Lecture 3

1. Lecture 3 Announcements

2. Next week only: we’re in 506

3. Design checks • Sign up early • Sign up early • If you can’t make the one you signed up for, ask another student to switch • The sooner you go, the sooner you can get feedback and fix potential problems • There are exactly 20 slots, and exactly 20 of you • Sign up early

4. Tac 1+2 feedback • Use Vecs instead of x,y pair passing • It gets WAY worse in Tou • Beware cutting up sprites every time you draw • Don’t ignore compiler warnings! • Maybe nothing, but maybe dodging a bullet • Looking good! • Playtesting should be fun today!

5. Tou has been released! • Shoot-’em-up, heavy use of collision detection • Much more open-ended than previous projects • Easier than previous projects if you work smart • Should help those of you combating the snowball of death TAC II TAC I TIC

6. Announcements Questions?

7. Lecture 3 Game Worlds

8. Game Worlds Motivation

9. How you probably did Tac Viewport Unit Unit Unit Unit Unit Unit Tile[][] Unit Unit Unit Unit Unit Unit AIUnit Unit Unit Unit Unit Bullet GameScreen Application FrontEnd

10. How does this scale? • Imagine implementing four-player split-screen: Normal UI element Viewports

11. Solution: separate world+screen Viewport Unit Unit Unit Unit Unit Unit Tile[][] Unit Unit Unit Unit Unit AIUnit Unit Unit Unit Unit Unit Bullet GameScreen Application World FrontEnd

12. Game Worlds Responsibilities

13. What does a world do? • Represents all game objects (“entities”) in a single space • “Owns” game coord system • Centralizes entity management • Maintains list, add/remove via queues or iterating over copy • Passes through ticks and draws to entities • Handles global logic • Anything beyond the scope of a single entity • E.g. providing collision detection callbacks to entities Unit Unit Unit Unit Unit Unit Enemy Unit Unit Background Unit Unit Bullet Boss Player Entities World logic World

14. Entities Entity • Single logical “object” within the game • Stores all state relevant to itself, e.g. draw shape, collision shape, HP, attack cooldown… • Should always know what World it’s in • Lightweight superclass • High-level subclasses in engine, low-level in game • Receives events from World • More than just tick+draw! Physics Entity Background Bullet Damagable Enemy Player Boss

15. Entity responsibilities • May draw, may not • Spawners, timers, • May use ticks, may not • Static environment objects, background… • Most will probably do both though • Player, items/pickups, • Player input? • Might be better handled by World, your call • Effects on other entities? • Might be better handled by World, your call

16. Multi-pass logic • Ticking and drawing entities in the wrong order leads to undesirable behavior • Drawing background over everything else • Entities removing themselves during collision detection • World can selectively update state in order • E.g. tick all entities so they update position, *then* check for collisions

17. Game Worlds Questions?

18. Lecture 1 Collision Detection I

19. Collision Detection I Motivation

20. Collisions have consequences • Object collisions are extremelyimportant in everyday life • Extremely important in games too • Games that use collision detection vastly outnumber those that don’t

21. What do we want to collide? • Points • Circles • Axis-Aligned Boxes • Convex polygons • Covered next week • Compound shapes • Other shapes • Not covered P min P1 P2 max P4 P5 P3 C r dim = (w,h) h x w y

22. Collision Detection I Detection Algorithms

23. Point-Circle • Check if the distance between the point and the center is less than or equal to the radius P P C C r r d d

24. Circle-Circle • Check if the distance between the two centers is less than or equal to the sum of the radii C2 C2 C1 C1 d d r2 r2 r1 r1

25. Point-AAB • Check if point is within range on each axis • AND P P max min max min x x y y

26. AAB-AAB • Ensure overlap on each axis: • AND • AND

27. Circle-AAB • Check if closest point to circle on AAB is in circle • Closest point: clamp C.x, C.y to [min.x, max.x], [min.y, max.y] • Then just do point-circle collision P P max min min max max min d P=C C C r r r d d=0

28. Compound-anything • Compound shape checks against sub-shapes • Only compound shapes should ever need to iterate over children

29. Collision Detection I Questions?

30. Lecture 1 Tips for Tou I

31. Write a collision debugger • Collision math is tricky • You will make mistakes • Test your collision code before even putting it in a game • This is “working smart” • You can try TA collision debuggers: dpercy, zdavis

32. Watch out for key “ghosting” • Due to the way keyboards are built, sometimes keys don’t work while others are being held down • In many cases, as few as 3 can be held at a time • Hardware issue (can’t fix it), you must design around it • E.g. use mouse for some actions • More info, interactive demo:http://www.microsoft.com/appliedsciences/antighostingexplained.mspx

33. Double dispatch • If you have a Circle and an AAB but only know that they’re Shapes, how do you determine which method to call? void testCollide() { Shapes = new Circle(); Shapes2= new AAB(); s.collides(s2); } abstract classShape { collidesCircle(Circlec); collidesAAB(AABaab); collidesComp(Compm); collides(Shape o); } boolean collides(Shapeo) { if (o instanceofCircle) { return collides((Circle) o); } elseif (o instanceofAAB) { return collides((AAB) o); } elseif (o instanceofComp) { return collides((Comp) o); } else { thrownewIllegalArgumentException(); } }

34. Double dispatch interfaceShape { collides(Shape o); collidesCircle(Circle c); collidesAAB(AABaab); collidesComp(Comp m); } public class CircleimplementsShape { collides(Shapeo) { returno.collidesCircle(this); } collidesCircle(Circlec) { /*code*/ } collidesAAB(AABaab) { /*code*/ } collidesComp(Compm) { /*code*/ } } public class AABimplementsShape { collides(Shapeo) { returno.collidesAAB(this); } collidesCircle(Circlec) { /*code*/ } collidesAAB(AABaab) { /*code*/ } collidesComp(Compm) { /*code*/ } } public class CompimplementsShape { collides(Shapeo) { returno.collidesComp(this); } collidesCircle(Circlec) { /*code*/ } collidesAAB(AABaab) { /*code*/ } collidesComp(Compm) { /*code*/ } }

35. Tips for Tou I Java Tip of the Week

36. Variable arguments! • Wouldn’t it be nice if instead of this: List<Shape> l = new ArrayList<Shape>(); l.add(newCircle()); l.add(newAAB()); l.add(newAAB()); Compound c = newCompound(l); • Or this: Compound c = newCompound(newShape[] {newCircle(), newAAB(), newAAB()}); • You could do this? Compound c = newCompound(newCircle(), newAAB(), newAAB());

37. Well you can! • The type of the last argument of a constructor/method may end in ‘…’ to indicate “any number of arguments” public Compound(Color c, Shape... shapes) • The “variable” argument will actually be an array of arguments (it’s really just wrapping array creation): public Compound(Color c, Shape... shapes) { for (int i=0; i < shapes.length; ++i) { add(shapes[i]); } }

38. Passing arrays as varargs • Since it’s really just an array, it’s possible to just pass in an array if you already have one: static voidprintAllOnALine(Object... objs) { for (Object o : objs) System.out.print(“(”+o+”) ”); System.out.println(); } public static void main(String[] args) { printAllOnALine(args); List<?>ents = newWorld().getEntities(); printAllOnALine(list.toArray()); }

39. Tips for Tou I Questions?

40. Tac 2 playtesting! More fun than last week!