diff --git a/src/p_setup.c b/src/p_setup.c
index 798c6dbb3..1c2f8e06a 100644
--- a/src/p_setup.c
+++ b/src/p_setup.c
@@ -2092,22 +2092,22 @@ static void TextmapUnfixFlatOffsets(sector_t *sec)
 {
 	if (sec->floorpic_angle)
 	{
-		fixed_t pc = FINECOSINE(sec->floorpic_angle >> ANGLETOFINESHIFT);
-		fixed_t ps = FINESINE(sec->floorpic_angle >> ANGLETOFINESHIFT);
+		fixed_t pc = FINECOSINE(sec->floorpic_angle>>ANGLETOFINESHIFT);
+		fixed_t ps = -FINESINE (sec->floorpic_angle>>ANGLETOFINESHIFT);
 		fixed_t xoffs = sec->floor_xoffs;
 		fixed_t yoffs = sec->floor_yoffs;
-		sec->floor_xoffs = (FixedMul(xoffs, ps) % MAXFLATSIZE) + (FixedMul(yoffs, pc) % MAXFLATSIZE);
-		sec->floor_yoffs = (FixedMul(xoffs, pc) % MAXFLATSIZE) - (FixedMul(yoffs, ps) % MAXFLATSIZE);
+		sec->floor_xoffs = (FixedMul(xoffs, pc) % MAXFLATSIZE) - (FixedMul(yoffs, ps) % MAXFLATSIZE);
+		sec->floor_yoffs = (FixedMul(xoffs, ps) % MAXFLATSIZE) + (FixedMul(yoffs, pc) % MAXFLATSIZE);
 	}
 
 	if (sec->ceilingpic_angle)
 	{
-		fixed_t pc = FINECOSINE(sec->ceilingpic_angle >> ANGLETOFINESHIFT);
-		fixed_t ps = FINESINE(sec->ceilingpic_angle >> ANGLETOFINESHIFT);
+		fixed_t pc = FINECOSINE(sec->ceilingpic_angle>>ANGLETOFINESHIFT);
+		fixed_t ps = -FINESINE (sec->ceilingpic_angle>>ANGLETOFINESHIFT);
 		fixed_t xoffs = sec->ceiling_xoffs;
 		fixed_t yoffs = sec->ceiling_yoffs;
-		sec->ceiling_xoffs = (FixedMul(xoffs, ps) % MAXFLATSIZE) + (FixedMul(yoffs, pc) % MAXFLATSIZE);
-		sec->ceiling_yoffs = (FixedMul(xoffs, pc) % MAXFLATSIZE) - (FixedMul(yoffs, ps) % MAXFLATSIZE);
+		sec->ceiling_xoffs = (FixedMul(xoffs, pc) % MAXFLATSIZE) - (FixedMul(yoffs, ps) % MAXFLATSIZE);
+		sec->ceiling_yoffs = (FixedMul(xoffs, ps) % MAXFLATSIZE) + (FixedMul(yoffs, pc) % MAXFLATSIZE);
 	}
 }
 
@@ -2127,6 +2127,29 @@ static INT32 P_RGBAToColor(INT32 rgba)
 	return (r << 16) | (g << 8) | b;
 }
 
+static void TextmapWriteSlopeConstants(FILE *f, sector_t *sec)
+{
+	if (sec->f_slope != NULL)
+	{
+		const pslope_t *slope = sec->f_slope;
+
+		fprintf(f, "floorplane_a = %f;\n", FIXED_TO_FLOAT(slope->constants[0]));
+		fprintf(f, "floorplane_b = %f;\n", FIXED_TO_FLOAT(slope->constants[1]));
+		fprintf(f, "floorplane_c = %f;\n", FIXED_TO_FLOAT(slope->constants[2]));
+		fprintf(f, "floorplane_d = %f;\n", FIXED_TO_FLOAT(slope->constants[3]));
+	}
+
+	if (sec->c_slope != NULL)
+	{
+		const pslope_t *slope = sec->c_slope;
+
+		fprintf(f, "ceilingplane_a = %f;\n", FIXED_TO_FLOAT(slope->constants[0]));
+		fprintf(f, "ceilingplane_b = %f;\n", FIXED_TO_FLOAT(slope->constants[1]));
+		fprintf(f, "ceilingplane_c = %f;\n", FIXED_TO_FLOAT(slope->constants[2]));
+		fprintf(f, "ceilingplane_d = %f;\n", FIXED_TO_FLOAT(slope->constants[3]));
+	}
+}
+
 typedef struct
 {
 	mapthing_t *teleport;
@@ -2192,7 +2215,7 @@ static void P_WriteTextmapThing(FILE *f, mapthing_t *wmapthings, size_t i, size_
 			fprintf(f, "thingarg%s = %d;\n", sizeu1(j), wmapthings[i].thing_args[j]);
 	for (j = 0; j < NUM_MAPTHING_STRINGARGS; j++)
 		if (mapthings[i].thing_stringargs[j])
-			fprintf(f, "stringthingarg%s = \"%s\";\n", sizeu1(j), mapthings[i].thing_stringargs[j]);
+			fprintf(f, "thingstringarg%s = \"%s\";\n", sizeu1(j), mapthings[i].thing_stringargs[j]);
 	if (wmapthings[i].user.length > 0)
 	{
 		for (j = 0; j < wmapthings[i].user.length; j++)
@@ -2904,6 +2927,7 @@ static void P_WriteTextmap(void)
 					break;
 			}
 		}
+		TextmapWriteSlopeConstants(f, &wsectors[i]);
 		if (wsectors[i].action != 0)
 			fprintf(f, "action = %d;\n", wsectors[i].action);
 		for (j = 0; j < NUM_SCRIPT_ARGS; j++)
diff --git a/src/p_slopes.c b/src/p_slopes.c
index 771cdbc1c..0c253f730 100644
--- a/src/p_slopes.c
+++ b/src/p_slopes.c
@@ -143,6 +143,8 @@ void P_ReconfigureViaVertexes (pslope_t *slope, const vector3_t v1, const vector
 {
 	vector3_t vec1, vec2;
 
+	memset(slope->constants, 0, sizeof(slope->constants));
+
 	// Set origin.
 	FV3_Copy(&slope->o, &v1);
 
@@ -204,6 +206,11 @@ static void ReconfigureViaConstants (pslope_t *slope, const fixed_t a, const fix
 	fixed_t o = 0;
 	vector3_t *normal = &slope->normal;
 
+	slope->constants[0] = a;
+	slope->constants[1] = b;
+	slope->constants[2] = c;
+	slope->constants[3] = d;
+
 	if (c)
 		o = abs(c) <= FRACUNIT ? -FixedMul(d, FixedDiv(FRACUNIT, c)) : -FixedDiv(d, c);
 
diff --git a/src/r_defs.h b/src/r_defs.h
index de7f5e499..234f288c1 100644
--- a/src/r_defs.h
+++ b/src/r_defs.h
@@ -321,6 +321,9 @@ struct pslope_t
 	fixed_t lowz;
 	fixed_t highz;
 
+	// The ABCD constants used to define this slope
+	fixed_t constants[4];
+
 	// Light offsets (see seg_t)
 	SINT8 lightOffset;
 #ifdef HWRENDER