Loops: correct origin point over duration of loop

- Players moving into a gate with a lot of momentum would
  offset the origin point from its intended position
- Find intersection of player movement and gate
- Correct origin point between loop entry and exit by
  interpolating the difference between player position on
  entry and intersection point

src/d_player.h

@@ -471,6 +471,7 @@ typedef struct {
 	fixed_t revolution, min_revolution, max_revolution;
 	angle_t yaw;
 	vector3_t origin;
+	vector2_t origin_shift;
 	vector2_t shift;
 	boolean flip;
 } sonicloopvars_t;

src/objects/loops.cpp

@@ -10,6 +10,8 @@
 /// \file  loop-endpoint.c
 /// \brief Sonic loops, start and end points
 
+#include <optional>
+
 #include "../doomdef.h"
 #include "../k_kart.h"
 #include "../taglist.h"
@@ -149,6 +151,81 @@ get_binary_direction
 	}
 }
 
+static std::optional<vector2_t>
+intersect
+(		const mobj_t * anchor,
+		const mobj_t * toucher)
+{
+	struct Line
+	{
+		angle_t a;
+		vector2_t o;
+
+		angle_t k = AbsAngle(a);
+
+		Line(vector2_t o_, angle_t a_) : a(a_), o(o_) {}
+
+		bool vertical() const { return k == ANGLE_90; }
+
+		fixed_t m() const
+		{
+			// tangent table is offset 90 degrees
+			return FTAN(a - ANGLE_90);
+		}
+
+		fixed_t b() const
+		{
+			return o.y - FixedMul(o.x, m());
+		}
+
+		fixed_t y(fixed_t x) const
+		{
+			return FixedMul(m(), x) + b();
+		}
+	};
+
+	if (toucher->momx == 0 && toucher->momy == 0)
+	{
+		// undefined angle
+		return {};
+	}
+
+	Line a({toucher->x, toucher->y},
+		R_PointToAngle2(0, 0, toucher->momx, toucher->momy));
+
+	Line b({anchor->x, anchor->y}, anchor->angle + ANGLE_90);
+
+	if (a.k == b.k)
+	{
+		// parallel lines do not intersect
+		return {};
+	}
+
+	vector2_t v;
+
+	auto v_intersect = [&v](Line &a, Line &b)
+	{
+		if (a.vertical())
+		{
+			return false;
+		}
+
+		v.x = b.o.x;
+		v.y = a.y(v.x);
+
+		return true;
+	};
+
+	if (!v_intersect(a, b) && !v_intersect(b, a))
+	{
+		// untested!
+		v.x = FixedDiv(a.b() - b.b(), b.m() - a.m());
+		v.y = a.y(v.x);
+	}
+
+	return v;
+}
+
 mobj_t *
 Obj_FindLoopCenter (const mtag_t tag)
 {
@@ -226,8 +303,8 @@ Obj_LoopEndpointCollide
 	fixed_t radius;
 
 	/* predict movement for a smooth transition */
-	const fixed_t px = toucher->x + toucher->momx;
-	const fixed_t py = toucher->y + toucher->momy;
+	const fixed_t px = toucher->x + 2 * toucher->momx;
+	const fixed_t py = toucher->y + 2 * toucher->momy;
 
 	SINT8 flip;
 
@@ -251,6 +328,11 @@ Obj_LoopEndpointCollide
 	if (!P_MobjWasRemoved(anchor_other(anchor)))
 	{
 		set_shiftxy(player, anchor);
+
+		vector2_t i = intersect(anchor, toucher)
+			.value_or(vector2_t {px, py});
+
+		s->origin_shift = {i.x - px, i.y - py};
 	}
 
 	flip = get_binary_direction(pitch, toucher);
@@ -276,4 +358,7 @@ Obj_LoopEndpointCollide
 	/* cancel the effects of K_Squish */
 	toucher->spritexscale = FRACUNIT;
 	toucher->spriteyscale = FRACUNIT;
+
+	toucher->momx = 0;
+	toucher->momy = 0;
 }

src/p_loop.c

@@ -24,15 +24,11 @@ get_pitch (fixed_t revolution)
 }
 
 static inline fixed_t
-get_shift_curve (const sonicloopvars_t *s)
+normal_revolution (const sonicloopvars_t *s)
 {
-	const angle_t th = get_pitch(FixedDiv(
-				s->revolution - s->min_revolution,
-				s->max_revolution - s->min_revolution));
-
-	// XY shift is transformed on wave scale; less movement
-	// at start and end of rotation, more halfway.
-	return FSIN((th / 2) - ANGLE_90);
+	return FixedDiv(
+			s->revolution - s->min_revolution,
+			s->max_revolution - s->min_revolution);
 }
 
 void P_HaltPlayerOrbit(player_t *player)
@@ -98,6 +94,7 @@ boolean P_PlayerOrbit(player_t *player)
 	sonicloopvars_t *s = &player->loop;
 
 	angle_t pitch;
+	angle_t pitch_normal;
 
 	fixed_t xy, z;
 	fixed_t xs, ys;
@@ -125,15 +122,25 @@ boolean P_PlayerOrbit(player_t *player)
 	}
 
 	pitch = get_pitch(s->revolution);
+	pitch_normal = get_pitch(normal_revolution(s) / 2);
+
 
 	xy = FixedMul(abs(s->radius), FSIN(pitch));
 	z = FixedMul(abs(s->radius), -(FCOS(pitch)));
 
-	th = get_shift_curve(s);
+	// XY shift is transformed on wave scale; less movement
+	// at start and end of rotation, more halfway.
+	th = FSIN(pitch_normal - ANGLE_90);
 
 	xs = FixedMul(s->shift.x, th);
 	ys = FixedMul(s->shift.y, th);
 
+	// Interpolate 0-1 over entire rotation.
+	th = FSIN(pitch_normal / 2);
+
+	xs += FixedMul(s->origin_shift.x, th);
+	ys += FixedMul(s->origin_shift.y, th);
+
 	xs += FixedMul(xy, FCOS(s->yaw));
 	ys += FixedMul(xy, FSIN(s->yaw));
 

src/p_saveg.c

@@ -674,6 +674,8 @@ static void P_NetArchivePlayers(savebuffer_t *save)
 		WRITEFIXED(save->p, players[i].loop.origin.x);
 		WRITEFIXED(save->p, players[i].loop.origin.y);
 		WRITEFIXED(save->p, players[i].loop.origin.z);
+		WRITEFIXED(save->p, players[i].loop.origin_shift.x);
+		WRITEFIXED(save->p, players[i].loop.origin_shift.y);
 		WRITEFIXED(save->p, players[i].loop.shift.x);
 		WRITEFIXED(save->p, players[i].loop.shift.y);
 		WRITEUINT8(save->p, players[i].loop.flip);
@@ -1196,6 +1198,8 @@ static void P_NetUnArchivePlayers(savebuffer_t *save)
 		players[i].loop.origin.x = READFIXED(save->p);
 		players[i].loop.origin.y = READFIXED(save->p);
 		players[i].loop.origin.z = READFIXED(save->p);
+		players[i].loop.origin_shift.x = READFIXED(save->p);
+		players[i].loop.origin_shift.y = READFIXED(save->p);
 		players[i].loop.shift.x = READFIXED(save->p);
 		players[i].loop.shift.y = READFIXED(save->p);
 		players[i].loop.flip = READUINT8(save->p);

src/tables.h

@@ -139,6 +139,7 @@ void FM_Rotate(matrix_t *dest, angle_t angle, fixed_t x, fixed_t y, fixed_t z);
 // FSIN(ANGLE_90) = FRACUNIT
 #define FSIN(n) FINESINE(ANGLETOFINE(n))
 #define FCOS(n) FINECOSINE(ANGLETOFINE(n))
+#define FTAN(n) FINETANGENT(((n) >> ANGLETOFINESHIFT) & ((FINEANGLES / 2) - 1))
 
 #ifdef __cplusplus
 } // extern "C"