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: an Elecrow 5" 800×480 panel, 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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