Allegro Manual: Joystick routines

# Joystick routines

- calibrate_joystick — Calibrates the specified joystick.
- calibrate_joystick_name — Returns the next calibration text string.
- initialise_joystick — Deprecated version of install_joystick().
- install_joystick — Initialises the joystick.
- joy — Global array of joystick state information.
- load_joystick_data — Loads joystick calibration data.
- num_joysticks — Global variable saying how many joysticks there are.
- poll_joystick — Polls the joystick.
- remove_joystick — Removes the joystick handler.
- save_joystick_data — Saves joystick calibration data.

Unlike keyboard or mouse input, which are usually read through hardware interrupts by Allegro, joystick input functions have to be polled because there are no hardware interrupts for them on most platforms. This doesn't mean that you have to poll the joysticks on each line of code you want to read their values, but you should make sure to poll them at least once per frame in your game loop. Otherwise you face the possibility of reading stale incorrect data.

int install_joystick(int type);
Installs Allegro's joystick handler, and calibrates the centre position values. The type parameter should usually be JOY_TYPE_AUTODETECT, or see the platform specific documentation for a list of the available drivers. You must call this routine before using any other joystick functions, and you should make sure that all joysticks are in the middle position at the time. Example:

>           textout_centre_ex(screen, font,
>                             "Center the joystick and press a key",
>                             SCREEN_W/2, SCREEN_H/2, red_color, -1);
>           readkey();
>           if (install_joystick(JOY_TYPE_AUTODETECT) != 0)
>              abort_on_error("Error initialising joystick!");

Return value: Returns zero on success. As soon as you have installed the joystick module, you will be able to read the button state and digital (on/off toggle) direction information, which may be enough for some games. If you want to get full analogue input, though, you need to use the calibrate_joystick() functions to measure the exact range of the inputs: see below.

> See also: remove_joystick, num_joysticks, load_joystick_data, calibrate_joystick, calibrate_joystick_name, poll_joystick, Standard config variables, JOY_TYPE_*/DOS, JOY_TYPE_*/Windows, JOY_TYPE_*/Linux.

> Examples using this: exjoy.

void remove_joystick();
Removes the joystick handler. You don't normally need to bother calling this, because allegro_exit() will do it for you.

> See also: install_joystick, allegro_exit.

int poll_joystick();
The joystick handler is not interrupt driven, so you need to call this function every now and again to update the global position values. Example:

>        do {
>           /* Get joystick input */
>           poll_joystick();

>           /* Process input for the first joystick */
>           if (joy[0].button[0].b)
>              first_button_pressed();

>           if (joy[0].button[1].b)
>              second_button_pressed();
>           ...
>        } while(!done);

Return value: Returns zero on success or a negative number on failure (usually because no joystick driver was installed).

> See also: install_joystick, joy, num_joysticks.

> Examples using this: exjoy.

extern int num_joysticks;
Global variable containing the number of active joystick devices. The current drivers support a maximum of eight controllers.

> See also: install_joystick, joy.

> Examples using this: exjoy.

extern JOYSTICK_INFO joy[n];
Global array of joystick state information, which is updated by the poll_joystick() function. Only the first num_joysticks elements will contain meaningful information. The JOYSTICK_INFO structure is defined as:

>           typedef struct JOYSTICK_INFO
>           {
>              int flags;                       - status flags for this
>                                                 joystick
>              int num_sticks;                  - how many stick inputs?
>              int num_buttons;                 - how many buttons?
>              JOYSTICK_STICK_INFO stick[n];    - stick state information
>              JOYSTICK_BUTTON_INFO button[n];  - button state information
>           } JOYSTICK_INFO;

The button status is stored in the structure:

>           typedef struct JOYSTICK_BUTTON_INFO
>           {
>              int b;                           - boolean on/off flag
>              char *name;                      - description of this
>                                                 button
>           } JOYSTICK_BUTTON_INFO;

You may wish to display the button names as part of an input configuration screen to let the user choose what game function will be performed by each button, but in simpler situations you can safely assume that the first two elements in the button array will always be the main trigger controls.

Each joystick will provide one or more stick inputs, of varying types. These can be digital controls which snap to specific positions (eg. a gamepad controller, the coolie hat on a Flightstick Pro or Wingman Extreme, or a normal joystick which hasn't yet been calibrated), or they can be full analogue inputs with a smooth range of motion. Sticks may also have different numbers of axes, for example a normal directional control has two, but the Flightstick Pro throttle is only a single axis, and it is possible that the system could be extended in the future to support full 3d controllers. A stick input is described by the structure:

>           typedef struct JOYSTICK_STICK_INFO
>           {
>              int flags;                       - status flags for this
>                                                 input
>              int num_axis;                    - how many axes do we
>                                                 have? (note the misspelling)
>              JOYSTICK_AXIS_INFO axis[n];      - axis state information
>              char *name;                      - description of this
>                                                 input
>           } JOYSTICK_STICK_INFO;

A single joystick may provide several different stick inputs, but you can safely assume that the first element in the stick array will always be the main directional controller.

Information about each of the stick axis is stored in the substructure:

>           typedef struct JOYSTICK_AXIS_INFO
>           {
>              int pos;                         - analogue axis position
>              int d1, d2;                      - digital axis position
>              char *name;                      - description of this axis
>           } JOYSTICK_AXIS_INFO;

This provides both analogue input in the pos field (ranging from -128 to 128 or from 0 to 255, depending on the type of the control), and digital values in the d1 and d2 fields. For example, when describing the X-axis position, the pos field will hold the horizontal position of the joystick, d1 will be set if it is moved left, and d2 will be set if it is moved right. Allegro will fill in all these values regardless of whether it is using a digital or analogue joystick, emulating the pos field for digital inputs by snapping it to the min, middle, and maximum positions, and emulating the d1 and d2 values for an analogue stick by comparing the current position with the centre point.

The joystick flags field may contain any combination of the bit flags:

JOYFLAG_DIGITAL
This control is currently providing digital input.

JOYFLAG_ANALOGUE
This control is currently providing analogue input.

JOYFLAG_CALIB_DIGITAL
This control will be capable of providing digital input once it has been calibrated, but is not doing this at the moment.

JOYFLAG_CALIB_ANALOGUE
This control will be capable of providing analogue input once it has been calibrated, but is not doing this at the moment.

JOYFLAG_CALIBRATE
Indicates that this control needs to be calibrated. Many devices require multiple calibration steps, so you should call the calibrate_joystick() function from a loop until this flag is cleared.

JOYFLAG_SIGNED
Indicates that the analogue axis position is in signed format, ranging from -128 to 128. This is the case for all 2d directional controls.

JOYFLAG_UNSIGNED
Indicates that the analogue axis position is in unsigned format, ranging from 0 to 255. This is the case for all 1d throttle controls.

Note for people who spell funny: in case you don't like having to type "analogue", there are some #define aliases in allegro/joystick.h that will allow you to write "analog" instead.

> See also: install_joystick, poll_joystick, num_joysticks, calibrate_joystick, calibrate_joystick_name.

> Examples using this: exjoy.

const char *calibrate_joystick_name(int n);
Pass the number of the joystick you want to calibrate as the parameter.

Return value: Returns a text description for the next type of calibration that will be done on the specified joystick, or NULL if no more calibration is required.

> See also: install_joystick, calibrate_joystick, joy, num_joysticks.

> Examples using this: exjoy.

int calibrate_joystick(int n);
Most joysticks need to be calibrated before they can provide full analogue input. This function performs the next operation in the calibration series for the specified stick, assuming that the joystick has been positioned in the manner described by a previous call to calibrate_joystick_name(), returning zero on success. For example, a simple routine to fully calibrate all the joysticks might look like:

>           int i;

>           for (i=0; i<;num_joysticks; i++) {
>              while (joy[i].flags & JOYFLAG_CALIBRATE) {
>                 char *msg = calibrate_joystick_name(i);
>                 textprintf_ex(..., "%s, and press a key\n", msg);
>                 readkey();
>                 if (calibrate_joystick(i) != 0) {
>                    textprintf_ex(..., "oops!\n");
>                    readkey();
>                    exit(1);
>                 }
>              }
>           }

Return value: Returns zero on success, non-zero if the calibration could not be performed successfully.

> See also: install_joystick, calibrate_joystick_name, joy, num_joysticks.

> Examples using this: exjoy.

int save_joystick_data(const char *filename);
After all the headache of calibrating the joystick, you may not want to make your poor users repeat the process every time they run your program. Call this function to save the joystick calibration data into the specified configuration file, from which it can later be read by load_joystick_data(). Pass a NULL filename to write the data to the currently selected configuration file.

Return value: Returns zero on success, non-zero if the data could not be saved.

> See also: load_joystick_data, set_config_file.

int load_joystick_data(const char *filename);
Restores calibration data previously stored by save_joystick_data() or the setup utility. This sets up all aspects of the joystick code: you don't even need to call install_joystick() if you are using this function. Pass a NULL filename to read the data from the currently selected configuration file.

Return value: Returns zero on success: if it fails the joystick state is undefined and you must reinitialise it from scratch.

> See also: install_joystick, save_joystick_data, set_config_file.

int initialise_joystick();
Deprecated. Use install_joystick() instead.

> See also: install_joystick.

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