diff --git a/src/am_map.cpp b/src/am_map.cpp
index 7a556542a..a8076550c 100644
--- a/src/am_map.cpp
+++ b/src/am_map.cpp
@@ -2070,7 +2070,7 @@ void AM_drawSubsectors()
 				originx, originy,
 				scale / (FIXED2DBL(scalex) * float(1 << MAPBITS)),
 				scale / (FIXED2DBL(scaley) * float(1 << MAPBITS)),
-				rotation,
+				ANGLE2DBL(rotation),
 				colormap,
 				floorlight
 				);
diff --git a/src/v_draw.cpp b/src/v_draw.cpp
index 8d3453fa4..2d3b1e3e2 100644
--- a/src/v_draw.cpp
+++ b/src/v_draw.cpp
@@ -1152,7 +1152,7 @@ void DCanvas::Clear (int left, int top, int right, int bottom, int palcolor, uin
 //==========================================================================
 
 void DCanvas::FillSimplePoly(FTexture *tex, FVector2 *points, int npoints,
-	double originx, double originy, double scalex, double scaley, angle_t rotation,
+	double originx, double originy, double scalex, double scaley, DAngle rotation,
 	FDynamicColormap *colormap, int lightlevel)
 {
 #ifndef NO_SWRENDER
@@ -1163,8 +1163,7 @@ void DCanvas::FillSimplePoly(FTexture *tex, FVector2 *points, int npoints,
 	int i;
 	int y1, y2, y;
 	fixed_t x;
-	double rot = rotation * M_PI / double(1u << 31);
-	bool dorotate = rot != 0;
+	bool dorotate = rotation != 0.;
 	double cosrot, sinrot;
 
 	if (--npoints < 2 || Buffer == NULL)
@@ -1205,8 +1204,9 @@ void DCanvas::FillSimplePoly(FTexture *tex, FVector2 *points, int npoints,
 	scalex /= FIXED2DBL(tex->xScale);
 	scaley /= FIXED2DBL(tex->yScale);
 
-	cosrot = cos(rot);
-	sinrot = sin(rot);
+	// Use the CRT's functions here.
+	cosrot = cos(ToRadians(rotation));
+	sinrot = sin(ToRadians(rotation));
 
 	// Setup constant texture mapping parameters.
 	R_SetupSpanBits(tex);
diff --git a/src/v_video.h b/src/v_video.h
index 269641373..533adb52e 100644
--- a/src/v_video.h
+++ b/src/v_video.h
@@ -180,7 +180,7 @@ public:
 
 	// Fill a simple polygon with a texture
 	virtual void FillSimplePoly(FTexture *tex, FVector2 *points, int npoints,
-		double originx, double originy, double scalex, double scaley, angle_t rotation,
+		double originx, double originy, double scalex, double scaley, DAngle rotation,
 		struct FDynamicColormap *colormap, int lightlevel);
 
 	// Set an area to a specified color
diff --git a/src/win32/fb_d3d9.cpp b/src/win32/fb_d3d9.cpp
index 17c548c29..8a6e7e50f 100644
--- a/src/win32/fb_d3d9.cpp
+++ b/src/win32/fb_d3d9.cpp
@@ -3071,7 +3071,7 @@ void D3DFB::FlatFill(int left, int top, int right, int bottom, FTexture *src, bo
 
 void D3DFB::FillSimplePoly(FTexture *texture, FVector2 *points, int npoints,
 	double originx, double originy, double scalex, double scaley,
-	angle_t rotation, FDynamicColormap *colormap, int lightlevel)
+	DAngle rotation, FDynamicColormap *colormap, int lightlevel)
 {
 	// Use an equation similar to player sprites to determine shade
 	fixed_t shade = LIGHT2SHADE(lightlevel) - 12*FRACUNIT;
@@ -3083,8 +3083,7 @@ void D3DFB::FillSimplePoly(FTexture *texture, FVector2 *points, int npoints,
 	D3DCOLOR color0, color1;
 	float ox, oy;
 	float cosrot, sinrot;
-	float rot = float(rotation * M_PI / float(1u << 31));
-	bool dorotate = rot != 0;
+	bool dorotate = rotation != 0;
 
 	if (npoints < 3)
 	{ // This is no polygon.
@@ -3105,8 +3104,8 @@ void D3DFB::FillSimplePoly(FTexture *texture, FVector2 *points, int npoints,
 		return;
 	}
 
-	cosrot = cos(rot);
-	sinrot = sin(rot);
+	cosrot = (float)cos(ToRadians(rotation));
+	sinrot = (float)sin(ToRadians(rotation));
 
 	CheckQuadBatch(npoints - 2, npoints);
 	quad = &QuadExtra[QuadBatchPos];
diff --git a/src/win32/win32iface.h b/src/win32/win32iface.h
index 00a34578a..f8fcd1fba 100644
--- a/src/win32/win32iface.h
+++ b/src/win32/win32iface.h
@@ -265,7 +265,7 @@ public:
 	void DrawPixel(int x, int y, int palcolor, uint32 rgbcolor);
 	void FillSimplePoly(FTexture *tex, FVector2 *points, int npoints,
 		double originx, double originy, double scalex, double scaley,
-		angle_t rotation, FDynamicColormap *colormap, int lightlevel);
+		DAngle rotation, FDynamicColormap *colormap, int lightlevel);
 	bool WipeStartScreen(int type);
 	void WipeEndScreen();
 	bool WipeDo(int ticks);