‘samples’ were, e.g., full anime videos and were responsible for a large bandwidth burden when linked from various sources.

  We multimedia nerds are a hoarding lot, never willing to throw anything away. This will probably the most challenging proposal to implement.

• ffmpeg/avconv mp4 to webm libvorbis buffer overflow

  4 janvier 2013, par Halsafar

  Trying to convert a bunch of mp4 files into webm. So I run the following command. I tried a similar command with ffmpeg.

  avconv -i input.mp4 -threads 8 -s 1280x720 -pre libvpx-720p -b 3900k -pass 2 -acodec libvorbis -b:a 128k -ac 2 -f webm -y output/webm

  Results in :

      Input #0, mov,mp4,m4a,3gp,3g2,mj2, from 'input.mp4':
  
    Metadata:
  
      major_brand     : isom
  
      minor_version   : 512
  
      compatible_brands: isomiso2mp41
  
      creation_time   : 1970-01-01 00:00:00
  
      encoder         : Lavf52.32.0
  
    Duration: 00:01:02.90, start: 0.000000, bitrate: 1649 kb/s
  
      Stream #0.0(und): Video: mpeg4 (Simple Profile), yuv420p, 640x480 [PAR 4:3 DAR 16:9], 1492 kb/s, PAR 853:640 DAR 853:480, 23.94 fps, 30 tbr, 30 tbn, 30 tbc
  
      Metadata:
  
        creation_time   : 1970-01-01 00:00:00
  
      Stream #0.1(und): Audio: aac, 44100 Hz, stereo, s16, 152 kb/s
  
      Metadata:
  
        creation_time   : 1970-01-01 00:00:00
  
  [buffer @ 0x1232600] w:640 h:480 pixfmt:yuv420p
  
  [scale @ 0x123c300] w:640 h:480 fmt:yuv420p -> w:1280 h:720 fmt:yuv420p flags:0x4
  
  [libvpx @ 0x1256d60] v1.0.0
  
  Output #0, webm, to 'output.webm':
  
    Metadata:
  
      major_brand     : isom
  
      minor_version   : 512
  
      compatible_brands: isomiso2mp41
  
      creation_time   : 1970-01-01 00:00:00
  
      encoder         : Lavf53.21.0
  
      Stream #0.0(und): Video: libvpx, yuv420p, 1280x720 [PAR 2559:2560 DAR 853:480], q=11-51, pass 2, 3900 kb/s, 1k tbn, 30 tbc
  
      Metadata:
  
        creation_time   : 1970-01-01 00:00:00
  
      Stream #0.1(und): Audio: libvorbis, 44100 Hz, stereo, s16, 152 kb/s
  
      Metadata:
  
        creation_time   : 1970-01-01 00:00:00
  
  Stream mapping:
  
    Stream #0:0 -> #0:0 (mpeg4 -> libvpx)
  
    Stream #0:1 -> #0:1 (aac -> libvorbis)
  
  Press ctrl-c to stop encoding
  
  [libvorbis @ 0x1221240] libvorbis: buffer overflow.Audio encoding failed

  Notice the nice error. buffer overflow in libvorbis.

  Any assistance ? An Alternative conversion command ?

  UPDATE

  The first pass looks like this :

   avconv -i input.mp4 -threads 8 -s 1280x720 -pre libvpx-720p -b 3900k -pass 1 -an -f webm -y output.webm

  Thanks !

• Parsing The Clue Chronicles

  30 décembre 2018, par Multimedia Mike — Game Hacking

  A long time ago, I procured a 1999 game called Clue Chronicles : Fatal Illusion, based on the classic board game Clue, a.k.a. Cluedo. At the time, I was big into collecting old, unloved PC games so that I could research obscure multimedia formats.

  
  
  

  Surveying the 3 CD-ROMs contained in the box packaging revealed only Smacker (SMK) videos for full motion video which was nothing new to me or the multimedia hacking community at the time. Studying the mix of data formats present on the discs, I found a selection of straightforward formats such as WAV for audio and BMP for still images. I generally find myself more fascinated by how computer games are constructed rather than by playing them, and this mix of files has always triggered a strong “I could implement a new engine for this !” feeling in me, perhaps as part of the ScummVM project which already provides the core infrastructure for reimplementing engines for 2D adventure games.

  Tying all of the assets together is a custom high-level programming language. I have touched on this before in a blog post over a decade ago. The scripts are in a series of files bearing the extension .ini (usually reserved for configuration scripts, but we’ll let that slide). A representative sample of such a script can be found here :

  clue-chronicles-scarlet-1.txt

  What Is This Language ?
  At the time I first analyzed this language, I was still primarily a C/C++-minded programmer, with a decent amount of Perl experience as a high level language, and had just started to explore Python. I assessed this language to be “mildly object oriented with C++-type comments (‘//’) and reliant upon a number of implicit library functions”. Other people saw other properties. When I look at it nowadays, it reminds me a bit more of JavaScript than C++. I think it’s sort of a Rorschach test for programming languages.

  Strangely, I sort of had this fear that I would put a lot of effort into figuring out how to parse out the language only for someone to come along and point out that it’s a well-known yet academic language that already has a great deal of supporting code and libraries available as open source. Google for “spanish dolphins far side comic” for an illustration of the feeling this would leave me with.

  It doesn’t matter in the end. Even if such libraries exist, how easy would they be to integrate into something like ScummVM ? Time to focus on a workable approach to understanding and processing the format.

  Problem Scope
  So I set about to see if I can write a program to parse the language seen in these INI files. Some questions :

  1. How large is the corpus of data that I need to be sure to support ?
  2. What parsing approach should I take ?
  3. What is the exact language format ?
  4. Other hidden challenges ?

  To figure out how large the data corpus is, I counted all of the INI files on all of the discs. There are 138 unique INI files between the 3 discs. However, there are 146 unique INI files after installation. This leads to a hidden challenge described a bit later.

  What parsing approach should I take ? I worried a bit too much that I might not be doing this the “right” way. I’m trying to ignore doubts like this, like how “SQL Shame” blocked me on a task for a little while a few years ago as I concerned myself that I might not be using the purest, most elegant approach to the problem. I know I covered language parsing a lot time ago in university computer science education and there is a lot of academic literature to the matter. But sometimes, you just have to charge in and experiment and prototype and see what falls out. In doing so, I expect to have a better understanding of the problems that need to solved and the right questions to ask, not unlike that time that I wrote a continuous integration system from scratch because I didn’t actually know that “continuous integration” was the keyword I needed.

  Next, what is the exact language format ? I realized that parsing the language isn’t the first and foremost problem here– I need to know exactly what the language is. I need to know what the grammar are keywords are. In essence, I need to reverse engineer the language before I write a proper parser for it. I guess that fits in nicely with the historical aim of this blog (reverse engineering).

  Now, about the hidden challenges– I mentioned that there are 8 more INI files after the game installs itself. Okay, so what’s the big deal ? For some reason, all of the INI files are in plaintext on the CD-ROM but get compressed (apparently, according to file size ratios) when installed to the hard drive. This includes those 8 extra INI files. I thought to look inside the CAB installation archive file on the CD-ROM and the files were there… but all in compressed form. I suspect that one of the files forms the “root” of the program and is the launching point for the game.

  Parsing Approach
  I took a stab at parsing an INI file. My approach was to first perform lexical analysis on the file and create a list of 4 types : symbols, numbers, strings, and language elements ([]{}()=., :). Apparently, this is the kind of thing that Lex/Flex are good at. This prototyping tool is written in Python, but when I port this to ScummVM, it might be useful to call upon the services of Lex/Flex, or another lexical analyzer, for there are many. I have a feeling it will be easier to use better tools when I understand the full structure of the language based on the data available.
  
  The purpose of this tool is to explore all the possibilities of the existing corpus of INI files. To that end, I ran all 138 of the plaintext files through it, collected all of the symbols, and massaged the results, assuming that the symbols that occurred most frequently are probably core language features. These are all the symbols which occur more than 1000 times among all the scripts :

     6248 false
     5734 looping
     4390 scripts
     3877 layer
     3423 sequentialscript
     3408 setactive
     3360 file
     3257 thescreen
     3239 true
     3008 autoplay
     2914 offset
     2599 transparent
     2441 text
     2361 caption
     2276 add
     2205 ge
     2197 smackanimation
     2196 graphicscript
     2196 graphic
     1977 setstate
     1642 state
     1611 skippable
     1576 desc
     1413 delayscript
     1298 script
     1267 seconds
     1019 rect
  

  About That Compression
  I have sorted out at least these few details of the compression :

  bytes 0-3    "COMP" (a pretty strong sign that this is, in fact, compressed data)
  bytes 4-11   unknown
  bytes 12-15  size of uncompressed data
  bytes 16-19  size of compressed data (filesize - 20)
  bytes 20-    compressed payload
  

  The compression ratios are on the same order of gzip. I was hoping that it was stock zlib data. However, I have been unable to prove this. I wrote a Python script that scrubbed through the first 100 bytes of payload data and tried to get Python’s zlib.decompress to initialize– no luck. It’s frustrating to know that I’ll have to reverse engineer a compression algorithm that deals with just 8 total text files if I want to see this effort through to fruition.

  Update, January 15, 2019
  Some folks expressed interest in trying to sort out the details of the compression format. So I have posted a followup in which I post some samples and go into deeper details about things I have tried :

  Reverse Engineering Clue Chronicles Compression

  The post Parsing The Clue Chronicles first appeared on Breaking Eggs And Making Omelettes.