diff --git a/src/p_maputl.c b/src/p_maputl.c
index 6e010d84c..b4ae2f3ed 100644
--- a/src/p_maputl.c
+++ b/src/p_maputl.c
@@ -670,44 +670,36 @@ void P_LineOpening(line_t *linedef, mobj_t *mobj)
 	if (mobj)
 	{
 		// Check for collision with front side's midtexture if Effect 4 is set
-		if (linedef->flags & ML_EFFECT4 && !linedef->polyobj) // don't do anything for polyobjects! ...for now
-		{
-			side_t *side = &sides[linedef->sidenum[0]];
-			fixed_t textop, texbottom, texheight;
+		if ((linedef->flags & ML_EFFECT4 || (mobj->player && P_IsLineTripWire(linedef) && !K_TripwirePass(mobj->player)))
+			&& !linedef->polyobj // don't do anything for polyobjects! ...for now
+			) {
+			fixed_t textop, texbottom;
 			fixed_t texmid, delta1, delta2;
-			INT32 texnum = R_GetTextureNum(side->midtexture); // make sure the texture is actually valid
 
-			if (texnum) {
-				// Get the midtexture's height
-				texheight = textures[texnum]->height << FRACBITS;
+			if (P_GetMidtextureTopBottom(linedef, cross.x, cross.y, &textop, &texbottom))
+			{
+				texmid = texbottom+(textop-texbottom)/2;
 
-				// Set texbottom and textop to the Z coordinates of the texture's boundaries
-				{
-					if (linedef->flags & ML_EFFECT5 && !side->repeatcnt) { // "infinite" repeat
-						texbottom = openbottom + side->rowoffset;
-						textop = opentop + side->rowoffset;
-					} else if (!!(linedef->flags & ML_DONTPEGBOTTOM) ^ !!(linedef->flags & ML_EFFECT3)) {
-						texbottom = openbottom + side->rowoffset;
-						textop = texbottom + texheight*(side->repeatcnt+1);
-					} else {
-						textop = opentop + side->rowoffset;
-						texbottom = textop - texheight*(side->repeatcnt+1);
+				delta1 = abs(mobj->z - texmid);
+				delta2 = abs(thingtop - texmid);
+
+				if (delta1 > delta2) { // Below
+					if (opentop > texbottom)
+					{
+						topedge[lo] -= ( opentop - texbottom );
+
+						opentop = texbottom;
+						openceilingstep = ( thingtop    - topedge[lo] );
+						openceilingdrop = ( topedge[hi] - topedge[lo] );
 					}
-				}
-				
-				if (P_GetMidtextureTopBottom(linedef, cross.x, cross.y, &textop, &texbottom))
-				{
-					texmid = texbottom+(textop-texbottom)/2;
+				} else { // Above
+					if (openbottom < textop)
+					{
+						botedge[hi] += ( textop - openbottom );
 
-					delta1 = abs(mobj->z - texmid);
-					delta2 = abs(thingtop - texmid);
-
-					if (delta1 > delta2) { // Below
-						if (opentop > texbottom)
-							opentop = texbottom;
-					} else { // Above
-						if (openbottom < textop)
-							openbottom = textop;
+						openbottom = textop;
+						openfloorstep = ( botedge[hi] - mobj->z );
+						openfloordrop = ( botedge[hi] - botedge[lo] );
 					}
 				}
 			}
@@ -736,14 +728,22 @@ void P_LineOpening(line_t *linedef, mobj_t *mobj)
 				delta2 = abs(thingtop - (polybottom + ((polytop - polybottom)/2)));
 
 				if (polybottom < opentop && delta1 >= delta2)
+				{
 					opentop = polybottom;
+				}
 				else if (polybottom < highceiling && delta1 >= delta2)
+				{
 					highceiling = polybottom;
+				}
 
 				if (polytop > openbottom && delta1 < delta2)
+				{
 					openbottom = polytop;
+				}
 				else if (polytop > lowfloor && delta1 < delta2)
+				{
 					lowfloor = polytop;
+				}
 			}
 			// otherwise don't do anything special, pretend there's nothing else there
 		}
@@ -755,6 +755,21 @@ void P_LineOpening(line_t *linedef, mobj_t *mobj)
 				ffloor_t *rover;
 				fixed_t delta1, delta2;
 
+				/* yuck */
+				struct
+				{
+					fixed_t top;
+					fixed_t bottom;
+					ffloor_t * ceilingrover;
+					ffloor_t *   floorrover;
+				} open[2] = {
+					{ INT32_MAX, INT32_MIN, NULL, NULL },
+					{ INT32_MAX, INT32_MIN, NULL, NULL },
+				};
+
+				const fixed_t oldopentop = opentop;
+				const fixed_t oldopenbottom = openbottom;
+
 				// Check for frontsector's fake floors
 				for (rover = front->ffloors; rover; rover = rover->next)
 				{
@@ -776,11 +791,11 @@ void P_LineOpening(line_t *linedef, mobj_t *mobj)
 
 					if (delta1 >= delta2 && (rover->flags & FF_INTANGIBLEFLATS) != FF_PLATFORM) // thing is below FOF
 					{
-						if (bottomheight < opentop) {
-							opentop = bottomheight;
+						if (bottomheight < open[FRONT].top) {
+							open[FRONT].top = bottomheight;
 							opentopslope = *rover->b_slope;
 							opentoppic = *rover->bottompic;
-							openceilingrover = rover;
+							open[FRONT].ceilingrover = rover;
 						}
 						else if (bottomheight < highceiling)
 							highceiling = bottomheight;
@@ -788,11 +803,11 @@ void P_LineOpening(line_t *linedef, mobj_t *mobj)
 
 					if (delta1 < delta2 && (rover->flags & FF_INTANGIBLEFLATS) != FF_REVERSEPLATFORM) // thing is above FOF
 					{
-						if (topheight > openbottom) {
-							openbottom = topheight;
+						if (topheight > open[FRONT].bottom) {
+							open[FRONT].bottom = topheight;
 							openbottomslope = *rover->t_slope;
 							openbottompic = *rover->toppic;
-							openfloorrover = rover;
+							open[FRONT].floorrover = rover;
 						}
 						else if (topheight > lowfloor)
 							lowfloor = topheight;
@@ -820,11 +835,11 @@ void P_LineOpening(line_t *linedef, mobj_t *mobj)
 
 					if (delta1 >= delta2 && (rover->flags & FF_INTANGIBLEFLATS) != FF_PLATFORM) // thing is below FOF
 					{
-						if (bottomheight < opentop) {
-							opentop = bottomheight;
+						if (bottomheight < open[BACK].top) {
+							open[BACK].top = bottomheight;
 							opentopslope = *rover->b_slope;
 							opentoppic = *rover->bottompic;
-							openceilingrover = rover;
+							open[BACK].ceilingrover = rover;
 						}
 						else if (bottomheight < highceiling)
 							highceiling = bottomheight;
@@ -832,16 +847,54 @@ void P_LineOpening(line_t *linedef, mobj_t *mobj)
 
 					if (delta1 < delta2 && (rover->flags & FF_INTANGIBLEFLATS) != FF_REVERSEPLATFORM) // thing is above FOF
 					{
-						if (topheight > openbottom) {
-							openbottom = topheight;
+						if (topheight > open[BACK].bottom) {
+							open[BACK].bottom = topheight;
 							openbottomslope = *rover->t_slope;
 							openbottompic = *rover->toppic;
-							openfloorrover = rover;
+							open[BACK].floorrover = rover;
 						}
 						else if (topheight > lowfloor)
 							lowfloor = topheight;
 					}
 				}
+
+				lo = ( open[0].top > open[1].top );
+
+				if (open[lo].top <= oldopentop)
+				{
+					hi = ! lo;
+
+					topedge[lo] = P_GetFFloorBottomZAt(open[lo].ceilingrover, cross.x, cross.y);
+
+					if (open[hi].top < oldopentop)
+					{
+						topedge[hi] = P_GetFFloorBottomZAt(open[hi].ceilingrover, cross.x, cross.y);
+					}
+
+					opentop = open[lo].top;
+					openceilingrover = open[lo].ceilingrover;
+					openceilingstep = ( thingtop    - topedge[lo] );
+					openceilingdrop = ( topedge[hi] - topedge[lo] );
+				}
+
+				hi = ( open[0].bottom < open[1].bottom );
+
+				if (open[hi].bottom >= oldopenbottom)
+				{
+					lo = ! hi;
+
+					botedge[hi] = P_GetFFloorTopZAt(open[hi].floorrover, cross.x, cross.y);
+
+					if (open[lo].bottom > oldopenbottom)
+					{
+						botedge[lo] = P_GetFFloorTopZAt(open[lo].floorrover, cross.x, cross.y);
+					}
+
+					openbottom = open[hi].bottom;
+					openfloorrover = open[hi].floorrover;
+					openfloorstep = ( botedge[hi] - mobj->z );
+					openfloordrop = ( botedge[hi] - botedge[lo] );
+				}
 			}
 		}
 	}