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SDL/docs/README-touch.md
Ryan C. Gordon 5b7b3da701
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Touch

System Specific Notes

Linux: The linux touch system is currently based off event streams, and proc/bus/devices. The active user must be given permissions to read /dev/input/TOUCHDEVICE, where TOUCHDEVICE is the event stream for your device. Currently only Wacom tablets are supported. If you have an unsupported tablet contact me at jim.tla+sdl_touch@gmail.com and I will help you get support for it.

Mac: The Mac and iPhone APIs are pretty. If your touch device supports them then you'll be fine. If it doesn't, then there isn't much we can do.

iPhone: Works out of box.

Windows: Unfortunately there is no windows support as of yet. Support for Windows 7 is planned, but we currently have no way to test. If you have a Windows 7 WM_TOUCH supported device, and are willing to help test please contact me at jim.tla+sdl_touch@gmail.com

=========================================================================== Events

SDL_FINGERDOWN: Sent when a finger (or stylus) is placed on a touch device. Fields:

  • event.tfinger.touchId - the Id of the touch device.
  • event.tfinger.fingerId - the Id of the finger which just went down.
  • event.tfinger.x - the x coordinate of the touch (0..1)
  • event.tfinger.y - the y coordinate of the touch (0..1)
  • event.tfinger.pressure - the pressure of the touch (0..1)

SDL_FINGERMOTION: Sent when a finger (or stylus) is moved on the touch device. Fields: Same as SDL_FINGERDOWN but with additional:

  • event.tfinger.dx - change in x coordinate during this motion event.
  • event.tfinger.dy - change in y coordinate during this motion event.

SDL_FINGERUP: Sent when a finger (or stylus) is lifted from the touch device. Fields: Same as SDL_FINGERDOWN.

=========================================================================== Functions

SDL provides the ability to access the underlying SDL_Finger structures. These structures should never be modified.

The following functions are included from SDL_touch.h

To get a SDL_TouchID call SDL_GetTouchDevice(int index). This returns a SDL_TouchID. IMPORTANT: If the touch has been removed, or there is no touch with the given index, SDL_GetTouchDevice() will return 0. Be sure to check for this!

The number of touch devices can be queried with SDL_GetNumTouchDevices().

A SDL_TouchID may be used to get pointers to SDL_Finger.

SDL_GetNumTouchFingers(touchID) may be used to get the number of fingers currently down on the device.

The most common reason to access SDL_Finger is to query the fingers outside the event. In most cases accessing the fingers is using the event. This would be accomplished by code like the following:

  float x = event.tfinger.x;
  float y = event.tfinger.y;

To get a SDL_Finger, call SDL_GetTouchFinger(SDL_TouchID touchID, int index), where touchID is a SDL_TouchID, and index is the requested finger. This returns a SDL_Finger *, or NULL if the finger does not exist, or has been removed. A SDL_Finger is guaranteed to be persistent for the duration of a touch, but it will be de-allocated as soon as the finger is removed. This occurs when the SDL_FINGERUP event is added to the event queue, and thus before the SDL_FINGERUP event is polled. As a result, be very careful to check for NULL return values.

A SDL_Finger has the following fields:

  • x, y: The current coordinates of the touch.
  • pressure: The pressure of the touch.

=========================================================================== Notes

For a complete example see test/testgesture.c

Please direct questions/comments to: jim.tla+sdl_touch@gmail.com (original author, API was changed since)