V_AdjustXYWithSnap: refactor V_SLIDEIN, separate timer calculation, fix interpolation

- Move timer calculation to ST_CalculateFadeIn, cache in
  st_fadein
- Fix interpolation by using rendertimefrac instead of
  renderdeltatics

src/st_stuff.c

@@ -117,6 +117,7 @@ void ST_Ticker(boolean run)
 // 0 is default, any others are special palettes.
 INT32 st_palette = 0;
 UINT32 st_translucency = 10;
+fixed_t st_fadein = 0;
 
 void ST_doPaletteStuff(void)
 {
@@ -1424,6 +1425,35 @@ void ST_DrawServerSplash(boolean timelimited)
 	}
 }
 
+static fixed_t ST_CalculateFadeIn(player_t *player)
+{
+	const tic_t length = TICRATE/4;
+
+	if (player->tally.hudSlide != 0)
+	{
+		tic_t timer = length - player->tally.hudSlide;
+
+		return ((timer * FRACUNIT) + (FRACUNIT - rendertimefrac)) / length;
+	}
+
+	tic_t timer = lt_exitticker;
+
+	if (K_CheckBossIntro() == true || G_IsTitleCardAvailable() == false)
+	{
+		if (leveltime <= 16)
+			timer = 0;
+		else
+			timer = leveltime-16;
+	}
+
+	if (timer < length)
+	{
+		return ((timer * FRACUNIT) + rendertimefrac) / length;
+	}
+
+	return FRACUNIT;
+}
+
 void ST_Drawer(void)
 {
 	boolean stagetitle = false; // Decide whether to draw the stage title or not
@@ -1479,6 +1509,7 @@ void ST_Drawer(void)
 		for (i = 0; i <= r_splitscreen; i++)
 		{
 			stplyr = &players[displayplayers[i]];
+			st_fadein = ST_FadeIn(stplyr);
 			R_SetViewContext(VIEWCONTEXT_PLAYER1 + i);
 			R_InterpolateView(rendertimefrac); // to assist with object tracking
 			ST_overlayDrawer();

src/st_stuff.h

@@ -87,6 +87,7 @@ boolean ST_SameTeam(player_t *a, player_t *b);
 extern boolean st_overlay; // sb overlay on or off when fullscreen
 extern INT32 st_palette; // 0 is default, any others are special palettes.
 extern UINT32 st_translucency;
+extern fixed_t st_fadein; // transitioning value per player, FRACUNIT = fully in view
 
 extern lumpnum_t st_borderpatchnum;
 // patches, also used in intermission

src/v_video.cpp

@@ -573,33 +573,14 @@ void V_AdjustXYWithSnap(INT32 *x, INT32 *y, UINT32 options, INT32 dupx, INT32 du
 
 	if ((options & V_SLIDEIN))
 	{
-		const tic_t length = TICRATE/4;
+		if (st_fadein < FRACUNIT)
-		tic_t timer = lt_exitticker;
-
-		if (K_CheckBossIntro() == true || G_IsTitleCardAvailable() == false)
-		{
-			if (leveltime <= 16)
-				timer = 0;
-			else
-				timer = leveltime-16;
-		}
-
-		if (stplyr->tally.hudSlide != 0)
-		{
-			timer = length - stplyr->tally.hudSlide;
-		}
-
-		if (timer < length)
 		{
 			if ((options & (V_SNAPTORIGHT|V_SNAPTOLEFT|V_SPLITSCREEN)) != 0)
 			{
 				boolean slidefromright = false;
 
-				const INT32 offsetAmount = (screenwidth * FRACUNIT/2) / length;
+				const fixed_t offsetAmount = (screenwidth * FRACUNIT/2);
-				fixed_t offset = (screenwidth * FRACUNIT/2) - (timer * offsetAmount);
+				INT32 offset = (offsetAmount - FixedMul(offsetAmount, st_fadein)) / FRACUNIT;
-
-				offset += FixedMul(offsetAmount, renderdeltatics);
-				offset /= FRACUNIT;
 
 				if (r_splitscreen > 1)
 				{
@@ -621,11 +602,8 @@ void V_AdjustXYWithSnap(INT32 *x, INT32 *y, UINT32 options, INT32 dupx, INT32 du
 			}
 			else
 			{
-				const INT32 offsetAmount = (screenheight * FRACUNIT/2) / length;
+				const fixed_t offsetAmount = (screenheight * FRACUNIT/2);
-				fixed_t offset = (screenheight * FRACUNIT/2) - (timer * offsetAmount);
+				INT32 offset = (offsetAmount - FixedMul(offsetAmount, st_fadein)) / FRACUNIT;
-
-				offset += FixedMul(offsetAmount, renderdeltatics);
-				offset /= FRACUNIT;
 
 				if (options & V_SNAPTOBOTTOM)
 				{