Basic Tutorials

Basic Game Tutorial #11 - Joystick movement

Introduction

This tutorial will demonstrate how to use the joystick to move an image around the screen.

Compile and run tutorial17. You can control the ship by using the joystick's joypad or analog stick. Pressing the fire button will fire a bullet.

An in-depth look

The structure for the joystick is an SDL_Joystick and holds all the information related to the joystick. It is stored in the Game struct:

typedef struct Game
{
	SDL_Joystick *joystick;
	SDL_Surface *screen;
} Game;

#define DEAD_ZONE 3200

Before using a joystick in SDL it must be opened. This is performed in the SDL_Init function in init.c

if (SDL_Init(SDL_INIT_VIDEO|SDL_INIT_AUDIO|SDL_INIT_JOYSTICK) < 0)

joystickCount = SDL_NumJoysticks();

Now that we have detected a joystick, we need to open it and store the reference to the joystick. This is done with the following.

game.joystick = SDL_JoystickOpen(0);

buttonCount = SDL_JoystickNumButtons(game.joystick);

printf("Joystick has %d buttons\n", buttonCount);

printf("Joystick has %d axes\n", SDL_JoystickNumAxes(game.joystick));

Are simply for information. In your game you might wish to raise an error if, say, the number of detected buttons is less than the number that your game requires. The only other addition to init.c is the closing of the joystick in the cleanup function.

/* Close the joystick */

if (game.joystick != NULL)
{
	SDL_JoystickClose(game.joystick);
}

In input.c we add in some extra code to detect the events produced from the joystick

case SDL_JOYBUTTONDOWN:
	switch (event.jbutton.button)
	{
		case 0:
			input.fire = 1;
		break;
	}
	break;

case SDL_JOYBUTTONUP:
	switch (event.jbutton.button)
	{
		case 0:
			input.fire = 0;
		break;
	}
	break;

case SDL_JOYAXISMOTION:
	/* Horizontal movement */

	if (event.jaxis.axis == 0)
	{
		if (event.jaxis.value < -DEAD_ZONE)
		{
			input.left = 1;
		}

		else if (event.jaxis.value > DEAD_ZONE)
		{
			input.right = 1;
		}

		else
		{
			input.left = 0;
			input.right = 0;
		}
	}

	/* Vertical movement */

	if (event.jaxis.axis == 1)
	{
		if (event.jaxis.value < -DEAD_ZONE)
		{
			input.up = 1;
		}

		else if (event.jaxis.value > DEAD_ZONE)
		{
			input.down = 1;
		}

		else
		{
			input.up = 0;
			input.down = 0;
		}
	}
	break;

if (event.jaxis.value < -DEAD_ZONE)
{
	input.left = 1;
}

Conclusion

Adding joystick support is quite simple, but as you can see, the joystick buttons may not be mapped in a desirable fashion. In the next tutorial, we will look at adding support for configuring controls.

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