Use height difference between front and backside of line when stepping down

This should fix the janky falling off steep slopes and let you cling to them.
Because when falling off a sector the edge of the player must not intersect
the edge of the higher sector, it is trickier than step up. The height
difference at the line must be stored to be useful.

src/p_map.c

@@ -1707,6 +1707,7 @@ static boolean PIT_CheckLine(line_t *ld)
 		tmceilingrover = openceilingrover;
 		tmceilingslope = opentopslope;
 		tmceilingdiff = openceilingdiff;
+		tmthing->ceilingdrop = openceilingdrop;
 	}
 
 	if (openbottom > tmfloorz)
@@ -1715,6 +1716,7 @@ static boolean PIT_CheckLine(line_t *ld)
 		tmfloorrover = openfloorrover;
 		tmfloorslope = openbottomslope;
 		tmfloordiff = openfloordiff;
+		tmthing->floordrop = openfloordrop;
 	}
 
 	if (highceiling > tmdrpoffceilz)
@@ -2505,10 +2507,25 @@ boolean P_TryMove(mobj_t *thing, fixed_t x, fixed_t y, boolean allowdropoff)
 			if (thing->eflags & MFE_VERTICALFLIP)
 			{
 				if (thing->z < tmfloorz)
+				{
 					return false; // mobj must raise itself to fit
+				}
+				else if (thing->z > thing->floorz)
+				{
+					thing->floordrop = 0;
+				}
+			}
+			else
+			{
+				if (tmceilingz < thingtop)
+				{
+					return false; // mobj must lower itself to fit
+				}
+				else if (thingtop < thing->ceilingz)
+				{
+					thing->ceilingdrop = 0;
+				}
 			}
-			else if (tmceilingz < thingtop)
-				return false; // mobj must lower itself to fit
 
 			// Ramp test
 			if ((maxstep > 0) && !(P_MobjTouchingSectorSpecial(thing, 1, 14, false)))
@@ -2518,11 +2535,12 @@ boolean P_TryMove(mobj_t *thing, fixed_t x, fixed_t y, boolean allowdropoff)
 
 				if (thing->eflags & MFE_VERTICALFLIP)
 				{
-					if (thingtop == thing->ceilingz && tmceilingz > thingtop && tmceilingz - thingtop <= maxstep)
+					if (thingtop == thing->ceilingz && tmceilingz > thingtop && thing->ceilingdrop <= maxstep)
 					{
 						thing->z = (thing->ceilingz = thingtop = tmceilingz) - thing->height;
 						thing->ceilingrover = tmceilingrover;
 						thing->eflags |= MFE_JUSTSTEPPEDDOWN;
+						thing->ceilingdrop = 0;
 					}
 					else if (tmceilingz < thingtop && tmceilingdiff <= maxstep)
 					{
@@ -2531,11 +2549,12 @@ boolean P_TryMove(mobj_t *thing, fixed_t x, fixed_t y, boolean allowdropoff)
 						thing->eflags |= MFE_JUSTSTEPPEDDOWN;
 					}
 				}
-				else if (thing->z == thing->floorz && tmfloorz < thing->z && thing->z - tmfloorz <= maxstep)
+				else if (thing->z == thing->floorz && tmfloorz < thing->z && thing->floordrop <= maxstep)
 				{
 					thing->z = thing->floorz = tmfloorz;
 					thing->floorrover = tmfloorrover;
 					thing->eflags |= MFE_JUSTSTEPPEDDOWN;
+					thing->floordrop = 0;
 				}
 				else if (tmfloorz > thing->z && tmfloordiff <= maxstep)
 				{

src/p_maputl.c

@@ -281,6 +281,8 @@ pslope_t *opentopslope, *openbottomslope;
 ffloor_t *openfloorrover, *openceilingrover;
 fixed_t openfloordiff;
 fixed_t openceilingdiff;
+fixed_t openfloordrop;
+fixed_t openceilingdrop;
 
 // P_CameraLineOpening
 // P_LineOpening, but for camera
@@ -466,46 +468,50 @@ void P_LineOpening(line_t *linedef, mobj_t *mobj)
 	}
 	else
 	{ // Set open and high/low values here
-		fixed_t frontheight, backheight;
-		sector_t * sector;
+		fixed_t          height[2];
+		const sector_t * sector[2] = { front, back };
 
-		frontheight = P_GetCeilingZ(mobj, front, tmx, tmy, linedef);
-		backheight = P_GetCeilingZ(mobj, back, tmx, tmy, linedef);
+		int high;
 
-		if (frontheight < backheight)
-		{
-			opentop = frontheight;
-			highceiling = backheight;
-			sector = front;
-		}
-		else
-		{
-			opentop = backheight;
-			highceiling = frontheight;
-			sector = back;
-		}
+#define low ! high
 
-		opentopslope = sector->c_slope;
-		openceilingdiff = ( mobj->z + mobj->height - P_GetSectorCeilingZAt(sector, cross.x, cross.y) );
+		height[0] = P_GetCeilingZ(mobj, front, tmx, tmy, linedef);
+		height[1] = P_GetCeilingZ(mobj, back,  tmx, tmy, linedef);
 
-		frontheight = P_GetFloorZ(mobj, front, tmx, tmy, linedef);
-		backheight = P_GetFloorZ(mobj, back, tmx, tmy, linedef);
+		high = ( height[0] < height[1] );
 
-		if (frontheight > backheight)
-		{
-			openbottom = frontheight;
-			lowfloor = backheight;
-			sector = front;
-		}
-		else
-		{
-			openbottom = backheight;
-			lowfloor = frontheight;
-			sector = back;
-		}
+		opentop      = height[low];
+		highceiling  = height[high];
+		opentopslope = sector[low]->c_slope;
 
-		openbottomslope = sector->f_slope;
-		openfloordiff = ( P_GetSectorFloorZAt(sector, cross.x, cross.y) - mobj->z );
+		openceilingdiff = ( mobj->z + mobj->height -
+				P_GetSectorCeilingZAt(sector[low], cross.x, cross.y) );
+
+		openceilingdrop =
+			(
+					P_GetSectorCeilingZAt(sector[high], cross.x, cross.y) -
+					P_GetSectorCeilingZAt(sector[low],  cross.x, cross.y)
+			);
+
+		height[0] = P_GetFloorZ(mobj, front, tmx, tmy, linedef);
+		height[1] = P_GetFloorZ(mobj, back,  tmx, tmy, linedef);
+
+		high = ( height[0] < height[1] );
+
+		openbottom      = height[high];
+		lowfloor        = height[low];
+		openbottomslope = sector[high]->f_slope;
+
+		openfloordiff =
+			( P_GetSectorFloorZAt(sector[high], cross.x, cross.y) - mobj->z );
+
+		openfloordrop =
+			(
+					P_GetSectorFloorZAt(sector[high], cross.x, cross.y) -
+					P_GetSectorFloorZAt(sector[low],  cross.x, cross.y)
+			);
+
+#undef low
 	}
 
 	if (mobj)

src/p_maputl.h

@@ -60,6 +60,8 @@ extern pslope_t *opentopslope, *openbottomslope;
 extern ffloor_t *openfloorrover, *openceilingrover;
 extern fixed_t openfloordiff;
 extern fixed_t openceilingdiff;
+extern fixed_t openfloordrop;
+extern fixed_t openceilingdrop;
 
 void P_LineOpening(line_t *plinedef, mobj_t *mobj);
 

src/p_mobj.h

@@ -326,6 +326,8 @@ typedef struct mobj_s
 	fixed_t ceilingz; // Nearest ceiling above.
 	struct ffloor_s *floorrover; // FOF referred by floorz
 	struct ffloor_s *ceilingrover; // FOF referred by ceilingz
+	fixed_t floordrop;
+	fixed_t ceilingdrop;
 
 	// For movement checking.
 	fixed_t radius;