Lazy Foo' Productions


Joysticks

Last Updated 12/28/09
Here we'll learn to use get input from a joystick/gamepad. This program is basically the motion tutorial, only this time a joystick will move around the dot instead of the keyboard.

A Game pads and joysticks tutorial with SDL 2 is now available.
//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 offsets of the dot int x, y; //The velocity of the dot int xVel, yVel; public: //Initializes Dot(); //Handles joystick void handle_input(); //Moves the dot void move(); //Shows the dot 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 an 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 in this program since the only initialized joystick is joystick 0, but in real games you'll need to check multiple joysticks.

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 here.

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