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# Allegro 5 Collision Detection

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Uhm, hello everyone, this is my first post on these forums. I'd like to receive some help on adding simple collision to a small pong project which I'm trying to make with Allegro5. Most of the code has just been assembled by reading the tutorials on the Allegro 5 wiki. Thing is, I don't know where/how to implement bounding box collision between, let's say, paddle 1 and bouncer. Here's the code, it's not too long.

#include <iostream> #include <cstdio> #include <allegro5/allegro.h> using namespace std; const float FPS = 60; const int SCREEN_W = 1024; const int SCREEN_H = 768; const int BOUNCER_SIZE = 40; const int PADDLE_SIZE_X = 40; const int PADDLE_SIZE_Y = 160; enum MYKEYS{KEY_W, KEY_S}; int bounding_box_collision ( int b1_x, int b1_y, int b1_w, int b1_h, int b2_x, int b2_y, int b2_w, int b2_h ) { if ( ( b1_x > b2_x + b2_w - 1 ) || // is b1 on the right side of b2? ( b1_y > b2_y + b2_h - 1 ) || // is b1 under b2? ( b2_x > b1_x + b1_w - 1 ) || // is b2 on the right side of b1? ( b2_y > b1_y + b1_h - 1 ) ) // is b2 under b1? { // no collision return 0; } // collision return 1; } int main(int argc, char **argv) { ALLEGRO_DISPLAY *display = NULL; ALLEGRO_EVENT_QUEUE *event_queue = NULL; ALLEGRO_TIMER *timer = NULL; ALLEGRO_BITMAP *bouncer = NULL; ALLEGRO_BITMAP *p1Paddle = NULL; ALLEGRO_BITMAP *p2Paddle = NULL; int randNum; float bouncer_x = SCREEN_W / 2.0 - BOUNCER_SIZE / 2.0; float bouncer_y = SCREEN_H / 2.0 - BOUNCER_SIZE / 2.0; float p1Paddle_x = 10; float p1Paddle_y = SCREEN_H / 2.0 - PADDLE_SIZE_Y / 2.0; float p2Paddle_x = SCREEN_W - 10 - PADDLE_SIZE_X; float p2Paddle_y = SCREEN_H / 2.0 - PADDLE_SIZE_Y / 2.0; float bouncer_dx = -4.0, bouncer_dy = 4.0; bool key[2] = {false, false}; bool doexit = false; bool redraw = true; if (!al_init()) { fprintf(stderr, "failed to initialize allegro!\n"); return -1; } if(!al_install_keyboard()) { fprintf(stderr, "failed to initialize the keyboard!\n"); return -1; } timer = al_create_timer(1.0 / FPS); if (!timer) { fprintf(stderr, "failed to create timer!\n"); return -1; } display = al_create_display(SCREEN_W, SCREEN_H); if (!display) { fprintf(stderr, "failed to create display!\n"); return -1; } bouncer = al_create_bitmap(BOUNCER_SIZE, BOUNCER_SIZE); if (!bouncer) { fprintf(stderr, "failed to create bouncer bitmap!\n"); al_destroy_display(display); al_destroy_timer(timer); return -1; } p1Paddle = al_create_bitmap(PADDLE_SIZE_X, PADDLE_SIZE_Y); if (!p1Paddle) { fprintf(stderr, "failed to create player one paddle bitmap!\n"); al_destroy_display(display); al_destroy_timer(timer); return -1; } p2Paddle = al_create_bitmap(PADDLE_SIZE_X, PADDLE_SIZE_Y); if (!p2Paddle) { fprintf(stderr, "failed to create player two paddle bitmap!\n"); return -1; } al_set_target_bitmap(p1Paddle); al_clear_to_color(al_map_rgb(0, 191, 255)); al_set_target_bitmap(p2Paddle); al_clear_to_color(al_map_rgb(0, 255, 191)); al_set_target_bitmap(bouncer); al_clear_to_color(al_map_rgb(255, 0, 255)); al_set_target_bitmap(al_get_backbuffer(display)); al_start_timer(timer); event_queue = al_create_event_queue(); if (!event_queue) { fprintf(stderr, "failed to create event_queue!\n"); al_destroy_bitmap(bouncer); al_destroy_display(display); al_destroy_timer(timer); 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) { bounding_box_collision ( p1Paddle_x, p1Paddle_y, PADDLE_SIZE_X, PADDLE_SIZE_Y, bouncer_x, bouncer_y, BOUNCER_SIZE, BOUNCER_SIZE ); if (bouncer_x < 0 || bouncer_x > SCREEN_W - BOUNCER_SIZE) { bouncer_dx = -bouncer_dx; } if (bouncer_y < 0 || bouncer_y > SCREEN_H - BOUNCER_SIZE) { bouncer_dy = -bouncer_dy; } bouncer_x += bouncer_dx; bouncer_y += bouncer_dy; if (bouncer_y + PADDLE_SIZE_Y / 2 >= SCREEN_H) { p2Paddle_y = SCREEN_H - PADDLE_SIZE_Y / 2; } else if(bouncer_y - PADDLE_SIZE_Y / 2 < 0) { p2Paddle_y = PADDLE_SIZE_Y / 2; } else { p2Paddle_y = bouncer_y; } if(key[KEY_W] && p1Paddle_y > 0 ) { p1Paddle_y -= 4; } if(key[KEY_S] && p1Paddle_y < SCREEN_H - PADDLE_SIZE_Y) { p1Paddle_y += 4; } redraw = true; } else if (ev.type == ALLEGRO_EVENT_DISPLAY_CLOSE) { break; } else if(ev.type == ALLEGRO_EVENT_KEY_DOWN) { switch(ev.keyboard.keycode) { case ALLEGRO_KEY_W: key[KEY_W] = true; break; case ALLEGRO_KEY_S: key[KEY_S] = true; break; } } else if(ev.type == ALLEGRO_EVENT_KEY_UP) { switch(ev.keyboard.keycode) { case ALLEGRO_KEY_W: key[KEY_W] = false; break; case ALLEGRO_KEY_S: key[KEY_S] = false; break; 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)); al_draw_bitmap(p1Paddle, p1Paddle_x, p1Paddle_y, 0); al_draw_bitmap(p2Paddle, p2Paddle_x, p2Paddle_y - PADDLE_SIZE_Y / 2, 0); al_draw_bitmap(bouncer, bouncer_x, bouncer_y, 0); al_flip_display(); } } al_destroy_bitmap(bouncer); al_destroy_bitmap(p1Paddle); al_destroy_bitmap(p2Paddle); al_destroy_timer(timer); al_destroy_display(display); al_destroy_event_queue(event_queue); return 0; }

Thank you for your time. And one more thing: would you advise me to continue working further with Allegro, or should I get another library set?

FallenShard

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All you need to do is: check in your logic code if paddle1 and bouncer's collision boxes are touching. You can assume they're always touching, and use the following statements to set collision to false.

 collision = true; if (paddle1_bottom < bouncer_top) { collision = false; } if (paddle1_top > bouncer_bottom) { collision = false; } if (paddle1_right < bouncer_left) { collision = false; } if (paddle1_left > bouncer_right) { collision = false; } 

Use whatever tools you feel complete the job. New programmers spend way too much time on what tools and languages they should be using, and almost no time on completing actual projects. If the games you're currently making surpass the ability of Allegro, then yes, move on! This is ultimately a choice you will have to make, as you will be the one coding with that library.

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