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• Subtitling Sierra RBT Files

  2 juin 2016, par Multimedia Mike — Game Hacking

  This is part 2 of the adventure started in my Subtitling Sierra VMD Files post. After I completed the VMD subtitling, The Translator discovered a wealth of animation files in a format called RBT (this apparently stands for “Robot” but I think “Ribbit” format could be more fun). What are we going to do ? We had come so far by solving the VMD subtitling problem for Phantasmagoria. It would be a shame if the effort ground to a halt due to this.

  Fortunately, the folks behind the ScummVM project already figured out enough of the format to be able to decode the RBT files in Phantasmagoria.

  In the end, I was successful in creating a completely standalone tool that can take a Robot file and a subtitle file and create a new Robot file with subtitles. The source code is here (subtitle-rbt.c). Here’s what the final result looks like :

  
  
  “What’s in the refrigerator ?” I should note at this juncture that I am not sure if this particular Robot file even has sound or dialogue since I was conducting these experiments on a computer with non-working audio.
  

  The RBT Format
  I have created a new MultimediaWiki page describing the Robot Animation format based on the ScummVM source code. I have not worked with a format quite like this before. These are paletted animations which consist of a sequence of independent frames that are designed to be overlaid on top of static background. Because of these characteristics, each frame encodes its own unique dimensions and origin coordinate within the frame. While the Phantasmagoria VMD files are usually 288×144 (which are usually double-sized for the benefit of a 640×400 Super VGA canvas), these frames are meant to be plotted on a game field that was roughly 576×288 (288×144 doublesized).
  

  For example, 2 minimalist animation frames from a desk investigation Robot file :

  
  
  100×147

  
  101×149
  

  As for compression, my first impression was that the algorithm was the same as VMD. This is wrong. It evidently uses an unmodified version of a standard algorithm called Lempel-Ziv-Stac (LZS). It shows up in several RFCs and was apparently used in MS-DOS’s transparent disk compression scheme.

  Approach
  Thankfully, many of the lessons I learned from the previous project are applicable to this project, including : subtitle library interfacing, subtitling in the paletted colorspace, and replacing encoded frames from the original file instead of trying to create a new file.

  Here is the pitch for this project :

  • Create a C program that can traverse through an input file, piece by piece, and generate an output file. The result of this should be a bitwise identical file.
  • Adapt the LZS compression decoding algorithm from ScummVM into the new tool. Make the tool dump raw Portable NetMap (PNM) files of varying dimensions and ensure that they look correct.
  • Compress using LZS.
  • Stretch the frames and draw subtitles.
  • More compression. Find the minimum window for each frame.

  Compression
  Normally, my first goal is to decompress the video and store the data in a raw form. However, this turned out to be mathematically intractable. While the format does support both compressed and uncompressed frames (even though ScummVM indicates that the uncompressed path is yet unexercised), the goal of this project requires making the frames so large that they overflow certain parameters of the file.

  A Robot file has a sequence of frames and 2 tables describing the size of each frame. One table describes the entire frame size (audio + video) while the second table describes just the video frame size. Since these tables only use 16 bits to specify a size, the maximum frame size is 65536 bytes. Leaving space for the audio portion of the frame, this only leaves a per-frame byte budget of about 63000 bytes for the video. Expanding the frame to 576×288 (165,888 pixels) would overflow this limit.

  Anyway, the upshot is that I needed to compress the data up front.

  Fortunately, the LZS compressor is pretty straightforward, at least if you have experience writing VLC-oriented codecs. While the algorithm revolves around back references, my approach was to essentially write an RLE encoder. My compressor would search for runs of data (plentiful when I started to stretch the frame for subtitling purposes). When a run length of n=3 or more of the same pixel is found, encode the pixel by itself, and then store a back reference of offset -1 and length (n-1). It took a little while to iron out a few problems, but I eventually got it to work perfectly.

  I have to say, however, that the format is a little bit weird in how it codes very large numbers. The length encoding is somewhat Golomb-like, i.e., smaller values are encoded with fewer bits. However, when it gets to large numbers, it starts encoding counts of 15 as blocks of 1111. For example, 24 is bigger than 7. Thus, emit 1111 into the bitstream and subtract 8 from 23 -> 16. Still bigger than 15, so stuff another 1111 into the bitstream and subtract 15. Now we’re at 1, so stuff 0001. So 24 is 11111111 0001. 12 bits is not too horrible. But the total number of bytes (value / 30). So a value of 300 takes around 10 bytes (80 bits) to encode.

  Palette Slices
  As in the VMD subtitling project, I took the subtitle color offered in the subtitle spec file as a suggestion and used Euclidean distance to match to the closest available color in the palette. One problem, however, is that the palette is a lot smaller in these animations. According to my notes, for the set of animations I scanned, only about 80 colors were specified, starting at palette index 55. I hypothesize that different slices of the palette are reserved for different uses. E.g., animation, background, and user interface. Thus, there is a smaller number of colors to draw upon for subtitling purposes.

  Scaling
  One bit of residual weirdness in this format is the presence of a per-frame scale factor. While most frames set this to 100 (100% scale), I have observed 70%, 80%, and 90%. ScummVM is a bit unsure about how to handle these, so I am as well. However, I eventually realized I didn’t really need to care, at least not when decoding and re-encoding the frame. Just preserve the scale factor. I intend to modify the tool further to take scale factor into account when creating the subtitle.

  The Final Resolution
  Right around the time that I was composing this post, The Translator emailed me and notified me that he had found a better way to subtitle the Robot files by modifying the scripts, rendering my entire approach moot. The result is much cleaner :

  
  
  Turns out that the engine supported subtitles all along
  

  It’s a good thing that I enjoyed the challenge or I might be annoyed at this point.

  See Also

  • Subtitling Sierra VMD Files : My effort to subtitle the main FMV files found in Sierra games.

• 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.

• Revision 36104 : Une petite erreur sur un message de la mutu grossissant pour pas grand ...

  11 mars 2010, par kent1@… — Log

  Une petite erreur sur un message de la mutu grossissant pour pas grand chose le texte