Homebrew Tablet

I'm playing at making a tablet based on a Raspberry Pi Zero 2 W and a USB/HDMI touchscreen by Elecrow (Amazon link).

I want to boot to a usable app screen as quickly as possible, and it all has to fit into the Pi's 512MB RAM...so the software will need to be really lightweight.

At the minute, I'm intending to avoid using X or Wayland and write apps which use EGL directly. I've also been a little worried by the screen's product information saying it doesn't support multi-touch on Linux.

So I'm looking to handle the touchscreen input myself, which led me to investigating how to use libevdev in C/C++.

Resources I've encountered so far:

• The freedesktop.org documentation is helpful, but not great.

• The libevdev source code has a lot of useful comments.

• I've found the blackle examples incredibly useful.

• The Magic Touchscreen Repo has also helped a huge amount.

What I've done so far:

I have managed to make a program which scans for the first multi-touch device, then connects to it and dumps to the console any input events from it.

The Elecrow screen seems to send through all the Linux input messages that I'd need (ABS_MT_SLOT, ABS_MT_TRACKING_ID, ABS_MT_POSITION_X/Y). Handling the EV_SYN event might take a little thinking to get right, but at the moment it all feels possible.

Where next?

So my initial thoughts on design for the touchscreen system are to split it into three layers:

• a getEvent() routine which passes through just the events I'm interested in, handling EV_SYN correctly.

• a system to manage a set of touch points, combining the filtered evdev events into structured events for a new touch point, movement of a pre-existing one, and for when it stops existing.

• a state-machine system to consume these structured touchpoint events and convert them into gesture/click/pan-zoom events that can be consumed by the application code.

I think my next step is now to design a simple gesture-control interface to navigate a proof-of-concept tablet app.

Or maybe the next step is to check I can actually get EGL working properly on the Pi with GPU acceleration.

Either way - it's progress! :D