Joysticks

Last Updated 2/25/06

Handling joysticks events is slightly different from handling other input. Here we're going to redo to motion tutorial to handle joysticks instead of key events.

//The joystick that will be used SDL_Joystick *stick = NULL;

Joysticks have their own data type in SDL which is SDL_Joystick.

In this program we declare our joystick as a global variable.

//The dot class Dot { private: //The X and Y offsets of the dot int x, y; //The velocity of the dot int xVel, yVel; public: //Initializes the variables Dot(); //Takes key presses and adjusts the dot's velocity void handle_input(); //Shows the dot on the screen void show(); };

As you can see, the Dot class has pretty much stayed the same. The only thing that has changed is how we handle the input.

bool init() { //Initialize all SDL subsystems if( SDL_Init( SDL_INIT_EVERYTHING ) == -1 ) { return false; } //Set up the screen screen = SDL_SetVideoMode( SCREEN_WIDTH, SCREEN_HEIGHT, SCREEN_BPP, SDL_SWSURFACE ); //If there was in error in setting up the screen if( screen == NULL ) { return false; } //Check if there's any joysticks if( SDL_NumJoysticks() < 1 ) { return false; } //Open the joystick stick = SDL_JoystickOpen( 0 ); //If there's a problem opening the joystick if( stick == NULL ) { return false; } //Set the window caption SDL_WM_SetCaption( "Move the Dot", NULL ); //If everything initialized fine return true; }

A key difference between using keys for input and using joysticks is that joysticks have to be initialized.

In the initialization function we use SDL_NumJoysticks() to check how many joysticks are plugged in. If at least 1 joystick is plugged in, we open the first joystick available using SDL_JoystickOpen(). The first available joystick is joystick 0 since in programming we always start counting at 0. When there's problem in opening the joystick, SDL_JoystickOpen() returns NULL.

void clean_up() { //Free the surface SDL_FreeSurface( dot ); //Close the joystick SDL_JoystickClose( stick ); //Quit SDL SDL_Quit(); }

In the clean up function, we have to call SDL_JoystickClose() to close the joystick that was opened.

void Dot::handle_input() { //If a axis was changed if( event.type == SDL_JOYAXISMOTION ) { //If joystick 0 has moved if( event.jaxis.which == 0 ) { //If the X axis changed if( event.jaxis.axis == 0 ) { //If the X axis is neutral if( ( event.jaxis.value > -8000 ) && ( event.jaxis.value < 8000 ) ) { xVel = 0; } //If not else { //Adjust the velocity if( event.jaxis.value < 0 ) { xVel = -DOT_WIDTH / 2; } else { xVel = DOT_WIDTH / 2; } } }

When a joystick moves, a SDL_JOYAXISMOTION occurs.

First we check if the joystick that has moved is joystick 0. It's kind of pointless, since we only initialized joystick is joystick 0, but its a good habit to check no matter what.

Then we check which axis it has moved on. On most modern joysticks, the X axis is 0, and the Y axis is 1.

After that we check if the joystick X value is between -8000, and 8000. If it is, it's neutral and the dot stays still.

You may be thinking "how the hell is such a large range considered neutral?". The thing is a joystick's axis have a range of -32768 to 32767. You could have the joystick at 0 and if you sneezed on it, it would be at like 200.

If the joystick is not in the neutral range, we set the X velocity accordingly.

//If the Y axis changed else if( event.jaxis.axis == 1 ) { //If the Y axis is neutral if( ( event.jaxis.value > -8000 ) && ( event.jaxis.value < 8000 ) ) { yVel = 0; } //If not else { //Adjust the velocity if( event.jaxis.value < 0 ) { yVel = -DOT_HEIGHT / 2; } else { yVel = DOT_HEIGHT / 2; } } } } } }

Then pretty much the same thing is done to the Y axis.

Handling SDL_JoyAxisEvent is the hardest event to handle when dealing with joysticks. Handling other events like SDL_JoyBallEvent, SDL_JoyHatEvent, and SDL_JoyButtonEvent should be easy to figure out with a quick look at the SDL API documentation.

Download the media and source code for this tutorial .