Basic Tutorials

Basic Game Tutorial #5 - Firing bullets

Introduction

Now that we can move a sprite around the screen, we will look into firing bullets. These tutorials will go into less detail about loading images and sounds because they have been covered in earlier tutorials and the theory is essentially the same.

Compile and run tutorial05. You can control the ship by using the arrow keys (not the ones on the numeric pad). Pressing the space key will fire a bullet, which will move to the right from the ship's current position.

An in-depth look

To deal with the player and the bullets, we update the Entity structure as follows:

typedef struct Entity
{
    int active;
    int x, y, thinkTime;
    SDL_Surface *sprite;
    void (*action)(void);
    void (*draw)(void);
} Entity;

The Control function has another value, fire which is used to let us know that we want to attempt to fire a bullet.

We will first look at entites.c. This file has 5 functions in it. We will look at these one at a time.
The clearEntities function loops through all of the available Entities and marks them as inactive using the C function memset. This ensures that at the start of the program, there are empty slots to fire bullets. In a larger game you would need to reset everything at the start of a level to make sure that when it starts there aren't enemies, bullets etc. appearing randomly on the screen.

getFreeEntity is a function that will be commonly used. It will loop through all the available Entities and return the index of the first one it finds that is marked as inactive. It will also initialise the Entity to ensure that all of its variables and functions are set to 0. If the function cannot find a free Entity, it will return -1 and you would deal with it as appropriate. In our code we will simply flag an error and carry on.

doEntities is used in the main game loop. This function loops through all the Entities and point its memory address to the self Entity global pointer, which we will go into more detail later on. If the Entity is marked as active, we call its action function.

drawEntities is exactly the same as doEntities, except that we call the draw function instead.

The drawStandardEntity function is similar to the drawPlayer functions from previous tutorials. Note that we are using the self pointer in this function. You will see how this is used later.

In input.c we add add the reading of the space key to the inputs.

We'll now look at player.c, specifically the doPlayer function. At the start of the function we decrease the player's thinkTime, and if it is less than or equal to 0, we set it to 0.

player.thinkTime--;

if (player.thinkTime <= 0)
{
    player.thinkTime = 0;
}

if (input.fire == 1)
{
    /* You can only fire when the thinkTime is 0 or less */

    if (player.thinkTime <= 0)
    {
        addBullet(player.x + player.sprite->w, player.y + (player.sprite->h / 2));

        player.thinkTime = MAX_RELOAD_TIME;
    }
}

bullet.c handles the creation and movement of the bullets. We will look at each of the functions in this file in turn:

void addBullet(int x, int y)
{
    int i = getFreeEntity();

    if (i == -1)
    {
        printf("Couldn't get a free slot for a bullet!\n");

        return;
    }

    entity[i].x = x;
    entity[i].y = y;
    entity[i].action = &moveStandardBullet;
    entity[i].draw = &drawStandardEntity;
    entity[i].sprite = getSprite(BULLET_SPRITE);

    /* Play a sound when the shot is fired */

    playSound(BULLET_SOUND);
}

static void moveStandardBullet()
{
    /* Move the bullet across the screen */

    self->x += BULLET_SPEED;

    /* Kill the bullet if it moves off the screen */

    if (self->x >= SCREEN_WIDTH)
    {
        self->active = 0;
    }
}

Conclusion

Like the previous tutorial, firing a basic bullet is quite simple. In the next tutorial, we will look into collision detection.

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Source Code