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• Run ffmpeg from its parent directory in node js application in its own directory

  3 septembre 2022, par Belb

  I have a node js application which needs to access ffmpeg. My approach to this has been to use exec from the child_processes module built into Node. The problem is that exec always starts the cmd line from the current directory and I can't figure out how to point the directory context to where ffmpeg.exe is located so that I can access the program. Is my approach flawed ? How can I access a seperate CLI application from Node ?
  
This code returns "'ffmpeg' is not recognized as an internal or external command" because I'm obviously in Node's execution context which is not where ffmpeg is located.
  
I also do not want to store the node application in the directory of ffmpeg.exe because that's just lazy and impractical.

  


  exec(`ffmpeg -i ${filepathToMedia} --vf fps=1 ${outputdirectory}/out%d.png`, (error, stdout, stderr) => {
if (error) {
    console.log(`error: ${error.message}`);
    return;
}
if (stderr) {
    console.log(`stderr: ${stderr}`);
    return;
}
if(stdout) {
    console.log(`success: ${stdout}`)
} });


  


• PES structure of mpeg4 AVC video for encapsulation into Transport Stream

  30 septembre 2015, par CompNet

  I want to understand how I,B,P pictures are packetized and multiplexed when mpeg4 AVC/H.264 coded video is encapsulated to a Transport Stream container (for streaming protocols like HTTP Live Streaming). For mpeg2 codec video as I understood that each PES starts in a new TS packet but there can be overlap of I,B,P pictures in a single PES.

  But for mpeg4 AVC video can anyone explain how I,B,P frames are multiplexed in to PES ? Can they overlap in a PES which means a single TS packet loss can potentially lose multiple I/B/P frames ? I tried to go through the payload structures from the RFC and some other documents but could not understand clearly.

• Approaches To Modifying Game Resource Files

  16 août 2016, par Multimedia Mike — Game Hacking

  I have been assisting The Translator in the translation of another mid-1990s adventure game. This one isn’t quite as multimedia-heavy as the last title, and the challenges are a bit different. I wanted to compose this post in order to describe my thought process and mental model in approaching this problem. Hopefully, this will help some others understand my approach since what I’m doing here often appears as magic to some of my correspondents.

  High Level Model
  At the highest level, it is valuable to understand the code and the data at play. The code is the game’s engine and the data refers to the collection of resources that comprise the game’s graphics, sound, text, and other assets.

  
  
  Simplistic high-level game engine model
  

  Ideally, we want to change the data in such a way that the original game engine adopts it as its own because it has the same format as the original data. It is very undesirable to have to modify the binary engine executable in any way.

  Modifying The Game Data Directly
  How to modify the data ? If we modify the text strings for the sake of language translation, one approach might be to search for strings within the game data files and change them directly. This model assumes that the text strings are stored in a plain, uncompressed format. Some games might store these strings in a text format which can be easily edited with any text editor. Other games will store them as binary data.
  

  In the latter situation, a game hacker can scan through data files with utilities like Unix ‘strings’ to find the resources with the desired strings. Then, use a hex editor to edit the strings directly. For example, change “Original String”…

  0098F800   00 00 00 00  00 00 00 4F  72 69 67 69  6E 61 6C 20  .......Original 
  0098F810   53 74 72 69  6E 67 00 00  00 00 00 00  00 00 00 00  String..........
  

  …to “Short String” and pad the difference in string lengths using spaces (0x20) :

  0098F800   00 00 00 00  00 00 00 53  68 6F 72 74  20 53 74 72  .......Short Str
  0098F810   69 6E 67 20  20 20 00 00  00 00 00 00  00 00 00 00  ing   ..........
  

  This has some obvious problems. First, translated strings need to be of equal our smaller length compared to the original. What if we want to encode “Much Longer String” ?

  0098F800   00 00 00 00  00 00 00 4D  75 63 68 20  4C 6F 6E 67  .......Much Long
  0098F810   65 72 20 53  74 72 00 00  00 00 00 00  00 00 00 00  er Str..........
  

  It won’t fit. The second problem pertains to character set limitations. If the font in use was only designed for ASCII, it’s going to be inadequate for expressing nearly any other language.

  So a better approach is needed.

  Understanding The Data Structures
  An alternative to the approach outlined above is to understand the game’s resources so they can be modified at a deeper level. Here’s a model to motivate this investigation :

  
  
  Model of the game resource archive format
  

  This is a very common layout for such formats : there is a file header, a sequence of resource blocks, and a trailing index which describes the locations and types of the foregoing blocks.

  What use is understanding the data structures ? In doing so, it becomes possible to write new utilities that disassemble the data into individual pieces, modify the necessary pieces, and then reassemble them into a form that the original game engine likes.

  It’s important to take a careful, experimental approach to this since mistakes can be ruthlessly difficult to debug (unless you relish the thought of debugging the control flow through an opaque DOS executable). Thus, the very first goal in all of this is to create a program that can disassemble and reassemble the resource, thus creating an identical resource file. This diagram illustrates this complex initial process :

  
  
  Rewriting the game resource file
  

  So, yeah, this is one of the most complicated “copy file” operations that I can possibly code. But it forms an important basis, since the next step is to carefully replace one piece at a time.

  
  
  Modifying a specific game resource
  

  This diagram shows a simplistic model of a resource block that contains a series of message strings. The header contains pointers to each of the strings within the block. Instead of copying this particular resource block directly to the new file, a proposed modification utility will intercept it and rewrite the entire thing, writing new strings of arbitrary length and creating an adjusted header which will correctly point to the start of each new string. Thus, translated strings can be longer than the original strings.

  Further Work
  Exploiting this same approach, we can intercept and modify other game resources including fonts, images, and anything else that might need to be translated. I will explore specific examples in a later blog post.

  Followup

  • Translating Return to Ringworld, in which I apply the ideas expressed in this post.

  The post Approaches To Modifying Game Resource Files first appeared on Breaking Eggs And Making Omelettes.