attempt to compensate for scheduling jitter

stepmania/src/RageSounds.cpp

@@ -8,6 +8,7 @@
 #include "TimingData.h"
 #include "NotesLoaderSM.h"
 #include "PrefsManager.h"
+#include "RageDisplay.h"
 
 RageSounds *SOUND = NULL;
 
@@ -308,6 +309,41 @@ RageSounds::~RageSounds()
 	delete g_Mutex;
 }
 
+static float GetFrameTimingAdjustment( float fDeltaTime )
+{
+	/*
+	 * We get one update per frame, and we're updated early, almost immediately after vsync,
+	 * near the beginning of the game loop.  However, it's very likely that we'll lose the
+	 * scheduler while waiting for vsync, and some other thread will be working.  Especially
+	 * with a low-resolution scheduler (Linux 2.4, Win9x), we may not get the scheduler back
+	 * immediately after the vsync; there may be up to a ~10ms delay.  This can cause jitter
+	 * in the rendered arrows.
+	 *
+	 * Compensate.  If vsync is enabled, and we're maintaining the refresh rate consistently,
+	 * we should have a very precise game loop interval.  If we have that, but we're off by
+	 * a small amount (less than the interval), adjust the time to line it up.  As long as we
+	 * adjust both the sound time and the timestamp, this won't adversely affect input timing.
+	 * If we're off by more than that, we probably had a frame skip, in which case we have
+	 * bigger skip problems, so don't adjust.
+	 */
+	static int iLastFPS = 0;
+	int iThisFPS = DISPLAY->GetFPS();
+
+	if( iThisFPS != DISPLAY->GetVideoModeParams().rate || iThisFPS != iLastFPS )
+	{
+		iLastFPS = iThisFPS;
+		return 0;
+	}
+
+	const float fExpectedDelay = 1.0f / iThisFPS;
+	const float fExtraDelay = fDeltaTime - fExpectedDelay;
+	if( fabsf(fExtraDelay) >= fExpectedDelay )
+		return 0;
+
+	/* Subtract the extra delay.  (We might have delayed too little, in which case we'll return
+	 * a positive number; this is normal.) */
+	return -fExtraDelay;
+}
 
 void RageSounds::Update( float fDeltaTime )
 {
@@ -316,6 +352,8 @@ void RageSounds::Update( float fDeltaTime )
 	if( !g_UpdatingTimer )
 		return;
 
+	const float fAdjust = GetFrameTimingAdjustment( fDeltaTime );
+
 	if( !g_Playing->m_Music->IsPlaying() )
 	{
 		/* There's no song playing.  Fake it. */
@@ -367,7 +405,7 @@ void RageSounds::Update( float fDeltaTime )
 	}
 
 	CHECKPOINT_M( ssprintf("%f", fSeconds) );
-	GAMESTATE->UpdateSongPosition( fSeconds, g_Playing->m_Timing, tm );
+	GAMESTATE->UpdateSongPosition( fSeconds+fAdjust, g_Playing->m_Timing, tm+fAdjust );
 }