tablet: support tool-specific pressure offsets

If a tool wears out, it may have a pre-loaded pressure offset. In that case,
even when the tool is not physically in contact with the tablet surface it
will send pressure events.

Use automatic pressure offset detection, similar to what the X.Org wacom
driver does. On proximity-in, check the pressure and if the distance is above
50% of the range and the pressure is nonzero but below 20% of the range, use
that value as pressure offset.

Signed-off-by: Peter Hutterer <peter.hutterer@who-t.net>
Reviewed-by: Ping Cheng <pingc@wacom.com>

3 files changed, 106 insertions, 3 deletions

diff --git a/src/evdev-tablet.c b/src/evdev-tablet.c
index 7f6eecb4..2f4d6804 100644
--- a/src/evdev-tablet.c
+++ b/src/evdev-tablet.c
@@ -21,6 +21,7 @@
  * CONNECTION WITH THE USE OR PERFORMANCE OF THIS SOFTWARE.
  */
 #include "config.h"
+#include "libinput-version.h"
 #include "evdev-tablet.h"
 
 #include <assert.h>
@@ -202,7 +203,7 @@ tablet_update_tool(struct tablet_dispatch *tablet,
 }
 
 static inline double
-normalize_pressure_dist_slider(const struct input_absinfo *absinfo)
+normalize_dist_slider(const struct input_absinfo *absinfo)
 {
 	double range = absinfo->maximum - absinfo->minimum;
 	double value = (absinfo->value - absinfo->minimum) / range;
@@ -211,6 +212,18 @@ normalize_pressure_dist_slider(const struct input_absinfo *absinfo)
 }
 
 static inline double
+normalize_pressure(const struct input_absinfo *absinfo,
+		   struct libinput_tablet_tool *tool)
+{
+	double range = absinfo->maximum - absinfo->minimum;
+	int offset = tool->has_pressure_offset ?
+			tool->pressure_offset : 0;
+	double value = (absinfo->value - offset - absinfo->minimum) / range;
+
+	return value;
+}
+
+static inline double
 normalize_tilt(const struct input_absinfo *absinfo)
 {
 	double range = absinfo->maximum - absinfo->minimum;
@@ -398,10 +411,12 @@ tablet_check_notify_axes(struct tablet_dispatch *tablet,
 						axis_to_evcode(a));
 
 		switch (a) {
-		case LIBINPUT_TABLET_TOOL_AXIS_DISTANCE:
 		case LIBINPUT_TABLET_TOOL_AXIS_PRESSURE:
+			tablet->axes[a] = normalize_pressure(absinfo, tool);
+			break;
+		case LIBINPUT_TABLET_TOOL_AXIS_DISTANCE:
 		case LIBINPUT_TABLET_TOOL_AXIS_SLIDER:
-			tablet->axes[a] = normalize_pressure_dist_slider(absinfo);
+			tablet->axes[a] = normalize_dist_slider(absinfo);
 			break;
 		case LIBINPUT_TABLET_TOOL_AXIS_TILT_X:
 		case LIBINPUT_TABLET_TOOL_AXIS_TILT_Y:
@@ -809,6 +824,8 @@ tablet_get_tool(struct tablet_dispatch *tablet,
 			.refcount = 1,
 		};
 
+		tool->pressure_offset = 0;
+		tool->has_pressure_offset = false;
 		tool_set_bits(tablet, tool);
 
 		list_insert(tool_list, &tool->link);
@@ -922,6 +939,67 @@ sanitize_tablet_axes(struct tablet_dispatch *tablet)
 		set_bit(tablet->changed_axes, LIBINPUT_TABLET_TOOL_AXIS_ROTATION_Z);
 }
 
+static inline int
+axis_range_percentage(const struct input_absinfo *a, int percent)
+{
+	return (a->maximum - a->minimum) * percent/100 + a->minimum;
+}
+
+static void
+detect_pressure_offset(struct tablet_dispatch *tablet,
+		       struct evdev_device *device,
+		       struct libinput_tablet_tool *tool)
+{
+	const struct input_absinfo *pressure, *distance;
+	int offset;
+
+	if (!bit_is_set(tablet->changed_axes,
+			LIBINPUT_TABLET_TOOL_AXIS_PRESSURE))
+		return;
+
+	pressure = libevdev_get_abs_info(device->evdev, ABS_PRESSURE);
+	distance = libevdev_get_abs_info(device->evdev, ABS_DISTANCE);
+
+	if (!pressure || !distance)
+		return;
+
+	offset = pressure->value - pressure->minimum;
+
+	if (tool->has_pressure_offset) {
+		if (offset < tool->pressure_offset)
+			tool->pressure_offset = offset;
+		return;
+	}
+
+	/* we only set a pressure offset on proximity in */
+	if (!tablet_has_status(tablet, TABLET_TOOL_ENTERING_PROXIMITY))
+		return;
+
+	/* If we're closer than 50% of the distance axis, skip pressure
+	 * offset detection, too likely to be wrong */
+	if (distance->value < axis_range_percentage(distance, 50))
+		return;
+
+	if (offset > axis_range_percentage(pressure, 20)) {
+		log_error(device->base.seat->libinput,
+			 "Ignoring pressure offset greater than 20%% detected on tool %s (serial %#x). "
+			 "See http://wayland.freedesktop.org/libinput/doc/%s/tablet-support.html\n",
+			 tablet_tool_type_to_string(tool->type),
+			 tool->serial,
+			 LIBINPUT_VERSION);
+		return;
+	}
+
+	log_info(device->base.seat->libinput,
+		 "Pressure offset detected on tool %s (serial %#x).  "
+		 "See http://wayland.freedesktop.org/libinput/doc/%s/tablet-support.html\n",
+		 tablet_tool_type_to_string(tool->type),
+		 tool->serial,
+		 LIBINPUT_VERSION);
+	tool->pressure_offset = offset;
+	tool->has_pressure_offset = true;
+}
+
 static void
 tablet_flush(struct tablet_dispatch *tablet,
 	     struct evdev_device *device,
@@ -946,6 +1024,7 @@ tablet_flush(struct tablet_dispatch *tablet,
 			tablet_set_status(tablet, TABLET_TOOL_LEAVING_CONTACT);
 	} else if (tablet_has_status(tablet, TABLET_AXES_UPDATED) ||
 		   tablet_has_status(tablet, TABLET_TOOL_ENTERING_PROXIMITY)) {
+		detect_pressure_offset(tablet, device, tool);
 		sanitize_tablet_axes(tablet);
 		tablet_check_notify_axes(tablet, device, time, tool);
 
diff --git a/src/evdev-tablet.h b/src/evdev-tablet.h
index 162b5366..4dcbccc6 100644
--- a/src/evdev-tablet.h
+++ b/src/evdev-tablet.h
@@ -178,4 +178,25 @@ tablet_tool_to_evcode(enum libinput_tablet_tool_type type)
 
 	return code;
 }
+
+static inline const char *
+tablet_tool_type_to_string(enum libinput_tablet_tool_type type)
+{
+	const char *str;
+
+	switch (type) {
+	case LIBINPUT_TABLET_TOOL_TYPE_PEN:	  str = "pen";		break;
+	case LIBINPUT_TABLET_TOOL_TYPE_ERASER:	  str = "eraser";	break;
+	case LIBINPUT_TABLET_TOOL_TYPE_BRUSH:	  str = "brush";	break;
+	case LIBINPUT_TABLET_TOOL_TYPE_PENCIL:	  str = "pencil";	break;
+	case LIBINPUT_TABLET_TOOL_TYPE_AIRBRUSH:  str = "airbrush";	break;
+	case LIBINPUT_TABLET_TOOL_TYPE_MOUSE:	  str = "mouse";	break;
+	case LIBINPUT_TABLET_TOOL_TYPE_LENS:	  str = "lens";		break;
+	default:
+		abort();
+	}
+
+	return str;
+}
+
 #endif
diff --git a/src/libinput-private.h b/src/libinput-private.h
index 38a14b8f..f5b26483 100644
--- a/src/libinput-private.h
+++ b/src/libinput-private.h
@@ -259,6 +259,9 @@ struct libinput_tablet_tool {
 	unsigned char buttons[NCHARS(KEY_MAX) + 1];
 	int refcount;
 	void *user_data;
+
+	int pressure_offset;
+	bool has_pressure_offset;
 };
 
 struct libinput_event {