Infinum Android Talks #15 - How to develop a simple 2D game with physics engine
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The document outlines the development of a simple 2D game featuring falling balls and a scoring system based on the numbers on the balls. It discusses key components such as object spawning, physics (gravity and collisions), and game architecture primarily implemented as a custom view. Key topics include collision detection methods, impulse resolution, scoring mechanics, and handling various screen sizes.
Infinum Android Talks #15 - How to develop a simple 2D game with physics engine
- 1. How to develop a simple 2D game with physics engine IVAN KUŠT, ANDROID TEAM LEADER
- 2. THE GAME • Balls with random numbers 0 - 9 are falling from the sky • User must catch as many as possible in the bucket that can move along the bottom • Number of the ball defines score for catching the ball
- 3. SO WHAT DO WE NEED? • A way to spawn the objects on the top • Simple physics - gravity and collisions • Scoring and time limit
- 5. OBJECTS • Each object in physics engine is either: • circle • rectangle • Gravity can be applied or not • Current position • Current speed vector (x, y components)
- 6. SPAWNING OBJECTS • Balls are spawned at a time interval • Random size (with min and max values) • Random position • Once spawned, object is added to physics engine (which applies gravity and resolves collisions of the object) SPAWNING AREA
- 7. 02COLLISIONS
- 9. AXIS ALIGNED BOUNDING BOXES X Y • Rectangle that encloses an object in the game • Used for collision detection: • easiest • fastest
- 10. AABB COLLISION DETECTION • Rectangles intersect • Extreme case: • one rect encloses another
- 11. CIRCLES • Distance between centers <= sum of radiuses
- 12. CIRCLE VS AABB • Distance between closest point of rect and circle center <= circle radius
- 14. OBJECTS COLLIDED - NOW WHAT? • Calculate new speed vectors for each object • Two dimensional elastic collision • Every object has an impulse or momentum: Impulse = mass * velocity
- 15. WHEN OBJECTS COLLIDE • The relative velocity of one particle with respect to the other is reversed by the collision • The average of the momenta before and after the collision is the same for both particles
- 16. CIRCLE VS CIRCLE TANGENT NORMAL • Overall velocity split into two perpendicular velocities • One along common normal (line of collision) • One perpendicular to common normal X Y
- 17. CIRCLE VS AABB TANGENT NORMAL • Calculate new velocities depending on the masses of the object • Recalculate new velocities to the x and y aligned velocity components X Y
- 18. SIMULATING REALITY • In reality, some kinetic energy is lost on collision (the ball never bounces back from the ground to the same height) • Restitution coefficient • multiply calculated velocities • Feels more “real”
- 19. IN THE GAME • Two rectangles on each side of the bucket that collide with the balls • mass = infinite • Simulates bucket collisions
- 20. 02.3SINKING
- 21. SINKING • What if an object with low restitution hits a wall (or floor) with infinite mass? • It begins to sink (the expected behavior would be for the ball to stay next to the wall or floor) • due to floating point error that accumulates over time
- 22. SOLUTION TANGENT NORMAL • Solution: linear projection • move each object by a small percentage of the mass along collision normal
- 23. 03SCORING
- 24. SCORING • Goal of the game is to collect as many balls as possible in a limited amount of time • Score for a “catch” is equal to number shown on the ball
- 25. SCORING • A rectangular region is defined • if a ball is in the region it is removed from physics engine and score is increased SCORING REGION
- 27. ENGINE ARCHITECTURE PHYSICS ENGINESOLID SOLID CIRCLE SOLID RECT OBJECT GENERATOR BALL GENERATOR
- 28. GAME IMPLEMENTATION • Implemented as a custom View (no OpenGL) • Constant redraw is forced • invalidate() posted in a loop using Handler • On each frame: 1. appliy gravity to all objects 2. check and resolve collisions 3. generate new objects 4. draw new frame based on state of objects
- 29. CONCLUSIONS • Simple game can be implemented without using OpenGL • Various screen sizes must be taken into account: • easier to catch if resolution is higher
- 30. REFERENCES • http://gamedevelopment.tutsplus.com/tutorials/how-to- create-a-custom-2d-physics-engine-the-basics-and- impulse-resolution--gamedev-6331 • http://gamedevelopment.tutsplus.com/tutorials/when- worlds-collide-simulating-circle-circle-collisions-- gamedev-769 • https://en.wikipedia.org/wiki/Elastic_collision • http://hypertextbook.com/facts/2006/restitution.shtml