Touchscreen patch

Optional. Not part of the instrument. Apply it only if you have the symptom below.

Who needs this

Your touchscreen reports touches at the wrong place — usually compressed into one corner or scaled toward one edge — while the same screen displays correctly.

That happens when the panel's touch controller reports coordinates in its own native resolution while Zynthian's UI is running at a different configured display size. Zynthian passes the raw touch value straight through, so a panel reporting 0-4095 on a 800×480 UI puts every touch in the top-left eighth of the screen.

This was the case on the rig this guide was written from: first an Elecrow 5" 800×480 panel, and now a 10.1" 1024×600 panel with the same USB touch controller. Both use HDMI for video and USB for touch.

If your touch already lands where you press, skip this page. The patch edits a Zynthian core file, and there is no reason to carry a change you do not need.

The patch

It scales the raw touch value onto Zynthian's configured display size, guarding against a zero maximum so an unreported range cannot divide by zero.

File: /zynthian/zynthian-ui/zyngui/multitouch.py, in MultiTouch's evdev event loop.

@@ class MultiTouch(object):
                  elif evdev_event.code == ecodes.ABS_MT_POSITION_X:
 -                    if self._invert_x:
 -                        self._current_touch.x_root = self.max_x - evdev_event.value
 -                    else:
 -                        self._current_touch.x_root = evdev_event.value
 +                    raw = self.max_x - evdev_event.value if self._invert_x else evdev_event.value
 +                    self._current_touch.x_root = int(raw * zynthian_gui_config.display_width / self.max_x) if self.max_x else raw
                      if self._current_touch not in self.events:
                          self.events.append(self._current_touch)
                  elif evdev_event.code == ecodes.ABS_MT_POSITION_Y:
 -                    if self._invert_y:
 -                        self._current_touch.y_root = self.max_y - evdev_event.value
 -                    else:
 -                        self._current_touch.y_root = evdev_event.value
 +                    raw = self.max_y - evdev_event.value if self._invert_y else evdev_event.value
 +                    self._current_touch.y_root = int(raw * zynthian_gui_config.display_height / self.max_y) if self.max_y else raw
                      if self._current_touch not in self.events:
                          self.events.append(self._current_touch)

Both hunks keep the existing _invert_x / _invert_y handling and only add the scaling on top of it.

Applying it

Back the file up first — this is a Zynthian core file, not one of ours.

ssh root@<pi>
cp /zynthian/zynthian-ui/zyngui/multitouch.py \
   /zynthian/zynthian-ui/zyngui/multitouch.py.bak
# edit the two handlers as shown above
systemctl restart zynthian

Verify: press each corner of the screen and confirm the UI responds at that corner.

To undo it:

cp /zynthian/zynthian-ui/zyngui/multitouch.py.bak \
   /zynthian/zynthian-ui/zyngui/multitouch.py
systemctl restart zynthian

Two warnings

A Zynthian update overwrites it. multitouch.py is tracked in the zynthian-ui checkout, so any update replaces your edit. Keep the .bak, and expect to re-apply after upgrading.

Do not run git checkout or git reset in /zynthian/zynthian-ui to undo this. That directory also holds this project's three driver files as untracked drop-ins; a hard reset removes the instrument along with the patch. Restore from the .bak copy instead.

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