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• Grand Unified Theory of Compact Disc

  1er février 2013, par Multimedia Mike — General

  This is something I started writing about a decade ago (and I almost certainly have some of it wrong), back when compact discs still had a fair amount of relevance. Back around 2002, after a few years investigating multimedia technology, I took an interest in compact discs of all sorts. Even though there may seem to be a wide range of CD types, I generally found that they’re all fundamentally the same. I thought I would finally publishing something, incomplete though it may be.

  Physical Perspective
  There are a lot of ways to look at a compact disc. First, there’s the physical format, where a laser detects where pits/grooves have disturbed the smooth surface (a.k.a. lands). A lot of technical descriptions claim that these lands and pits on a CD correspond to ones and zeros. That’s not actually true, but you have to decide what level of abstraction you care about, and that abstraction is good enough if you only care about the discs from a software perspective.

  Grand Unified Theory (Software Perspective)
  Looking at a disc from a software perspective, I have generally found it useful to view a CD as a combination of a 2 main components :

  • table of contents (TOC)
  • a long string of sectors, each of which is 2352 bytes long

  I like to believe that’s pretty much all there is to it. All of the information on a CD is stored as a string of sectors that might be chopped up into a series of anywhere from 1-99 individual tracks. The exact sector locations where these individual tracks begin are defined in the TOC.

  Audio CDs (CD-DA / Red Book)
  The initial purpose for the compact disc was to store digital audio. The strange sector size of 2352 bytes is an artifact of this original charter. “CD quality audio”, as any multimedia nerd knows, is formally defined as stereo PCM samples that are each 16 bits wide and played at a frequency of 44100 Hz.
  

  (44100 audio frames / 1 second) * (2 samples / audio frame) * 
    (16 bits / 1 sample) * (1 byte / 8 bits) = 176,400 bytes / second
  (176,400 bytes / 1 second) / (2352 bytes / 1 sector) = 75
  

  75 is the number of sectors required to store a single second of CD-quality audio. A single sector stores 1/75th of a second, or a ‘frame’ of audio (though I think ‘frame’ gets tossed around at all levels when describing CD formats).

  The term “red book” is thrown around in relation to audio CDs. There is a series of rainbow books that define various optical disc standards and the red book describes audio CDs.

  Basic Data CD-ROMs (Mode 1 / Yellow Book)
  Somewhere along the line, someone decided that general digital information could be stored on these discs. Hence, the CD-ROM was born. The standard model above still applies– TOC and string of 2352-byte sectors. However, it’s generally only useful to have a single track on a CD-ROM. Thus, the TOC only lists a single track. That single track can easily span the entire disc (something that would be unusual for a typical audio CD).

  While the model is mostly the same, the most notable difference between and audio CD and a plain CD-ROM is that, while each sector is 2352 bytes long, only 2048 bytes are used to store actual data payload. The remaining bytes are used for synchronization and additional error detection/correction.

  At least, the foregoing is true for mode 1 / form 1 CD-ROMs (which are the most common). “Mode 1″ CD-ROMs are defined by a publication called the yellow book. There is also mode 1 / form 2. This forgoes the additional error detection and correction afforded by form 1 and dedicates 2336 of the 2352 sector bytes to the data payload.

  CD-ROM XA (Mode 2 / Green Book)
  From a software perspective, these are similar to mode 1 CD-ROMs. There are also 2 forms here. The first form gives a 2048-byte data payload while the second form yields a 2324-byte data payload.

  Video CD (VCD / White Book)
  These are CD-ROM XA discs that carry MPEG-1 video and audio data.

  Photo CD (Beige Book)
  This is something I have never personally dealt with. But it’s supposed to conform to the CD-ROM XA standard and probably fits into my model. It seems to date back to early in the CD-ROM era when CDs were particularly cost prohibitive.

  Multisession CDs (Blue Book)
  Okay, I admit that this confuses me a bit. Multisession discs allow a user to burn multiple sessions to a single recordable disc. I.e., burn a lump of data, then burn another lump at a later time, and the final result will look like all the lumps were recorded as the same big lump. I remember this being incredibly useful and cost effective back when recordable CDs cost around US$10 each (vs. being able to buy a spindle of 100 CD-Rs for US$10 or less now). Studying the cdrom.h file for the Linux OS, I found a system call named CDROMMULTISESSION that returns the sector address of the start of the last session. If I were to hypothesize about how to make this fit into my model, I might guess that the TOC has some hint that the disc was recorded in multisession (which needs to be decided up front) and the CDROMMULTISESSION call is made to find the last session. Or it could be that a disc read initialization operation always leads off with the CDROMMULTISESSION query in order to determine this.

  I suppose I could figure out how to create a multisession disc with modern software, or possibly dig up a multisession disc from 15+ years ago, and then figure out how it should be read.

  CD-i
  This type puzzles my as well. I do have some CD-i discs and I thought that I could read them just fine (the last time I looked, which was many years ago). But my research for this blog post has me thinking that I might not have been seeing the entire picture when I first studied my CD-i samples. I was able to see some of the data, but sources indicate that only proper CD-i hardware is able to see all of the data on the disc (apparently, the TOC doesn’t show all of the sectors on disc).

  Hybrid CDs (Data + Audio)
  At some point, it became a notable selling point for an audio CD to have a data track with bonus features. Even more common (particularly in the early era of CD-ROMs) were computer and console games that used the first track of a disc for all the game code and assets and the remaining tracks for beautifully rendered game audio that could also be enjoyed outside the game. Same model : TOC points to the various tracks and also makes notes about which ones are data and which are audio.

  There seems to be 2 distinct things described above. One type is the mixed mode CD which generally has the data in the first track and the audio in tracks 2..n. Then there is the enhanced CD, which apparently used multisession recording and put the data at the end. I think that the reasoning for this is that most audio CD player hardware would only read tracks from the first session and would have no way to see the data track. This was a positive thing. By contrast, when placing a mixed-mode CD into an audio player, the data track would be rendered as nonsense noise.

  Subchannels
  There’s at least one small detail that my model ignores : subchannels. CDs can encode bits of data in subchannels in sectors. This is used for things like CD-Text and CD-G. I may need to revisit this.

  In Summary
  There’s still a lot of ground to cover, like how those sectors might be formatted to show something useful (e.g., filesystems), and how the model applies to other types of optical discs. Sounds like something for another post.

• avcodec/cuvid : don’t align frame size

  4 octobre 2016, par Timo Rothenpieler

  avcodec/cuvid : don’t align frame size
  hwcontext_cuda was changed to take care of proper alignment internally
  

  • [DH] libavcodec/cuvid.c

• Translating Return To Ringworld

  17 août 2016, par Multimedia Mike — Game Hacking

  As indicated in my previous post, the Translator has expressed interest in applying his hobby towards another DOS adventure game from the mid 1990s : Return to Ringworld (henceforth R2RW) by Tsunami Media. This represents significantly more work than the previous outing, Phantasmagoria.

  
  
  Return to Ringworld Title Screen
  

  I have been largely successful thus far in crafting translation tools. I have pushed the fruits of these labors to a Github repository named improved-spoon (named using Github’s random name generator because I wanted something more interesting than ‘game-hacking-tools’).

  Further, I have recorded everything I have learned about the game’s resource format (named RLB) at the XentaxWiki.

  New Challenges
  The previous project mostly involved scribbling subtitle text on an endless series of video files by leveraging a separate software library which took care of rendering fonts. In contrast, R2RW has at least 30k words of English text contained in various blocks which require translation. Further, the game encodes its own fonts (9 of them) which stubbornly refuse to be useful for rendering text in nearly any other language.

  Thus, the immediate 2 challenges are :
  

  1. Translating volumes of text to Spanish
  2. Expanding the fonts to represent Spanish characters

  Normally, “figuring out the file format data structures involved” is on the list as well. Thankfully, understanding the formats is not a huge challenge since the folks at the ScummVM project already did all the heavy lifting of reverse engineering the file formats.

  The Pitch
  Here was the plan :

  • Create a tool that can dump out the interesting data from the game’s master resource file.
  • Create a tool that can perform the elaborate file copy described in the previous post. The new file should be bit for bit compatible with the original file.
  • Modify the rewriting tool to repack some modified strings into the new resource file.
  • Unpack the fonts and figure out a way to add new characters.
  • Repack the new fonts into the resource file.
  • Repack message strings with Spanish characters.

  Showing The Work : Modifying Strings
  First, I created the tool to unpack blocks of message string resources. I elected to dump the strings to disk as JSON data since it’s easy to write and read JSON using Python, and it’s quick to check if any mistakes have crept in.

  The next step is to find a string to focus on. So I started the game and looked for the first string I could trigger :

  
  
  

  This shows up in the JSON string dump as :

    
      "Spanish" : " !0205Your quarters on the Lance of Truth are spartan, in accord with your mercenary lifestyle.",
      "English" : " !0205Your quarters on the Lance of Truth are spartan, in accord with your mercenary lifestyle."
    ,
  

  As you can see, many of the strings are encoded with an ID key as part of the string which should probably be left unmodified. I changed the Spanish string :

    
      "Spanish" : " !0205Hey, is this thing on ?",
      "English" : " !0205Your quarters on the Lance of Truth are spartan, in accord with your mercenary lifestyle."
    ,
  

  And then I wrote the repacking tool to substitute this message block for the original one. Look ! The engine liked it !

  
  
  

  Little steps, little steps.

  Showing The Work : Modifying Fonts
  The next little step is to find a place to put the new characters. First, a problem definition : The immediate goal is to translate the game into Spanish. The current fonts encoded in the game resource only support 128 characters, corresponding to 7-bit ASCII. In order to properly express Spanish, 16 new characters are required : á, é, í, ó, ú, ü, ñ (each in upper and lower case for a total of 14 characters) as well as the inverted punctuation symbols : ¿, ¡.

  Again, ScummVM already documents (via code) the font coding format. So I quickly determined that each of the 9 fonts is comprised of 128 individual bitmaps with either 1 or 2 bits per pixel. I wrote a tool to unpack each character into an individual portable grey map (PGM) image. These can be edited with graphics editors or with text editors since they are just text files.

  Where to put the 16 new Spanish characters ? ASCII characters 1-31 are non-printable, so my first theory was that these characters would be empty and could be repurposed. However, after dumping and inspecting, I learned that they represent the same set of characters as seen in DOS Code Page 437. So that’s a no-go (so I assumed ; I didn’t check if any existing strings leveraged those characters).

  My next plan was hope that I could extend the font beyond index 127 and use positions 128-143. This worked superbly. This is the new example string :

    
      "Spanish" : " !0205¿Ves esto ? ¡La puntuacion se hace girar !",
      "English" : " !0205Your quarters on the Lance of Truth are spartan, in accord with your mercenary lifestyle."
    ,
  

  Fortunately, JSON understands UTF-8 and after mapping the 16 necessary characters down to the numeric range of 128-143, I repacked the new fonts and the new string :

  
  
  Translation : “See this ? The punctuation is rotated !”
  

  Another victory. Notice that there are no diacritics in this string. None are required for this translation (according to Google Translate). But adding the diacritics to the 14 characters isn’t my department. My tool does help by prepopulating [aeiounAEIOUN] into the right positions to make editing easier for the Translator. But the tool does make the effort to rotate the punctuation since that is easy to automate.

  Next Steps and Residual Weirdness
  There is another method for storing ASCII text inside the R2RW resource called strip resources. These store conversation scripts. There are plenty of fields in the data structures that I don’t fully understand. So, following the lessons I learned from my previous translation outing, I was determined to modify as little as possible. This means copying over most of the original data structures intact, but changing the field representing the relative offset that points to the corresponding string. This works well since the strings are invariably stored NULL-terminated in a concatenated manner.

  I wanted to document for the record that the format that R2RW uses has some weirdness in they way it handles residual bytes in a resource. The variant of the resource format that R2RW uses requires every block to be aligned on a 16-byte boundary. If there is space between the logical end of the resource and the start of the next resource, there are random bytes in that space. This leads me to believe that these bytes were originally recorded from stale/uninitialized memory. This frustrates me because when I write the initial file copy tool which unpacks and repacks each block, I want the new file to be identical to the original. However, these apparent nonsense bytes at the end thwart that effort.

  But leaving those bytes as 0 produces an acceptable resource file.

  Text On Static Images
  There is one last resource type we are working on translating. There are various bits of text that are rendered as images. For example, from the intro :

  
  
  

  It’s possible to locate and extract the exact image that is overlaid on this scene, though without the colors :

  
  
  

  The palettes are stored in a separate resource type. So it seems the challenge is to figure out the palette in use for these frames and render a transparent image that uses the same palette, then repack the new text-image into the new resource file.