diff --git a/source/sw/src/interpso.cpp b/source/sw/src/interpso.cpp
new file mode 100644
index 000000000..3ef340adc
--- /dev/null
+++ b/source/sw/src/interpso.cpp
@@ -0,0 +1,341 @@
+//-------------------------------------------------------------------------
+/*
+Copyright (C) 1997, 2005 - 3D Realms Entertainment
+
+This file is part of Shadow Warrior version 1.2
+
+Shadow Warrior is free software; you can redistribute it and/or
+modify it under the terms of the GNU General Public License
+as published by the Free Software Foundation; either version 2
+of the License, or (at your option) any later version.
+
+This program is distributed in the hope that it will be useful,
+but WITHOUT ANY WARRANTY; without even the implied warranty of
+MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.
+
+See the GNU General Public License for more details.
+
+You should have received a copy of the GNU General Public License
+along with this program; if not, write to the Free Software
+Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA  02110-1301, USA.
+
+Original Source: 1997 - Frank Maddin and Jim Norwood
+Prepared for public release: 03/28/2005 - Charlie Wiederhold, 3D Realms
+*/
+//-------------------------------------------------------------------------
+
+#include "compat.h"
+#include "pragmas.h"
+
+#include "game.h"
+#include "interp.h"
+#include "interpso.h"
+
+#define SO_MAXINTERPOLATIONS MAXINTERPOLATIONS
+
+static struct so_interp
+{
+    struct interp_data
+    {
+        void *curipos;
+        int32_t oldipos;
+        int32_t bakipos;
+        int32_t lastipos;
+        int32_t lastoldipos;
+        int32_t lastangdiff;
+        int32_t spriteofang;
+    } data[SO_MAXINTERPOLATIONS];
+
+    int32_t numinterpolations;
+    int32_t tic, lasttic;
+} so_interpdata[MAX_SECTOR_OBJECTS];
+
+static void so_setpointinterpolation(so_interp *interp, int32_t *posptr)
+{
+    int32_t i;
+    if (interp->numinterpolations >= SO_MAXINTERPOLATIONS)
+        return;
+
+    for (i = 0; i < interp->numinterpolations; i++)
+        if (interp->data[i].curipos == posptr)
+            return;
+
+    so_interp::interp_data *data = &interp->data[interp->numinterpolations++];
+
+    data->curipos = posptr;
+    data->oldipos = *posptr;
+    data->lastipos = *posptr;
+    data->lastoldipos = *posptr;
+    data->spriteofang = -1;
+}
+
+static void so_setspriteanginterpolation(so_interp *interp, int16_t *posptr, int32_t spritenum)
+{
+    int32_t i;
+    if (interp->numinterpolations >= SO_MAXINTERPOLATIONS)
+        return;
+
+    for (i = 0; i < interp->numinterpolations; i++)
+        if (interp->data[i].curipos == posptr)
+            return;
+
+    so_interp::interp_data *data = &interp->data[interp->numinterpolations++];
+
+    data->curipos = posptr;
+    data->oldipos = *posptr;
+    data->lastipos = *posptr;
+    data->lastoldipos = *posptr;
+    data->lastangdiff = 0;
+    data->spriteofang = spritenum;
+}
+
+// Covers points and angles altogether
+static void so_stopdatainterpolation(so_interp *interp, void *posptr)
+{
+    int32_t i;
+
+    for (i = 0; i < interp->numinterpolations; i++)
+        if (interp->data[i].curipos == posptr)
+            break;
+
+    if (i == interp->numinterpolations)
+        return;
+
+    interp->data[i] = interp->data[--(interp->numinterpolations)];
+}
+
+void so_addinterpolation(SECTOR_OBJECTp sop)
+{
+    SECTORp *sectp;
+    int32_t startwall, endwall;
+    int32_t i;
+
+    so_interp *interp = &so_interpdata[sop - SectorObject];
+    interp->numinterpolations = 0;
+
+    for (sectp = sop->sectp; *sectp; sectp++)
+    {
+        startwall = (*sectp)->wallptr;
+        endwall = startwall + (*sectp)->wallnum - 1;
+
+        for (i = startwall; i <= endwall; i++)
+        {
+            int32_t nextwall = wall[i].nextwall;
+
+            so_setpointinterpolation(interp, &wall[i].x);
+            so_setpointinterpolation(interp, &wall[i].y);
+
+            if (nextwall >= 0)
+            {
+                so_setpointinterpolation(interp, &wall[wall[nextwall].point2].x);
+                so_setpointinterpolation(interp, &wall[wall[nextwall].point2].y);
+            }
+        }
+
+        so_setpointinterpolation(interp, &(*sectp)->ceilingz);
+        so_setpointinterpolation(interp, &(*sectp)->floorz);
+    }
+
+    // interpolate midpoint, for aiming at a remote controlled SO
+    so_setpointinterpolation(interp, &sop->xmid);
+    so_setpointinterpolation(interp, &sop->ymid);
+    so_setpointinterpolation(interp, &sop->zmid);
+
+    interp->tic = 0;
+    interp->lasttic = synctics;
+}
+
+void so_setspriteinterpolation(SECTOR_OBJECTp sop, spritetype *sp)
+{
+    so_interp *interp = &so_interpdata[sop - SectorObject];
+
+    so_setpointinterpolation(interp, &sp->x);
+    so_setpointinterpolation(interp, &sp->y);
+    so_setpointinterpolation(interp, &sp->z);
+    so_setspriteanginterpolation(interp, &sp->ang, sp - sprite);
+}
+
+void so_stopspriteinterpolation(SECTOR_OBJECTp sop, spritetype *sp)
+{
+    so_interp *interp = &so_interpdata[sop - SectorObject];
+
+    so_stopdatainterpolation(interp, &sp->x);
+    so_stopdatainterpolation(interp, &sp->y);
+    so_stopdatainterpolation(interp, &sp->z);
+    so_stopdatainterpolation(interp, &sp->ang);
+}
+
+void so_setinterpolationtics(SECTOR_OBJECTp sop, int16_t locktics)
+{
+    so_interp *interp = &so_interpdata[sop - SectorObject];
+
+    interp->tic = 0;
+    interp->lasttic = locktics;
+}
+
+void so_updateinterpolations(void) // Stick at beginning of domovethings
+{
+    int32_t i;
+    SECTOR_OBJECTp sop;
+    so_interp *interp;
+    so_interp::interp_data *data;
+    SWBOOL interpolating = gs.InterpolateSO && !CommEnabled; // If changing from menu
+
+    for (sop = SectorObject, interp = so_interpdata;
+         sop < &SectorObject[MAX_SECTOR_OBJECTS]; sop++, interp++)
+    {
+        if (sop->xmid == INT32_MAX /*|| sop->xmid == MAXSO*/)
+            continue;
+        if (interp->tic < interp->lasttic)
+            interp->tic += synctics;
+        for (i = 0, data = interp->data; i < interp->numinterpolations; i++, data++)
+        {
+            if (data->spriteofang >= 0)
+            {
+                USERp u = User[data->spriteofang];
+                if (u)
+                    u->oangdiff = 0;
+                if (!interpolating)
+                    data->lastangdiff = 0;
+                data->oldipos = *(int16_t *)(data->curipos);
+            }
+            else
+                data->oldipos = *(int32_t *)(data->curipos);
+
+            if (!interpolating)
+                data->lastipos = data->lastoldipos = data->oldipos;
+        }
+    }
+}
+
+// must call restore for every do interpolations
+// make sure you don't exit
+void so_dointerpolations(int32_t smoothratio)                      // Stick at beginning of drawscreen
+{
+    int32_t i, delta;
+    SECTOR_OBJECTp sop;
+    so_interp *interp;
+    so_interp::interp_data *data;
+
+    // Set the bakipos values separately, in case a point is shared.
+    // Also set lastipos if there's been an actual change in a point.
+    for (sop = SectorObject, interp = so_interpdata;
+         sop < &SectorObject[MAX_SECTOR_OBJECTS]; sop++, interp++)
+    {
+        if (sop->xmid == INT32_MAX /*|| sop->xmid == MAXSO*/)
+            continue;
+
+        for (i = 0; i < interp->numinterpolations; i++)
+            interp->data[i].bakipos = (interp->data[i].spriteofang >= 0) ?
+                                      *(int16_t *)(interp->data[i].curipos) :
+                                      *(int32_t *)(interp->data[i].curipos);
+
+        if (interp->tic == 0) // Only if the SO has just moved
+        {
+            for (i = 0, data = interp->data; i < interp->numinterpolations; i++, data++)
+            {
+                data->lastipos = data->bakipos;
+                data->lastoldipos = data->oldipos;
+                if (data->spriteofang >= 0)
+                {
+                    USERp u = User[data->spriteofang];
+                    data->lastangdiff = u ? u->oangdiff : 0;
+                }
+            }
+        }
+    }
+
+    for (sop = SectorObject, interp = so_interpdata;
+         sop < &SectorObject[MAX_SECTOR_OBJECTS]; sop++, interp++)
+    {
+        if (sop->xmid == INT32_MAX /*|| sop->xmid == MAXSO*/)
+            continue;
+
+        for (i = 0, data = interp->data; i < interp->numinterpolations; i++, data++)
+        {
+            int32_t ratio = smoothratio * synctics + 65536 * interp->tic;
+            ratio /= interp->lasttic;
+            ratio = (interp->tic == interp->lasttic) ? 65536 : ratio;
+            if (data->spriteofang >= 0)
+                *(int16_t *)(data->curipos) = NORM_ANGLE(data->lastoldipos + mulscale16(data->lastangdiff, ratio));
+            else
+            {
+                delta = data->lastipos - data->lastoldipos;
+                *(int32_t *)(data->curipos) = data->lastoldipos + mulscale16(delta, ratio);
+            }
+        }
+    }
+}
+
+void so_restoreinterpolations(void)                 // Stick at end of drawscreen
+{
+    int32_t i;
+    SECTOR_OBJECTp sop;
+    so_interp *interp;
+    so_interp::interp_data *data;
+
+    for (sop = SectorObject, interp = so_interpdata;
+         sop < &SectorObject[MAX_SECTOR_OBJECTS]; sop++, interp++)
+    {
+        if (sop->xmid == INT32_MAX /*|| sop->xmid == MAXSO*/)
+            continue;
+
+        for (i = 0, data = interp->data; i < interp->numinterpolations; i++, data++)
+            if (data->spriteofang >= 0)
+                *(int16_t *)(data->curipos) = data->bakipos;
+            else
+                *(int32_t *)(data->curipos) = data->bakipos;
+    }
+}
+
+int SaveSymDataInfo(MFILE_WRITE fil, void *ptr);
+
+SWBOOL so_writeinterpolations(MFILE_WRITE fil)
+{
+    int32_t i;
+    SECTOR_OBJECTp sop;
+    const so_interp *interp;
+    SWBOOL saveisshot = FALSE;
+
+    for (sop = SectorObject, interp = so_interpdata;
+         sop < &SectorObject[MAX_SECTOR_OBJECTS]; sop++, interp++)
+    {
+        const so_interp::interp_data *data = interp->data;
+        MWRITE(&interp->numinterpolations,sizeof(interp->numinterpolations),1,fil);
+        for (i = 0; i < interp->numinterpolations; i++, data++)
+        {
+            saveisshot |= SaveSymDataInfo(fil, data->curipos);
+            MWRITE(&data->oldipos,sizeof(data->oldipos),1,fil);
+            MWRITE(&data->spriteofang,sizeof(data->spriteofang),1,fil);
+        }
+    }
+    return saveisshot;
+}
+
+int LoadSymDataInfo(MFILE_READ fil, void **ptr);
+
+SWBOOL so_readinterpolations(MFILE_READ fil)
+{
+    int32_t i;
+    SECTOR_OBJECTp sop;
+    so_interp *interp;
+    SWBOOL saveisshot = FALSE;
+
+    for (sop = SectorObject, interp = so_interpdata;
+         sop < &SectorObject[MAX_SECTOR_OBJECTS]; sop++, interp++)
+    {
+        so_interp::interp_data *data = interp->data;
+        MREAD(&interp->numinterpolations,sizeof(interp->numinterpolations),1,fil);
+        for (i = 0; i < interp->numinterpolations; i++, data++)
+        {
+            saveisshot |= LoadSymDataInfo(fil, (void **)&data->curipos);
+            MREAD(&data->oldipos,sizeof(data->oldipos),1,fil);
+            MREAD(&data->spriteofang,sizeof(data->spriteofang),1,fil);
+            data->lastipos = data->lastoldipos = data->oldipos;
+            data->lastangdiff = 0;
+        }
+        interp->tic = 0;
+        interp->lasttic = synctics;
+    }
+    return saveisshot;
+}
diff --git a/source/sw/src/interpso.h b/source/sw/src/interpso.h
new file mode 100644
index 000000000..a509782f8
--- /dev/null
+++ b/source/sw/src/interpso.h
@@ -0,0 +1,45 @@
+//-------------------------------------------------------------------------
+/*
+Copyright (C) 1997, 2005 - 3D Realms Entertainment
+
+This file is part of Shadow Warrior version 1.2
+
+Shadow Warrior is free software; you can redistribute it and/or
+modify it under the terms of the GNU General Public License
+as published by the Free Software Foundation; either version 2
+of the License, or (at your option) any later version.
+
+This program is distributed in the hope that it will be useful,
+but WITHOUT ANY WARRANTY; without even the implied warranty of
+MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.
+
+See the GNU General Public License for more details.
+
+You should have received a copy of the GNU General Public License
+along with this program; if not, write to the Free Software
+Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA  02110-1301, USA.
+
+Original Source: 1997 - Frank Maddin and Jim Norwood
+Prepared for public release: 03/28/2005 - Charlie Wiederhold, 3D Realms
+*/
+//-------------------------------------------------------------------------
+
+#ifndef INTERPSO_H
+#define INTERPSO_H
+
+#include "mfile.h"
+
+extern int32_t so_numinterpolations;
+
+void so_addinterpolation(SECTOR_OBJECTp sop);
+void so_setspriteinterpolation(SECTOR_OBJECTp sop, spritetype *sp);
+void so_stopspriteinterpolation(SECTOR_OBJECTp sop, spritetype *sp);
+void so_setinterpolationangdiff(SECTOR_OBJECTp sop, int16_t angdiff);
+void so_setinterpolationtics(SECTOR_OBJECTp sop, int16_t locktics);
+void so_updateinterpolations(void);
+void so_dointerpolations(int32_t smoothratio);
+void so_restoreinterpolations(void);
+SWBOOL so_writeinterpolations(MFILE_WRITE fil);
+SWBOOL so_readinterpolations(MFILE_READ fil);
+
+#endif