Tag Archives: AI

Missions of the Reliant: Watch it, you’ve got one on your tail!

Missions of the Reliant version 3.0 now has the framework for enemies, enemy AI, and those infinitely annoying little fighters that everything lauches in droves at you and you can only hit by draining all the charge from your laser couplings. It took some work, let me tell you.

Mike’s original code expresses differences between facing angles as a function of which sprite is being displayed. Efficient. My code expresses differences between facing angles as atan2(-distanceY, distanceX). Mathematically correct and conceptually accurate.

‘Course, then “am I facing the player within a 120-degree arc?” becomes a completely different numerical test. Didn’t help that you kept setting variables you never used anywhere, Mike :-). Originally, I tried to use pretty much identical code for enemy movement as player movement, but it became clear that there were just too many quirks to it.

The fighters especially have an interesting AI about moving:

  1. Calculate distance from me to the player. If too high, return to origin, else continue. (Target determination.)
  2. Calculate angle from me to the player. If I’m not facing that angle exactly (difference between facing and calculated != 0), turn towards it. (Aim.)
  3. If I’m facing the player within 120° of arc, apply thrust N. (Thrust.)
  4. If I’m within distance D1 of the player, reduce speed by 10%. (Falloff.)
  5. If I’m within distance D2 of the player, and I have a loaded torpedo, and I’m facing the player exactly as in step 2, fire my weapon. (Attack.)
  6. Repeat every tick (or every other tick depending on preference setting) of the game timer.

Sounds simple? The fighter has to track its target, its current vector, its ideal vector, the difference between those two, its distance from target, and its firing delay, and constantly adjust all parameters accordingly. The code specifically intended for fighters is 118 lines including comments. Add 134 lines for code that applies to every enemy, 176 lines for code that applies to all game objects, 50 lines for code that applies to anything physics-capable, and 87 more lines for code that applies to everything that needs to do things based on the game timer. Subtract roughly 50 from that for comments and you get 500 lines of code just to drive that simple little AI, not counting the game timer itself or any of the drawing logic.

And I haven’t implemented the weapon, collision detection for the weapon, or the return to origin logic yet. And it’s got a couple of glitches as is, like if the fighter happens to spawn at exactly a 90, 180, 270, or 0-degree angle to the player while said player is at a full stop, it moves in a straight line rather than arcing like it should. Not sure if that bug’s in the original game since it’s next to impossible to set that particular scenario up deliberately without hacking source.

Did I mention that due to several stupid errors on my part, I ended up having to go back to the original fighter model in Infini-D and re-render it, then use Photoshop Elements to recreate the 36-phase sprite from that render? That was a fun two hours. On the other hand, the fighter model looks more “cool” now. It’s also a little harder to see. Sigh.

If anyone’s interested, here’s a couple of screenshots. I’d post a movie but I don’t have any instantly available way of making one and I’m too lazy to pursue the less instant ways.

Missions of the Reliant Fighter 1 Missions of the Reliant Fighter 2