Joysticks
Last Updated 11/15/07
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.//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.
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.
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 the 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.
First we check if the joystick that has moved is joystick 0. It's kind of pointless, since the 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.
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.