From 7d1f2df404711986a3cc742dad1f9e6e0ac69cde Mon Sep 17 00:00:00 2001
From: Boondorl <59555366+Boondorl@users.noreply.github.com>
Date: Tue, 22 Jul 2025 10:35:20 -0400
Subject: [PATCH] Added scale and alpha interpolation

Similar to angles, needs to be enabled via flags to prevent breaking existing sprites.
---
 src/playsim/actor.h                           | 14 +++++++++++++-
 src/playsim/actorinlines.h                    |  2 ++
 src/rendering/hwrenderer/scene/hw_sprites.cpp |  7 ++++---
 src/scripting/thingdef_data.cpp               |  2 ++
 4 files changed, 21 insertions(+), 4 deletions(-)

diff --git a/src/playsim/actor.h b/src/playsim/actor.h
index 2a0db55b9..37c27a820 100644
--- a/src/playsim/actor.h
+++ b/src/playsim/actor.h
@@ -510,6 +510,8 @@ enum ActorRenderFlag2
 	RF2_STRETCHPIXELS			= 0x0200,	// don't apply SQUAREPIXELS for ROLLSPRITES
 	RF2_LIGHTMULTALPHA			= 0x0400,	// attached lights use alpha as intensity multiplier
 	RF2_ANGLEDROLL				= 0x0800,	// Sprite roll amount depends on (actor.Angle - actor.AngledRollOffset)
+	RF2_INTERPOLATESCALE		= 0x1000,
+	RF2_INTERPOLATEALPHA		= 0x2000,
 };
 
 // This translucency value produces the closest match to Heretic's TINTTAB.
@@ -1396,6 +1398,8 @@ public:
 	// [RH] Used to interpolate the view to get >35 FPS
 	DVector3 Prev;
 	DRotator PrevAngles;
+	DVector2 PrevScale;
+	double PrevAlpha;
 	DAngle   PrevFOV;
 	TArray<FDynamicLight *> AttachedLights;
 	TDeletingArray<FLightDefaults *> UserLights;
@@ -1544,7 +1548,7 @@ public:
 	DVector3 InterpolatedPosition(double ticFrac) const
 	{
 		if (renderflags & RF_DONTINTERPOLATE) return Pos();
-		else return Prev + (ticFrac * (Pos() - Prev));
+		else return Prev * (1.0 - ticFrac) + Pos() * ticFrac;
 	}
 	DRotator InterpolatedAngles(double ticFrac) const
 	{
@@ -1554,6 +1558,14 @@ public:
 		result.Roll = PrevAngles.Roll + deltaangle(PrevAngles.Roll, Angles.Roll) * ticFrac;
 		return result;
 	}
+	DVector2 InterpolatedScale(double ticFrac) const
+	{
+		return (renderflags2 & RF2_INTERPOLATESCALE) ? PrevScale * (1.0 - ticFrac) + Scale * ticFrac : Scale;
+	}
+	double InterpolatedAlpha(double ticFrac) const
+	{
+		return (renderflags2 & RF2_INTERPOLATEALPHA) ? PrevAlpha * (1.0 - ticFrac) + Alpha * ticFrac : Alpha;
+	}
 	float GetSpriteOffset(bool y) const
 	{
 		if (y)	return (float)(renderflags2 & RF2_FLIPSPRITEOFFSETY ? SpriteOffset.Y : -SpriteOffset.Y);
diff --git a/src/playsim/actorinlines.h b/src/playsim/actorinlines.h
index bbc173801..95e7d314f 100644
--- a/src/playsim/actorinlines.h
+++ b/src/playsim/actorinlines.h
@@ -30,6 +30,8 @@ inline void AActor::ClearInterpolation()
 {
 	Prev = Pos();
 	PrevAngles = Angles;
+	PrevScale = Scale;
+	PrevAlpha = Alpha;
 	if (Sector) PrevPortalGroup = Sector->PortalGroup;
 	else PrevPortalGroup = 0;
 }
diff --git a/src/rendering/hwrenderer/scene/hw_sprites.cpp b/src/rendering/hwrenderer/scene/hw_sprites.cpp
index 54253ee9e..d29766350 100644
--- a/src/rendering/hwrenderer/scene/hw_sprites.cpp
+++ b/src/rendering/hwrenderer/scene/hw_sprites.cpp
@@ -808,7 +808,8 @@ void HWSprite::Process(HWDrawInfo *di, AActor* thing, sector_t * sector, area_t
 	const auto &vp = di->Viewpoint;
 	AActor *camera = vp.camera;
 
-	if (thing->renderflags & RF_INVISIBLE || !thing->RenderStyle.IsVisible(thing->Alpha))
+	const double alpha = thing->InterpolatedAlpha(vp.TicFrac);
+	if (thing->renderflags & RF_INVISIBLE || !thing->RenderStyle.IsVisible(alpha))
 	{
 		if (!(thing->flags & MF_STEALTH) || !di->isStealthVision() || thing == camera)
 			return;
@@ -821,7 +822,7 @@ void HWSprite::Process(HWDrawInfo *di, AActor* thing, sector_t * sector, area_t
 		return;
 
 	int spritenum = thing->sprite;
-	DVector2 sprscale(thing->Scale.X, thing->Scale.Y);
+	DVector2 sprscale = thing->InterpolatedScale(vp.TicFrac);
 	if (thing->player != nullptr)
 	{
 		P_CheckPlayerSprite(thing, spritenum, sprscale);
@@ -1281,7 +1282,7 @@ void HWSprite::Process(HWDrawInfo *di, AActor* thing, sector_t * sector, area_t
 	translation = thing->Translation;
 
 	OverrideShader = -1;
-	trans = thing->Alpha;
+	trans = alpha;
 	hw_styleflags = STYLEHW_Normal;
 
 	if (RenderStyle.BlendOp >= STYLEOP_Fuzz && RenderStyle.BlendOp <= STYLEOP_FuzzOrRevSub)
diff --git a/src/scripting/thingdef_data.cpp b/src/scripting/thingdef_data.cpp
index 2c49d102a..db69e09b9 100644
--- a/src/scripting/thingdef_data.cpp
+++ b/src/scripting/thingdef_data.cpp
@@ -392,6 +392,8 @@ static FFlagDef ActorFlagDefs[]=
 	DEFINE_FLAG(RF2, STRETCHPIXELS, AActor, renderflags2),
 	DEFINE_FLAG(RF2, LIGHTMULTALPHA, AActor, renderflags2),
 	DEFINE_FLAG(RF2, ANGLEDROLL, AActor, renderflags2),
+	DEFINE_FLAG(RF2, INTERPOLATESCALE, AActor, renderflags2),
+	DEFINE_FLAG(RF2, INTERPOLATEALPHA, AActor, renderflags2),
 
 	// Bounce flags
 	DEFINE_FLAG2(BOUNCE_Walls, BOUNCEONWALLS, AActor, BounceFlags),