config: Add option to set the prediction target

etc/iptsd.conf

@@ -135,6 +135,13 @@
 ##
 # TipDistance = 0
 
+##
+## Reduces the latency of the stylus by using input prediction. This option is the amount
+## of milliseconds that the predictor will look into the future. Larger values will be more
+## unstable. A good starting point is 50 milliseconds.
+##
+# PredictionTarget = 0
+
 [Cone]
 ##
 ## The wideness of the cone in degrees.

src/apps/daemon/stylus.hpp

@@ -5,6 +5,7 @@
 
 #include "uinput-device.hpp"
 
+#include <common/chrono.hpp>
 #include <common/types.hpp>
 #include <core/generic/config.hpp>
 #include <core/generic/device.hpp>
@@ -43,8 +44,12 @@ class StylusDevice {
 	// Kalman-based input predictor to reduce latency.
 	prediction::SinglePointerPredictor m_predictor {};
 
+	// How many milliseconds the predictor will look ahead.
+	milliseconds<usize> m_prediction_target;
+
 public:
 	StylusDevice(const core::Config &config, const core::DeviceInfo &info)
+		: m_prediction_target {config.stylus_prediction_target}
 	{
 		m_uinput->set_name("IPTS Stylus");
 		m_uinput->set_vendor(info.vendor);
@@ -86,7 +91,7 @@ class StylusDevice {
 	void update(const ipts::StylusData &data)
 	{
 		// If the stylus moves to proximity, reset the predictor
-		if (!m_active && data.proximity)
+		if (!m_last.contact && data.contact)
 			m_predictor.reset();
 
 		m_active = data.proximity;
@@ -100,15 +105,17 @@ class StylusDevice {
 			f64 cy = data.y;
 			f64 cp = data.pressure;
 
-			m_predictor.update(Vector2<f64> {cx, cy}, cp);
+			// Run prediction if the user enabled it.
+			if (m_prediction_target.count() > 0 && data.contact) {
+				m_predictor.update(Vector2<f64> {cx, cy}, cp);
 
-			// If the predictor was able to predict something, update our values.
-			if (m_predictor.predict(100ms)) {
-				const Vector2<f64> pos = m_predictor.position();
+				if (m_predictor.predict(m_prediction_target)) {
+					const Vector2<f64> pos = m_predictor.position();
 
-				cx = pos.x();
-				cy = pos.y();
-				cp = m_predictor.pressure();
+					cx = pos.x();
+					cy = pos.y();
+					cp = m_predictor.pressure();
+				}
 			}
 
 			const Vector2<i32> tilt = calculate_tilt(data.altitude, data.azimuth);

src/core/generic/config.hpp

@@ -47,6 +47,7 @@ class Config {
 	// [Stylus]
 	bool stylus_disable = false;
 	f64 stylus_tip_distance = 0;
+	usize stylus_prediction_target = 0;
 
 	// [Cone]
 	f64 cone_angle = 30;

src/core/linux/config-loader.hpp

@@ -163,6 +163,7 @@ class ConfigLoader {
 
 		this->get(ini, "Stylus", "Disable", m_config.stylus_disable);
 		this->get(ini, "Stylus", "TipDistance", m_config.stylus_tip_distance);
+		this->get(ini, "Stylus", "PredictionTarget", m_config.stylus_prediction_target);
 
 		this->get(ini, "Cone", "Angle", m_config.cone_angle);
 		this->get(ini, "Cone", "Distance", m_config.cone_distance);