flt2vhs

A set of tools to convert Falcon BMS recordings from their initial format (.flt) to their replay format (.vhs) in seconds, not minutes.

How do I use it?

1. Extract the 7zip archive into BMS/User/Acmi.

2. Run patch-bms-novhs.exe to disable BMS's (slow) FLT to VHS conversion. You only need to do this once.

3. Record BMS flights normally with the AVTR switch in the cockpit, or by running BMS with -acmi. BMS should no longer pause to convert recordings when you exit 3D.

4. Close BMS (if you're running with -acmi, BMS often holds tiny FLT files open in 2D, preventing these tools from doing their jobs.)

5. Run convert-all-flts.exe to convert all FLT files to VHS, with names based on the times they were created. Or drag a FLT file file onto flt2vhs.exe to convert one at a time.

For the CLI-inclined, see each tool's --help for more options.

Why?

Running BMS with the -acmi flag or flipping the AVTR switch in the cockpit records your flight, but before it can be viewed in tools like Tacview, it needs to be converted from one format (FLT) to another (VHS).

As of version 4.35U1, BMS's conversion is painfully slow. For 30+ minute flights with lots of planes and vehicles moving around, the conversion takes several minutes, during which you stare at a black screen. It also chunks recordings into multiple files if they were larger than 1 GB, and Tacview (as of 1.8.6) sometimes has trouble merging these, leaving "ghost" planes hanging in the air where one file ended and the next began.

What?

Since 2017, Loitho has provided a third-party tool called ACMI-Compiler that:

1. Steals the FLT file from BMS in the brief time between when the game finishes writing it and when the game re-opens the file to start the conversion.

2. Performs the FLT -> VHS conversion itself in seconds.

This project does much of the same, but with several improvements:

1. Nothing to run in the background: Instead of running a background program to steal FLT files from BMS, flt2vhs ships with a tool (patch-bms-novhs) that just disables BMS's slow conversion.

2. Automatic merging: flt2vhs can merge multiple FLT files into a single VHS. This feature is still in the experimental stage but seems to work well so far, even in 20+ player events with hundreds of units moving around.

3. Even better performance:

   • flt2vhs memory maps the FLT files it reads and the VHS files it writes. This improves performance by working directly with the operating system's page cache instead of copying file data with each read() and write() system call.

   • ACMI-Compiler stores events in a series of large arrays, very similar to how they are stored in the VHS file. This simplifies actually writing the VHS, but complicates everything else. Nearly all of the data has to be sorted, and many steps need to search through the arrays to find the data they need.

     flt2vhs organizes its data more efficiently. Events for each entity (plane, vehicle, missile, etc.) are stored in a hash table, allowing us to look entities up in constant time instead of performing a search. This also reduces the amount of duplicated data - for example, events no longer needs to store the ID of the entity they belong to. Less data means the program is more cache-friendly, which is one of the most important ways you can improve performance on modern systems.

   These design choices make flt2vhs very fast - nearly twice as fast as ACMI-Compiler. Most of the time is spent waiting for the OS to put the file on the disk!

Additionally,

1. In the spirit of the Unix philosophy, "make each program do one thing well", functionality is split into a couple programs: patch-bms-novhs patches BMS, flt2vhs handles the actual FLT to VHS conversion, and convert-all-flts runs flt2vhs on each FLT file in the directory. A tool to print VHS files as JSON, vhscat, is also provided for debugging.

2. Everything but convert-all-flts is entirely cross-platform and can be built/run/tested on Linux or MacOS.

Thanks

This wouldn't be possible without Loitho. The ACMI-Compiler source - and his kind responses to my many silly questions - were instrumental to understanding the FLT and VHS formats. The legwork they went through to understand the formats in the first place is nothing short of impressive. Loitho was also kind enough to provide example FLT files and expected outputs, which gave me great test data.