Calibrating iptsd

The palm rejection of iptsd relies on measuring the size and aspect ratio of every contact and then comparing these values to a reference to determine whether the contact is valid or not. The default values will try to cover the finger size of most people, but it is possible (and recommended) to tune these values to better match your fingers. This by extension will also improve palm rejection.

The values that are interesting for calibration are:

• SizeMin (Default: 0.2 cm)
• SizeMax (Default: 2.0 cm)
• AspectMin (Default: 1.0)
• AspectMax (Default: 2.5)

To calibrate iptsd, you need to measure the size of your own fingers and then input those values into the configuration file of the daemon. But before you go away and grab a ruler (or just go away), wait! To make this easier, iptsd includes a tool that will do these measurements for you: iptsd-calibrate.

The first thing you need to do is stop iptsd.

sudo systemctl stop 'iptsd@*.service'

Once that is done you can run the calibration tool.

sudo iptsd-calibrate $(sudo iptsd-find-hidraw)

You will see something like this:

[17:46:55.678] [info] Connected to device 045E:0021
[17:46:55.678] [info] Samples: 0
[17:46:55.678] [info] Size:    0.000 (Min: 0.000; Max: 0.000)
[17:46:55.678] [info] Aspect:  0.000 (Min: 0.000; Max: 0.000)

The values will update as soon as you touch the display and iptsd registers a contact. You can now start the actual calibration process. Touch the screen with each of your fingers, one time pressing hard and one time pressing lightly. Then put all your fingers on the display at the same time. You should also do some common gestures like pinch-to-zoom or swiping across the display with three or four fingers, since gestures tend to deform the contact. If you only press your fingers straight on the display, iptsd will detect touches fine but will fail to detect gestures.

There is no need for extensively long contacts. Keeping the finger on the display for a long time won't improve calibration accuracy, since you only need a minimal and a maximal value. But it has the potential to introduce noise into the calibration process which will distort the Min/Max values. 3000 to 4000 samples should be plenty.

NOTE: During the calibration, palm rejection is not active! So dont put your palm on the display. If you can, detach the display from the keyboard.

When you are done with the calibration, you should see something like this in the terminal:

[17:46:55.678] [info] Connected to device 045E:0021
[17:46:57.186] [info] Samples: 3629
[17:46:57.186] [info] Size:    0.636 (Min: 0.425; Max: 0.859)
[17:46:57.187] [info] Aspect:  1.298 (Min: 1.021; Max: 2.323)

In this case the new values would be:

• SizeMin: 0.425 cm
• SizeMax: 0.859 cm
• AspectMin: 1.021
• AspectMax: 2.323

Create a file /etc/iptsd.d/90-calibration.conf with a text editor of your choice, and enter the above values like this:

[Contacts]
SizeMin = 0.425
SizeMax = 0.859
AspectMin = 1.021
AspectMax = 2.323

After you did that, you can restart iptsd and try out your new calibration.

sudo systemctl start $(sudo iptsd-find-service)

If you have issues with iptsd not detecting a particular gesture or contact, you should repeat this process and only do the gesture that iptsd has problems with. Then compare the reported values with the ones from your earlier run and adjust them accordingly.