Ping pong game collision detection error

Hello all, I'm a beginner, just trying to learn the elements of game development. I am creating a ping pong game clone, with allegro library. Following are the classes in my program:

1.Paddle

2.AiPaddle

3.Ball

1.Paddle.cpp code:


#include "paddle.h"
#include <iostream>

bool Paddle::init()
{
	p = new Position();
	p->x = 0.0;
	p->y = SCREEN_HEIGHT/2.0;

	v = new Velocity();
	v->x = 4.0;
	v->y = 4.0;

	image = al_load_bitmap("paddle1.png");

	if (!image) {
		std::cout<<"in padd init!"<<std::endl;
		return false;
	}
	return true;
}

ALLEGRO_BITMAP* Paddle::getBitmap()
{
	return image;
}

float Paddle::getHeight()
{
	return al_get_bitmap_height(image);
}

float Paddle::getWidth()
{
	return al_get_bitmap_width(image);
}

Position* Paddle::getPosition()
{
	return p;
}

Velocity*  Paddle::getVelocity()
{
	return v;
}

void Paddle::move(ALLEGRO_KEYBOARD_STATE* keyState)
{
	if ( al_key_down(keyState, ALLEGRO_KEY_DOWN) ) {
		if(p->y+getHeight() < SCREEN_HEIGHT ) {
			p->y += v->y;
		}
	} else if ( al_key_down(keyState, ALLEGRO_KEY_UP) ) {
		if ( (p->y - v->y) >= 0 ) {
			p->y -= v->y;
		}
	}
}

void Paddle::draw()
{
	al_draw_bitmap(image, p->x, p->y,0);
}

2. AiPaddle.cpp code:


#include "ai_paddle.h"
#include <iostream>

bool AiPaddle::init()
{
	p = new Position();
	p->y = SCREEN_HEIGHT/2.0;

	v = new Velocity();
	v->x = 4.0;
	v->y = 4.0;

	image = al_load_bitmap("paddle1.png");
	p->x = SCREEN_WIDTH - al_get_bitmap_width(image);

	if (!image) {
		std::cout<<"in padd init!"<<std::endl;
		return false;
	}
	return true;
}

ALLEGRO_BITMAP* AiPaddle::getBitmap()
{
	return image;
}

float AiPaddle::getHeight()
{
	return al_get_bitmap_height(image);
}

float AiPaddle::getWidth()
{
	return al_get_bitmap_width(image);
}

Position* AiPaddle::getPosition()
{
	return p;
}

Velocity*  AiPaddle::getVelocity()
{
	return v;
}

void AiPaddle::move(ALLEGRO_KEYBOARD_STATE* keyState)
{
	if ( al_key_down(keyState, ALLEGRO_KEY_DOWN) ) {
		if(p->y+getHeight() < SCREEN_HEIGHT ) {
			p->y += v->y;
		}
	} else if ( al_key_down(keyState, ALLEGRO_KEY_UP) ) {
		if ( (p->y - v->y) >= 0 ) {
			p->y -= v->y;
		}
	}
}

void AiPaddle::draw()
{
	al_draw_bitmap(image, p->x, p->y,0);
}

void AiPaddle::update(Velocity* ball_velocity, Position* ball_position)
{
	if ( ball_velocity->x > 0 ) {
		if ( ball_velocity->y > 0 && p->y + getHeight() < ball_position->y ) {
			if(p->y+getHeight() < SCREEN_HEIGHT ) {
				p->y += v->y;
			}
		} else {
			if ( (p->y - v->y) >= 0 && p->y > ball_position->y ) {
				p->y -= v->y;
			}
		}
	}		
}

3.Ball.cpp code:


#include "ball.h"

bool Ball::init()
{
	p = new Position();
	p->x = 0.0;
	p->y = 200.0;

	v = new Velocity();
	v->x = 3.0;
	v->y = 2.0;

	image = al_load_bitmap("ball1.png");
	
	if(!image) {
		return false;
	}
	return true;
}

bool Ball::collisionWithWall()
{
	bool collision_flag = false;

	if ( p->y <= 0 || p->y + getHeight() >= SCREEN_HEIGHT ) {
		collision_flag = true;
	}

	return collision_flag;
}

bool Ball::collisionWithPaddles(Paddle* pd, AiPaddle* aipd)
{
	bool collision_flag = false;

	if( p->y >= pd->getPosition()->y && p->y <= (pd->getPosition()->y+pd->getHeight()) ) {
		if ( p->x <= (pd->getPosition()->x+pd->getWidth()) ) {
			collision_flag = true;
		}
	} else if ( p->y >= aipd->getPosition()->y && p->y <= (aipd->getPosition()->y+aipd->getHeight()) ) {
		if ( p->x+getWidth() >= (aipd->getPosition()->x) ) {
			collision_flag = true;
		}
	}

	return collision_flag;
}

void Ball::update(Paddle* pd, AiPaddle* aipd)
{
	if ( collisionWithPaddles(pd, aipd) ) {
		v->x = -v->x;
	} else if ( p->y <= 0 || p->y + getHeight() >= SCREEN_HEIGHT ) {
		v->y = -v->y;
	}
	p->x += v->x;
	p->y += v->y;
}

Position* Ball::getPosition()
{
	return p;
}

Velocity* Ball::getVelocity()
{
	return v;
}

void Ball::draw()
{
	al_draw_bitmap(image, p->x, p->y, 0);
}

float Ball::getHeight()
{
	return al_get_bitmap_height(image);
}

float Ball::getWidth()
{
	return al_get_bitmap_width(image);
}

4. Main function:


#include <iostream>
#include <stdio.h>
#include <allegro5/allegro.h>
#include "allegro5/allegro_image.h"
#include "allegro5/allegro_native_dialog.h"
#include "ball.h"
#include "ai_paddle.h"
#include <iostream>

using namespace std;

const float FPS = 60;

const int BOUNCER_SIZE = 32;
enum MYKEYS { KEY_UP, KEY_DOWN, KEY_LEFT, KEY_RIGHT };

int main(int argc, char** argv)
{
	ALLEGRO_DISPLAY *display = NULL;
	ALLEGRO_EVENT_QUEUE* event_queue = NULL;
	ALLEGRO_TIMER* timer = NULL;
	bool redraw = true;
	bool doexit = false;
	bool key[4] = {false, false, false, false };
	ALLEGRO_KEYBOARD_STATE keyState;

	if(!al_init()) {
			fprintf(stderr, "failed to initialize!");
		return -1;
	}

	if(!al_init_image_addon()) {
      al_show_native_message_box(display, "Error", "Error", "Failed to initialize al_init_image_addon!", 
                                 NULL, ALLEGRO_MESSAGEBOX_ERROR);
      return 0;
   }

	if(!al_install_keyboard()) {
		fprintf(stderr, "failed to initialize keyboard!");
		return -1;
	}

	timer = al_create_timer(1.0/FPS);
	if(!timer) {
		fprintf(stderr, "failed to initialize timer!");
		return -1;
	}

	display = al_create_display(SCREEN_WIDTH, SCREEN_HEIGHT);
	if(!display) {
		fprintf(stderr, "display failed to initialize!");
		return -1;
	}

	 event_queue = al_create_event_queue();
   if(!event_queue) {
      fprintf(stderr, "failed to create event_queue!\n");
      al_destroy_display(display);
      al_destroy_timer(timer);
      return -1;
   }

	Paddle paddle;
	if ( !paddle.init() ) {
		cout<<"error";
		return -1;
	}
	 
	Ball ball;
	if ( !ball.init() ) {
		return -1;
	}

	AiPaddle aipaddle;
	if( !aipaddle.init() ) {
		return -1;
	}

	al_register_event_source(event_queue, al_get_display_event_source(display));
 
	al_register_event_source(event_queue, al_get_timer_event_source(timer));
 
	al_register_event_source(event_queue, al_get_keyboard_event_source());
 
	al_clear_to_color(al_map_rgb(0,0,0));
 
	al_flip_display(); 
	
	al_start_timer(timer);

	while(!doexit) {
		ALLEGRO_EVENT ev;

		al_wait_for_event(event_queue, &ev);

		if ( ev.type == ALLEGRO_EVENT_TIMER ) {
			al_get_keyboard_state(&keyState);
			paddle.move(&keyState);
			ball.update(&paddle, &aipaddle);
			aipaddle.update( ball.getVelocity(), ball.getPosition() );
			redraw = true;
		} else if ( ev.type == ALLEGRO_EVENT_DISPLAY_CLOSE ) {
			break;
		} else if(ev.type == ALLEGRO_EVENT_KEY_UP) {
			switch(ev.keyboard.keycode) {
				case ALLEGRO_KEY_ESCAPE:
					doexit = true;
					break;
			}
		}
		 
		if(redraw && al_is_event_queue_empty(event_queue)) {
			redraw = false;
 			al_clear_to_color(al_map_rgb(0,0,0));
			ball.draw();
			paddle.draw();
			aipaddle.draw();
			al_flip_display();
		}
	}

	return 0;
}

Along with these, i created a global header file, in which i put common structs:


#ifndef GLOBAL_H
#define GLOBAL_H


#include "allegro5/allegro.h"
#include "allegro5/allegro_image.h"

struct Position {
	float x;
	float y;
};

struct Velocity {
	float x;
	float y;
};

#define SCREEN_WIDTH 640
#define SCREEN_HEIGHT 480

#endif

The problem is, sometimes the ball goes through the AI paddle, dont seem to get it why.

Other issue is, sometimes the ball collides the player paddle but it just doesn't change the direction. It would be helpful if you could point out the error or where I'm going wrong.

Thank you!

Generally you want to move objects and then check collision due to the fact that it completely messes up collision resolution if only one object collides with another. Also what you're talking, the ball passing through the paddle is known as tunnelling. Normally that's just a thing related to objects moving faster than the update rate, but part of that I'm sure is caused by the way you're handling collision detection.

For your issue with the ball sometimes not reversing direction, taking a wild guess I'd say that happens when the ball moves into a paddle partially and then reverses its velocity and doesn't 'escape' the paddle on the next frame. Generally with collision detection you want to either move an object back to it's previous position when a collision is detected or you want to "nudge" it until it is free from the other object. Only then do you apply things like velocity changes. Unless you want colliding objects to travel through each other, collisions are basically an event that should be reversed in code before they're ever drawn.

For fixing tunneling your options are a little limited, you can slow the objects down by capping their velocity, adjust sprite size, change to a continuous form of collision detection, so on. With a game like pong unless you're letting the balls move supersonic speeds or your update rate is extremely low you shouldn't really have an issue.

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