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System Specific Notes
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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 API's 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

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Events
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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..touch.xres)
event.tfinger.y        - the y coordinate of the touch (0..touch.yres)
event.tfinger.pressure - the pressure of the touch (0..touch.pressureres)

SDL_FINGERMOTION:
Sent when a finger (or stylus) is moved on the touch device.
Fields:
Same as FINGERDOWN but with additional:
event.tfginer.dx       - change in x coordinate during this motion event.
event.tfginer.dy       - change in y coordinate during this motion event.

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


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Functions
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SDL provides the ability to access the underlying Touch and Finger structures.
These structures should _never_ be modified.

The following functions are included from SDL_Touch.h

To get a SDL_Touch device call SDL_GetTouch(touchId). 
This returns an SDL_Touch*. 
IMPORTANT: If the touch has been removed, or there is no touch with the given ID, SDL_GetTouch will return null. Be sure to check for this!

An SDL_Touch has the following fields:
>xres,yres,pressures:
	The resolution at which x,y, and pressure values are reported. Currently these will always be equal to 2^15, but this may not always be the case. 

>pressure_max, pressure_min, x_max, x_min, y_max, y_min
	Which give, respectively, the maximum and minumum values that the touch digitizer can return for pressure, x coordiniate, and y coordinate AS REPORTED BY THE OPERATING SYSTEM.
On Mac/iPhone systems _max will always be 0, and _min will always be 1. 

>native_xres,native_yres,native_pressureres:
	The native resolution of the touch device AS REPORTED BY THE OPERATING SYSTEM.
On Mac/iPhone systems these will always be 1.

>num_fingers:
	The number of fingers currently down on the device.

>fingers:
	An array of pointers to the fingers which are on the device.


The most common reason to access a touch device is to normalize inputs. This would be accomplished by code like the following:

      SDL_Touch* inTouch = SDL_GetTouch(event.tfinger.touchId);
      if(inTouch == NULL) continue; //The touch has been removed

      float x = ((float)event.tfinger.x)/inTouch->xres;
      float y = ((float)event.tfinger.y)/inTouch->yres;



To get an SDL_Finger, call SDL_GetFinger(touch,fingerId), where touch is a pointer to an SDL_Touch device, and fingerId is the id of the requested finger.
This returns an SDL_Finger*, or null if the finger does not exist, or has been removed.
An 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 FINGERUP event is polled. 
As a result, be very careful to check for null return values.

An SDL_Finger has the following fields:
>x,y,pressure:
	The current coordinates of the touch.
>xdelta,ydelta: 
	The change in position resulting from the last finger motion.
>last_x, last_y, last_pressure:
	 The previous coordinates of the touch.

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Notes
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For a complete example see test/testgesture.c

Please direct questions/comments to:
   jim.tla+sdl_touch@gmail.com