From 1b49a96eed927e80e019b580c5a94e13ae3e0640 Mon Sep 17 00:00:00 2001
From: James R <justsomejames2@gmail.com>
Date: Wed, 12 Jan 2022 03:36:02 -0800
Subject: [PATCH] Interpolate from time of previous tic

Previously interpolated from last 35th of a second, which
may be offset from game time due to connection lag.

Consider this the proper fix to 54148a0dd0 too.
---
 src/d_clisrv.c     | 16 ++++++++++++----
 src/d_clisrv.h     |  2 +-
 src/d_main.c       | 16 +++++++++++++---
 src/i_system.h     |  4 ++--
 src/sdl/i_system.c |  5 ++---
 5 files changed, 30 insertions(+), 13 deletions(-)

diff --git a/src/d_clisrv.c b/src/d_clisrv.c
index ac8bba608..2972502a7 100644
--- a/src/d_clisrv.c
+++ b/src/d_clisrv.c
@@ -5206,8 +5206,10 @@ static void SV_Maketic(void)
 	maketic++;
 }
 
-void TryRunTics(tic_t realtics)
+boolean TryRunTics(tic_t realtics)
 {
+	boolean ticking;
+
 	// the machine has lagged but it is not so bad
 	if (realtics > TICRATE/7) // FIXME: consistency failure!!
 	{
@@ -5251,10 +5253,14 @@ void TryRunTics(tic_t realtics)
 	}
 #endif
 
-	if (player_joining)
-		return;
+	ticking = neededtic > gametic;
 
-	if (neededtic > gametic)
+	if (player_joining)
+	{
+		return false;
+	}
+
+	if (ticking)
 	{
 		if (advancedemo)
 		{
@@ -5290,6 +5296,8 @@ void TryRunTics(tic_t realtics)
 					break;
 			}
 	}
+
+	return ticking;
 }
 
 /*
diff --git a/src/d_clisrv.h b/src/d_clisrv.h
index bf3f0b64f..c75f7f0bf 100644
--- a/src/d_clisrv.h
+++ b/src/d_clisrv.h
@@ -430,7 +430,7 @@ boolean Playing(void);
 void D_QuitNetGame(void);
 
 //? How many ticks to run?
-void TryRunTics(tic_t realtic);
+boolean TryRunTics(tic_t realtic);
 
 // extra data for lmps
 // these functions scare me. they contain magic.
diff --git a/src/d_main.c b/src/d_main.c
index 963439d66..155913320 100644
--- a/src/d_main.c
+++ b/src/d_main.c
@@ -697,6 +697,7 @@ void D_SRB2Loop(void)
 {
 	tic_t oldentertics = 0, entertic = 0, realtics = 0, rendertimeout = INFTICS;
 	static lumpnum_t gstartuplumpnum;
+	boolean ticked;
 
 	if (dedicated)
 		server = true;
@@ -783,11 +784,20 @@ void D_SRB2Loop(void)
 			realtics = 1;
 
 		// process tics (but maybe not if realtic == 0)
-		TryRunTics(realtics);
+		ticked = TryRunTics(realtics);
 
-		if (cv_frameinterpolation.value == 1)
+		if (cv_frameinterpolation.value == 1 && !(paused || P_AutoPause()))
 		{
-			fixed_t entertimefrac = I_GetTimeFrac();
+			static float tictime;
+			float entertime = I_GetTimeFrac();
+
+			fixed_t entertimefrac;
+
+			if (ticked)
+				tictime = entertime;
+
+			entertimefrac = FLOAT_TO_FIXED(entertime - tictime);
+
 			// renderdeltatics is a bit awkard to evaluate, since the system time interface is whole tic-based
 			renderdeltatics = realtics * FRACUNIT;
 			if (entertimefrac > rendertimefrac)
diff --git a/src/i_system.h b/src/i_system.h
index 0cdc7db85..93bb34a21 100644
--- a/src/i_system.h
+++ b/src/i_system.h
@@ -46,9 +46,9 @@ UINT32 I_GetFreeMem(UINT32 *total);
 */
 tic_t I_GetTime(void);
 
-/** \brief  Get the current time as a fraction of a tic since the last tic.
+/** \brief  Get the current time in tics including fractions.
 */
-fixed_t I_GetTimeFrac(void);
+float I_GetTimeFrac(void);
 
 /**	\brief	Returns precise time value for performance measurement.
   */
diff --git a/src/sdl/i_system.c b/src/sdl/i_system.c
index 2d99f5c1f..bf2e42cfd 100644
--- a/src/sdl/i_system.c
+++ b/src/sdl/i_system.c
@@ -2160,11 +2160,10 @@ tic_t I_GetTime(void)
 	return (tic_t) floor(elapsed_tics);
 }
 
-fixed_t I_GetTimeFrac(void)
+float I_GetTimeFrac(void)
 {
 	UpdateElapsedTics();
-	
-	return FLOAT_TO_FIXED((float) (elapsed_tics - floor(elapsed_tics)));
+	return elapsed_tics;
 }
 
 precise_t I_GetPreciseTime(void)