Recherche avancée

Sur d’autres sites (10485)

• More Cinepak Madness

  20 octobre 2011, par Multimedia Mike — Codec Technology

  Fellow digital archaeologist Clone2727 found a possible fifth variant of the Cinepak video codec. He asked me if I cared to investigate the sample. I assured him I wouldn’t be able to die a happy multimedia nerd unless I have cataloged all possible Cinepak variants known to exist in the wild. I’m sure there are chemistry nerds out there who are ecstatic when another element is added to the periodic table. Well, that’s me, except with weird multimedia formats.

  Background
  Cinepak is a video codec that saw widespread use in the early days of digital multimedia. To date, we have cataloged 4 variants of Cinepak in the wild. This distinction is useful when trying to write and maintain an all-in-one decoder. The variants are :

  1. The standard type : Most Cinepak data falls into this category. It decodes to a modified/simplified YUV 4:2:0 planar colorspace and is often seen in AVI and QuickTime/MOV files.
  2. 8-bit greyscale : Essentially the same as the standard type but with only a Y plane. This has only been identified in AVI files and is distinguished by the file header’s video bits/pixel field being set to 8 instead of 24.
  3. 8-bit paletted : Again, this is identified by the video header specifying 8 bits/pixel for a Cinepak stream. There is essentially only a Y plane in the data, however, each 8-bit value is a palette index. The palette is transported along with the video header. To date, only one known sample of this format has even been spotted in the wild, and it’s classified as NSFW. It is also a QuickTime/MOV file.
  4. Sega/FILM CPK data : Sega Saturn games often used CPK files which stored a variant of Cinepak that, while very close the standard Cinepak, couldn’t be decoded with standard decoder components.

  So, a flexible Cinepak decoder has to identify if the file’s video header specified 8 bits/pixel. How does it distinguish between greyscale and paletted ? If a file is paletted, a custom palette should have been included with the video header. Thus, if video bits/pixel is 8 and a palette is present, use paletted ; else, use greyscale. Beyond that, the Cinepak decoder has a heuristic to determine how to handle the standard type of data, which might deviate slightly if it comes from a Sega CPK file.

  The Fifth Variant ?
  Now, regarding this fifth variant– the reason this issue came up is because of that aforementioned heuristic. Basically, a Cinepak chunk is supposed to store the length of the entire chunk in its header. The data from a Sega CPK file plays fast and loose with this chunk size and the discrepancy makes it easy to determine if the data requires special handling. However, a handful of files discovered on a Macintosh game called “The Journeyman Project : Pegasus Prime” have chunk lengths which are sometimes in disagreement with the lengths reported in the containing QuickTime file’s stsz atom. This trips the heuristic and tries to apply the CPK rules against Cinepak data which, aside from the weird chunk length, is perfectly compliant.

  Here are the first few chunk sizes, as reported by the file header (stsz atom) and the chunk :

  size from stsz = 7880 (0x1EC8) ; from header = 3940 (0xF64)
  size from stsz = 3940 (0xF64) ; from header = 3940 (0xF64)
  size from stsz = 15792 (0x3DB0) ; from header = 3948 (0xF6C)
  size from stsz = 11844 (0x2E44) ; from header = 3948 (0xF6C)
  

  Hey, there’s a pattern here. If they don’t match, then the stsz size is an even multiple of the chunk size (2x, 3x, or 4x in my observation). I suppose I could revise the heuristic to state that if the stsz size is 2x, 3x, 4x, or equal to the chunk header, qualify it as compliant Cinepak data.

  Of course it feels impure, but software engineering is rarely about programmatic purity. A decade of special cases in the FFmpeg / Libav codebases are a testament to that.

  What’s A Variant ?
  Suddenly, I find myself contemplating what truly constitutes a variant. Maybe this was just a broken encoder program making these files ? And for that, I assign it the designation of distinct variation, like some sort of special, unique showflake ?

  Then again, I documented Magic Carpet FLIC as being a distinct variant of the broader FLIC format (which has an enormous number of variants as well).

• Origin Crusader Media

  14 février 2012, par Multimedia Mike — Game Hacking

  A gleaming copy of the old Origin game Crusader : No Remorse showed up today :

  
  
  

  Immediately, I delved in expecting to find Xan-encoded AVI files that would play perfectly using FFmpeg/Libav. Instead, I found a directory labeled flics/ that indeed has a lot of AVI files, but not in Xan. The programs attempt to interpret them as raw RGB. The strangest thing is the first frame often looks correct, if upside down :

  
  
  

  The first file I peered inside had the video FourCC ‘RRV1′. Searching for this led me to this discussion forum where people have already been hacking on this very format (Origin games invariably get a heap of lasting love). The forum participants have observed that 3 codecs are in play in this flics/ directory, including ‘RRV1′, ‘RRV2′, and ‘JYV1′, which apparently correspond to the initials of certain developers. The reason that the programs identify the files as raw RGB is because the FourCCs don’t appear everywhere that they’re supposed to. Additionally, there are several trailers for other Origin/EA games stored in Cinepak format elsewhere on the disc.

  It seems that I’m the person who added this title to the Xan wiki page, obviously with no first-hand evidence to back it up. Meanwhile, the forum participants speculate that the files are descended from the old Autodesk FLIC format (which would explain why they live in a directory called flics/). Corroborating strings extracted from the CRUSADER.EXE file include “FlicWait”, “FlicPlayer”, “Flic %s not found.”, “flicpath”, and “FLICPLAY.C”.

  The disc also features a sound/ directory which contains AMF files. Suxen Drol already documented these on the wiki as Asylum Media Format files. The disc contains an ASYLUM.DLL file as well as a utility called MOD2AMF.EXE. The latter works beautifully on a random MOD file I had laying around. The AMF file is a bit larger.

  Samples for all 3 FourCCs can be found here, while the AMF files and associated utilities are here.

• Playing With File

  8 septembre 2011, par Multimedia Mike — General

  I played with the ‘file’ utility a long time ago because I wanted to make it recognize a large number of multimedia formats. I had trouble getting my changes to take. But I’m prepared to try again after many years.

  Aiming at the Corpus
  In my local mirror of the MPlayerHQ samples archive, I find 9853 unique files. So I run all of them through the ‘file’ command :

    ’find /path/to/samples -type f -print0 | xargs -0 file —no-pad’
  

  My Ubuntu installation has file v5.04. I also tested against 5.07 and the latest, 5.08. Here is the number of files each version was unable to identify (generically marking as ‘data’) :

  5.04  1521
  5.07  1405
  5.08  1501
  

  That seems like a regression for v5.08 until I dug into the details and saw quite a few items like this, indicating that the MPEG detection could use some work :

  
   mov/mov-demux-infinite-loop.mpg : DOS-executable (
  +mov/mg-‹demux-infinite-loop.mpg : data
  
   image-samples/UNeedQT4.pntg : DOS-executable (
  +imY- samples/UNeedQT4.pntg : data
  

  Workflow
  These are just notes to myself and perhaps anyone else who wants to add new file formats to be identified by the ‘file’ command.

  First, download either the latest release from the FTP or clone from Github. Do the usual unpack, ‘./configure’, ‘make’ routine. To use this newly-built version and its associated magic file :

    ./src/file —magic-file magic/magic.mgc <file>
  

  To add a new format for ID, first, run the foregoing command to ensure that it’s not already identified. Then, check over the files in magic/Magdir and see which one might pertain to what you’re doing (it’s unlikely that your format will merit a new file in this directory). For example, for this round, I modified animation, audio, iff, and riff. Add or modify existing specs based on the copious examples in the directory and by consulting the appropriate man page (‘man 5 magic’).

  Finally, run ‘make’ again which will regenerate the magic file. Invoke the above command again to use the modified magic file.

  Before and After
  On a selection of formats taken from the samples archive (renamed and cut down to a kilobyte because detection typically only relies on the first few bytes), here is the “before” :

  amv :            RIFF (little-endian) data
  armovie :        data
  bbc-dirac :      data
  interplay-mve :  data
  mtv :            data
  nintendo-thp :   data
  nullsoft-video : data
  redcode :        data
  sega-film :      data
  smacker :        data
  trueaudio :      data
  vqa :            IFF data
  wavpack :        data
  wc3-mve :        IFF data
  wtv :            data
  

  And the “after” :

  amv :            RIFF (little-endian) data, AMV
  armovie :        ARMovie
  bbc-dirac :      BBC Dirac Video
  interplay-mve :  Interplay MVE Movie
  mtv :            MTV Multimedia File
  nintendo-thp :   Nintendo THP Multimedia
  nullsoft-video : Nullsoft Video
  redcode :        REDCode Video
  sega-film :      Sega FILM/CPK Multimedia, 320 x 224
  smacker :        RAD Game Tools Smacker Multimedia version 2, 320 x 200, 100 frames
  trueaudio :      True Audio Lossless Audio
  vqa :            IFF data, Westwood Studios VQA Multimedia, 418 video frames, 320 x 200
  wavpack :        WavPack Lossless Audio
  wc3-mve :        IFF data, Wing Commander III Video, PC version
  wtv :            Windows Television DVR Media
  

  After rerunning ‘file’ on the mphq corpus using the modified magic file, only 1329 files remain unidentified (down from 1501).

  Going Forward
  As mentioned, MPEG detection could probably be strengthened. However, a major weakness is QuickTime/MP4. Many files are not detected, probably owing to the many ways that QuickTime files can begin.