Friday, January 29, 2010

Roadblocks in the path of tSage

Those of you who have been following the IRC channels will know that I made some real progress in the disassembly of the tSage engine, used by the Ringworld - Revenge of the Patriarch game, as well as several others. I hadn't really gotten into any of the game logic, but I'd made great strides in decoding the image formats and graphics display, so much so that my engine on Scummvm-misc could display test dialogs, as well as the right-click game dialog with proper highlighting and everything. Sample screenshots below:

However, given the perversity of life, I should have known better than to announce on the channel that I was making real progress. :(. Because as I was getting into the game execution logic, I began to realise just how complicated they made the engine. Whilst using C++ has it's advantages from the perspective of RE, since it does group related functions close together with the variables they use, in this case, they seemed to go overboard with it.

A preliminary look at the main event loop of the game shows they implemented a stack of "action objects" - multiple independent event handlers can be registered and control swapped form one to the other. I'm not sure, but I presume that this is to allow for things like cutscenes, where standard game operation is superseded.

For even more complexity, they went with a class hierarchy for the action objects which is at least four levels deep!.
The classes, as far as I've been able to determine, are as follows:
  • They have a core 'SavedObject' class, which has some internal logic for registering each object instance in a central list. I presume this is for automating saving and loading.
  • A child class which I'll call "SmartPointer". This class seems to encapsulate a pointer to a secondary object, and provides core logic for 'fixing' up the pointer reference. I can't be sure yet, but there's a lot of 'fixup' code in the engine, which I presume handles restoring pointer references between objects when a savegame is loaded, and all the objects are re-constructed. This class also implement a basic virtual table of processing and dispatch methods that automatically defer to the foreign object if one is set
  • Yet another child class which introduces basic event manager functionality amongst other things
  • A concrete fourth child class which seems to further override some of the event manager functionality, and is what gets added to the game's "action list".

As you can imagine, all this makes it somewhat different to understand what the blazes is going on. I've also examined the engine from some other points, such as scene loading, and even that seems to be overly complex, with method calling method - they really went in the direction of generalised/generic functionality, with the result that the code seems to be much more complex than it should otherwise need to be.


In any case, the tSage engine was only meant to be a part-time RE effort for when I needed a break from the MADS engine. I did somewhat get carried away with it for the last few weeks, but that was only because the areas I was looking at proved so easy to reverse engineer. Now that I'm running into more complex code, I've decided that the break's over, and I'm returning to work on the MADS engine again. For that, at least, the code seems somewhat simpler, even if it can be a pain to run in the DOSBOX debugger because the segments keep getting shifted around in memory. ;)

For those of you who are Ringworld fans, don't worry, I'll still be working on the engine. It's just there won't be as rapid a progress on it as I'd recently hoped.

1 comment:

Unknown said...

They did something similar with The Last Express where they have a different set of event handlers for the menu, the inventory and the actual game.

There is also all sorts of crazyness in the logic code with some shared structs where they store the arguments to each function. And since the game allows you to rewind, they have this list of SavePoint structs to allow the game to get back to a previous state. So much fun!