Wednesday, February 10, 2010

Computer assisted Chinese (part IV): Touchscreen 2

I want to use Skritter with a pen, not a mouse. Commercial products are too expensive for me, so I want to make something using a touchscreen.

While I plan to eventually use a cheap Nintendo touchscreen, I want to start with a touchscreen that I already have, one that used to be in my EeePC. This way, I don't have to deal with as many variables at once.

The interface board for this touchscreen usually speaks to a proprietary kernel module, and proprietary X driver. I would rather not use proprietary software, as proprietary software doesn't let me change things if I need to.

Once upon a time, X was configured with a file called /etc/X11/xorg.conf. This file was a description of all the input and output devices that X had to speak to. You could tell X about a device such as a touchscreen, by having an InputDevice stanza in /etc/X11/xorg.conf, which would cause X to load a driver for that input device. The touchscreen maker provides a proprietary driver for this purpose.

Those days are now gone. In Fedora (and probably in other recent Linux distributions), there's no longer an /etc/X11/xorg.conf file. Instead, it's all done with auto-detection.

The Linux kernel has a subsystem called evdev. It unifies handling of keyboards and pointing devices (and can even "mix" different input devices into one virtual device). The kernel contains drivers for many input devices, which use the services of evdev. One of those drivers is for the touchscreen I have. And for each input device, there'll be a device file in /dev/input.

To start off, I did an ls of /dev/input for files matching event*:
mjd@blackcat [/] ls /dev/input/event*
/dev/input/event0  /dev/input/event2  /dev/input/event4
/dev/input/event1  /dev/input/event3  /dev/input/event5
After I plugged in the touchscreen, I saw this:
mjd@blackcat [/] ls /dev/input/event*
/dev/input/event0  /dev/input/event2  /dev/input/event4  /dev/input/event6
/dev/input/event1  /dev/input/event3  /dev/input/event5
So it appears that /dev/input/event6 is the device file for the touchscreen. I ran evtest on this device: (evtest is in the evtest package)
[root@blackcat /]# evtest /dev/input/event6
Input driver version is 1.0.0
Input device ID: bus 0x3 vendor 0xeef product 0x1 version 0x100
Input device name: "eGalax Inc. USB TouchController"
Supported events:
  Event type 0 (Sync)
  Event type 1 (Key)
    Event code 330 (Touch)
  Event type 3 (Absolute)
    Event code 0 (X)
      Value    249
      Min        0
      Max     2047
    Event code 1 (Y)
      Value   1397
      Min        0
      Max     2047
Testing ... (interrupt to exit)
Event: time 1265760809.718325, type 1 (Key), code 330 (Touch), value 1
Event: time 1265760809.718331, type 3 (Absolute), code 0 (X), value 246
Event: time 1265760809.718332, type 3 (Absolute), code 1 (Y), value 1390
Event: time 1265760809.718335, -------------- Report Sync ------------
Event: time 1265760809.770319, type 3 (Absolute), code 1 (Y), value 1389
Event: time 1265760809.770322, -------------- Report Sync ------------
Event: time 1265760809.810320, type 3 (Absolute), code 1 (Y), value 1388
Event: time 1265760809.810325, -------------- Report Sync ------------
Event: time 1265760809.822320, type 3 (Absolute), code 0 (X), value 303
Event: time 1265760809.822322, type 3 (Absolute), code 1 (Y), value 1331
Event: time 1265760809.822326, -------------- Report Sync ------------
Event: time 1265760809.834320, type 3 (Absolute), code 0 (X), value 331
Event: time 1265760809.834322, type 3 (Absolute), code 1 (Y), value 1303
Event: time 1265760809.834326, -------------- Report Sync ------------
Event: time 1265760809.846321, type 3 (Absolute), code 0 (X), value 403
Event: time 1265760809.846323, type 3 (Absolute), code 1 (Y), value 1235
Event: time 1265760809.846326, -------------- Report Sync ------------
Event: time 1265760809.878320, type 1 (Key), code 330 (Touch), value 0
Event: time 1265760809.878326, -------------- Report Sync ------------
[root@blackcat mjd]# 
When I touch the touchscreen, I get data! And the data looks sensible, with values for each axis from 0-2047!  Now to get it working with X...

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