Chris White - Emulation & Decompilation

Friday, November 27, 2009

Game Objects and Level Format

I've made a lot of progress. After a lot of hard work I've figured out the internal object format used by OutRun for most sprites and game objects which is a big breakthrough. I understand the complete format, with the exception of a single bit that eludes me right now. The internal format is a lot more complex than the values finally written directly to the sprite hardware, as it includes aspects like:

  • Both Screen Co-ordinates & World co-ordinates
  • Sprite Z Values
  • Independent priority settings in relation to the road layer and other sprites
  • Specific Sprite routines to use
  • X/Y Draw anchors
  • Sprite Type
  • Frame Number
  • The way in which the sprites utilise a series of lookup tables to extract more properties
  • All the usual things you'd expect: h-flip values, palette settings and so forth.
The traffic sprite format uses a version of this, but adds additional properties:
  • Bits to denote the traffic's position in relation to other traffic. This is used to control its speed and lane changing behaviour. 
  • Speed
  • Information regarding the side of the road the traffic has spawned on
Even after establishing the above, and also a lot of related code - one thing is pretty clear: the codebase to this game is advanced and complex.

I also understand the internal format used to store the scenery data for the entire set of levels. Writing a quick utility to spit this out in some kind of visual form would be an interesting exercise and a way of verifying this. Whilst all the code is documented and commented, looking at it actually hurts by brain. I'll follow up on this later.

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