feat(TimeSmoother): Smoother note falling on vsync mode

src/bms/player/beatoraja/skin/Skin.java

@@ -98,6 +98,8 @@ public class Skin {
 	public long pcntPrepare;
 	public long pcntDraw;
 
+	private VsyncSmoother vsyncSmoother;
+
 	public Skin(SkinHeader header) {
 		this.header = header;
 		Resolution org = header.getSourceResolution();
@@ -212,6 +214,12 @@ public void prepare(MainState state) {
 		objectarray = objects.toArray(SkinObject.class);
 		option.clear();
 
+        if (state.main.getConfig().isVsync()) {
+			vsyncSmoother = new VsyncSmoother();
+        } else {
+            vsyncSmoother = null;
+        }
+
 		for(SkinObject obj : objects) {
 			obj.load();
 		}
@@ -261,15 +269,17 @@ public void drawAllObjects(SpriteBatch sprite, MainState state) {
 		}
 
 		final long microtime = state.timer.getNowMicroTime();
+		final long smoothedNowTime = (vsyncSmoother != null)
+				? vsyncSmoother.smoothTime(state.timer.getNowTime())
+				: state.timer.getNowTime();
 
 		if (MainController.debug) {
 			if (nextpreparetime <= microtime) {
 				tempmap.forEach((c,l) -> Arrays.fill(l, 1, 6, 0L));
-				final long time = state.timer.getNowTime();
 				var startPrepare = System.nanoTime();
 				for (SkinObject obj : objectarray) {
 					var objPrepare = System.nanoTime();
-					obj.prepare(time, state);
+					obj.prepare(smoothedNowTime, state);
 					tempmap.get(obj.getClass())[1] += (System.nanoTime() - objPrepare);
 				}
 				pcntPrepare = (System.nanoTime() - startPrepare) / 1000;
@@ -298,9 +308,8 @@ public void drawAllObjects(SpriteBatch sprite, MainState state) {
 
 		} else {
 			if (nextpreparetime <= microtime) {
-				final long time = state.timer.getNowTime();
 				for (SkinObject obj : objectarray) {
-					obj.prepare(time, state);
+					obj.prepare(smoothedNowTime, state);
 				}
 
 				nextpreparetime += ((microtime - nextpreparetime) / prepareduration + 1) * prepareduration;

src/bms/player/beatoraja/skin/VsyncSmoother.java

@@ -0,0 +1,77 @@
+// Copyright (C) 2025 Park Hyunwoo
+//
+// This file is part of beatoraja.
+//
+// beatoraja is free software: you can redistribute it and/or modify
+// it under the terms of the GNU General Public License as published by
+// the Free Software Foundation, either version 3 of the License, or
+// (at your option) any later version.
+//
+// beatoraja is distributed in the hope that it will be useful,
+// but WITHOUT ANY WARRANTY; without even the implied warranty of
+// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+// GNU General Public License for more details.
+//
+// You should have received a copy of the GNU General Public License
+// along with beatoraja.  If not, see <https://www.gnu.org/licenses/>.
+
+package bms.player.beatoraja.skin;
+
+import com.badlogic.gdx.Gdx;
+
+public class VsyncSmoother {
+    private float targetTime = 0;  // Output value
+
+    public void reset() {
+        targetTime = 0;
+    }
+
+    /**
+     * Smooths out [newTime] to more evenly-spaced values.
+     *
+     * @param newTime Raw input time in milliseconds
+     * @return Smoothed out time.
+     */
+    public long smoothTime(long newTime) {
+        float timeInterval = 1000.0f / Gdx.graphics.getDisplayMode().refreshRate;
+
+        // Detect frame skip and advance targetTime
+        // Note: this might be due to display HZ change, but this will be averaged out in `averageDelta` as
+        // the time passes by, and `targetTime` will follow up.
+
+        // newTime = [true display time] - α
+        // we expect α to be in range of [0, averageDelta)
+        // where targetTime tries to estimate [true display time] - α. (There is no way to expect α as I know of)
+        //
+        // let targetTime = [true display time] - β
+        // abs(α - β) < averageDelta (must) -> if (a - b) > averageDelta → miss
+
+        // targetTime ~= [previous display time] - β
+        // newTime - targetTime = ([true display time] - α) - ([previous display time] - β)
+        //                      = averageDelta + (β - α) <= averageDelta + averageDelta
+        // if this inequality misses, we assume that the frame skip occurred
+        if (newTime - targetTime >= timeInterval * 2.02f) {
+            // Allow additional 0.02f to prevent false positive
+            long frameSkips = (long) (Math.floor((newTime - targetTime) / timeInterval) - 1);
+            targetTime += timeInterval * frameSkips;
+        }
+
+        // Some beatoraja code just spams a bunch of `time=0` before starting.
+        if (timeInterval == 0) {
+            return newTime;
+        }
+
+        float timeDiff = targetTime - newTime - timeInterval;
+        float targetTimeDiff = timeInterval;
+        if (timeDiff < -timeInterval) {
+            targetTimeDiff += Math.min(timeInterval, (-timeDiff - timeInterval) / 2 + 0.05f * timeInterval);
+        } else if (timeDiff < timeInterval) {
+            targetTimeDiff -= timeDiff * 0.05f;
+            // Do nothing
+        } else {
+            targetTimeDiff -= Math.min(timeInterval, (timeDiff - timeInterval) / 2 + 0.05f * timeInterval);
+        }
+        targetTime += targetTimeDiff;
+        return (long) Math.round(targetTime);
+    }
+}