1. CSE 380 – Computer Game Programming AI & Collision Strategy Erin Catto’s Box2D

2. The Big Picture Game::processGameLogic: void Game::processGameLogic() { // NOW PERFORM ALL AI ai->update(this); // NOW CORRECT FOR COLLISION DETECTION physics->update(this); // UPDATE THE WORLD, INCLUDING SPRITE AND // PARTICLE POSITIONS, FACTORING IN INPUT // AND AI world->update(this); } This should usually only change sprite velocities. Exception: spawning This will update position of all sprites This may do additional updates for special cases

3. So what’s our AI strategy? SpriteManager has access to all sprites –these are the only things that move –each frame we must test them against: each other collidable Background tiles User Input & AI –should typically only change a character’s velocity –there are exceptions of course (init positioning) Recommendation: –attach a Bot & BotType to each AnimatedSprite

4. One AI Possibility class AnimatedSprite HAS-A Bot instance variable class Bot HAS-A BotType instance variable –describes current AI state for sprite bot state, bot frame, bot counter, etc. –has an update method to update all variables updates sprite’s vX and vY, not position class BotType –describes a type of bot –types of states, state behaviors, etc. Each Frame, GameAI ’s update method: –goes through all sprites and calls update on bot

5. What about physics? Every time we detect a collision, we can store data about that event in one of these objects: class Collision { public: CollidableObject *co1; CollidableObject *co2; float timeOfCollision; float startTimeOfXCollision; float startTimeOfYCollision; float endTimeOfXCollision; float endTimeOfYCollision; };

6. Collision Construction Warning Each frame you’ll test for collisions You may find multiple collisions each frame You’ll want to store info about each collision in a Collision object. Why? –so you can sort them –use them to update the CollidableObjects DON’T construct new Collision objects each frame –recycle them instead

7. Recycling Collision objects When your game starts, construct an array of Collision objects. How many? –100 should do, make it 1000 to be over-safe –an array stack is easiest When a collision is detected: –take a collision object from array to use When collision resolved: –put it back

8. GamePhysics::update strategy 1.For each sprite: find all collisions with tiles, make a Collision object for each and compute time of collision. Add each Collision object to a collisions array 2.For each sprite, do the same as step 1 but for all Sprite-Sprite collisions 3.Sort the Collision array by time of collision 4.If collisions array is not empty, move all sprites to time of first collision (change their x, y according to vX, vY and % of frame) 5.Resolve collision (change vX, vY of sprites involved) …

9. GamePhysics::update strategy 6.Execute collision response code –Ex: sprite type 1 collides with sprite type 2, so reduce HP of sprite type 1 –this step may be combined with step 5. i.e., sometimes you might not want to change velocity of a sprite after collision 7.Remove all collisions from array involved in resolved collision 8.Perform steps 1 & 2 only for sprites involved in collisions Continue to do these steps until you reach the end of the frame NOTE: after each collision resolution, make sure the objects are not colliding. Why? –floating point error