diff --git a/stepmania/src/BGAnimation.cpp b/stepmania/src/BGAnimation.cpp
index 3cde785952..331e935964 100644
--- a/stepmania/src/BGAnimation.cpp
+++ b/stepmania/src/BGAnimation.cpp
@@ -59,7 +59,7 @@ void AddLayersFromAniDir( CString sAniDir, vector<Actor*> &layersAddTo, bool Gen
 		CString expr;
 		if( ini.GetValue( "BGAnimation", "Condition", expr ) || ini.GetValue( "BGAnimation", "Cond", expr ) )
 		{
-			if( !Lua::RunExpression( expr ) )
+			if( !Lua::RunExpressionB( expr ) )
 				return;
 		}
 	}
@@ -77,7 +77,7 @@ void AddLayersFromAniDir( CString sAniDir, vector<Actor*> &layersAddTo, bool Gen
 			CString expr;
 			if( ini.GetValue(sLayer,"Condition",expr) )
 			{
-				if( !Lua::RunExpression( expr ) )
+				if( !Lua::RunExpressionB( expr ) )
 					continue;
 			}
 
diff --git a/stepmania/src/LuaHelpers.cpp b/stepmania/src/LuaHelpers.cpp
index c420fa3fee..eaeb9949de 100644
--- a/stepmania/src/LuaHelpers.cpp
+++ b/stepmania/src/LuaHelpers.cpp
@@ -3,6 +3,7 @@
 #include "LuaFunctions.h"
 #include "RageUtil.h"
 #include "RageLog.h"
+#include "ScreenDimensions.h"
 
 #include <csetjmp>
 #include <cassert>
@@ -129,12 +130,15 @@ static int LuaPanic( lua_State *L )
 	longjmp( jbuf, 1 );
 }
 
-bool Lua::RunExpression( const CString &str )
+lua_State *L = NULL;
+
+void OpenLua()
 {
-try {
+	ASSERT( L == NULL );
+
 	if( setjmp(jbuf) )
 		RageException::Throw("%s", jbuf_error.c_str());
-	lua_State *L = lua_open();
+	L = lua_open();
 	ASSERT( L );
 
 	lua_atpanic( L, LuaPanic );
@@ -145,7 +149,43 @@ try {
 	lua_settop(L, 0); // luaopen_* pushes stuff onto the stack that we don't need
 
 	Lua::RegisterFunctions( L );
+}
 
+void CloseLua()
+{
+	ASSERT( L );
+
+	lua_close( L );
+	L = NULL;
+}
+
+void Lua::UpdateGlobals()
+{
+	if( L == NULL )
+		OpenLua();
+
+	ASSERT( L );
+
+	lua_pushnumber( L, SCREEN_WIDTH );
+	lua_setglobal( L, "SCREEN_WIDTH" );
+	lua_pushnumber( L, SCREEN_HEIGHT );
+	lua_setglobal( L, "SCREEN_HEIGHT" );
+	lua_pushnumber( L, SCREEN_LEFT );
+	lua_setglobal( L, "SCREEN_LEFT" );
+	lua_pushnumber( L, SCREEN_RIGHT );
+	lua_setglobal( L, "SCREEN_RIGHT" );
+	lua_pushnumber( L, SCREEN_TOP );
+	lua_setglobal( L, "SCREEN_TOP" );
+	lua_pushnumber( L, SCREEN_BOTTOM );
+	lua_setglobal( L, "SCREEN_BOTTOM" );
+	lua_pushnumber( L, CENTER_X );
+	lua_setglobal( L, "SCREEN_CENTER_X" );
+	lua_pushnumber( L, CENTER_Y );
+	lua_setglobal( L, "SCREEN_CENTER_Y" );
+}
+
+void RunExpression( const CString &str )
+{
 	LoadFromString( L, "return " + str );
 	ASSERT_M( lua_gettop(L) == 1, ssprintf("%i", lua_gettop(L)) );
 
@@ -163,16 +203,40 @@ try {
 	 * as a boolean, convert it before returning. */
 	if( lua_isfunction( L, -1 ) )
 		throw CString( "result is a function; did you forget \"()\"?" );
-
-	bool result = !!lua_toboolean( L, -1 );
-	lua_pop( L, -1 );
-
-	lua_close( L );
-
-	return result;
-} catch( const CString &err ) {
-	RageException::Throw( "Error running \"%s\": %s", str.c_str(), err.c_str() );
 }
+
+bool Lua::RunExpressionB( const CString &str )
+{
+	try {
+		if( L == NULL )
+			OpenLua();
+
+		RunExpression( str );
+
+		bool result = !!lua_toboolean( L, -1 );
+		lua_pop( L, -1 );
+
+		return result;
+	} catch( const CString &err ) {
+		RageException::Throw( "Error running \"%s\": %s", str.c_str(), err.c_str() );
+	}
+}
+
+float Lua::RunExpressionF( const CString &str )
+{
+	try {
+		if( L == NULL )
+			OpenLua();
+
+		RunExpression( str );
+
+		float result = lua_tonumber( L, -1 );
+		lua_pop( L, -1 );
+
+		return result;
+	} catch( const CString &err ) {
+		RageException::Throw( "Error running \"%s\": %s", str.c_str(), err.c_str() );
+	}
 }
 
 void Lua::Fail( lua_State *L, const CString &err )
diff --git a/stepmania/src/LuaHelpers.h b/stepmania/src/LuaHelpers.h
index d701f8c950..7860c2e30c 100644
--- a/stepmania/src/LuaHelpers.h
+++ b/stepmania/src/LuaHelpers.h
@@ -4,7 +4,8 @@
 struct lua_State;
 namespace Lua
 {
-	bool RunExpression( const CString &str );
+	bool RunExpressionB( const CString &str );
+	float RunExpressionF( const CString &str );
 
 	void Fail( lua_State *L, const CString &err );
 
@@ -17,6 +18,7 @@ namespace Lua
 	void PushStack( lua_State *L, const CString &out );
 	void PopStack( lua_State *L, CString &out );
 	bool GetStack( lua_State *L, int pos, int &out );
+	void UpdateGlobals(); // call this when the theme changes
 };
 
 #endif
diff --git a/stepmania/src/ScreenBranch.cpp b/stepmania/src/ScreenBranch.cpp
index a5460f0d86..41d1d3f943 100644
--- a/stepmania/src/ScreenBranch.cpp
+++ b/stepmania/src/ScreenBranch.cpp
@@ -22,7 +22,7 @@ ScreenBranch::ScreenBranch( CString sClassName ) : Screen( sClassName )
 		CString sChoice = Capitalize( as[i] );
 		CString sCondition = CONDITION(sChoice);
 
-		if( Lua::RunExpression(sCondition) )
+		if( Lua::RunExpressionB(sCondition) )
 		{
 			m_sChoice = sChoice;
 			HandleScreenMessage( SM_GoToNextScreen );
diff --git a/stepmania/src/ThemeManager.cpp b/stepmania/src/ThemeManager.cpp
index d8c9c64e5d..cd4ec45577 100644
--- a/stepmania/src/ThemeManager.cpp
+++ b/stepmania/src/ThemeManager.cpp
@@ -17,6 +17,7 @@
 #include "StepMania.h"
 #include "Foreach.h"
 #include "ThemeMetric.h"
+#include "LuaHelpers.h"
 
 
 ThemeManager*	THEME = NULL;	// global object accessable from anywhere in the program
@@ -247,6 +248,8 @@ void ThemeManager::SwitchThemeAndLanguage( CString sThemeName, CString sLanguage
 
 	// reload subscribers
 	FOREACH( IThemeMetric*, *g_pvpSubscribers, p ) (*p)->Read();
+
+	Lua::UpdateGlobals();
 }
 
 CString ThemeManager::GetThemeDirFromName( const CString &sThemeName )
@@ -611,19 +614,19 @@ int ThemeManager::GetMetricI( CString sClassName, CString sValueName )
 
 float ThemeManager::GetMetricF( CString sClassName, CString sValueName )
 {
-	return strtof( GetMetricRaw(sClassName,sValueName), NULL );
+	const CString str = GetMetricRaw( sClassName,sValueName );
+	return Lua::RunExpressionF( str );
 }
 
 // #include "LuaHelpers.h"
 bool ThemeManager::GetMetricB( CString sClassName, CString sValueName )
 {
-//	const CString str = GetMetricRaw( sClassName,sValueName );
-//	if( str == "0" )
-//		return false; /* optimization */
-//	if( str == "1" )
-//		return true; /* optimization */
-//	return Lua::RunExpression( str );
-	return atoi( GetMetricRaw(sClassName,sValueName) ) != 0;
+	const CString str = GetMetricRaw( sClassName,sValueName );
+	if( str == "0" )
+		return false; /* optimization */
+	if( str == "1" )
+		return true; /* optimization */
+	return Lua::RunExpressionB( str );
 }
 
 RageColor ThemeManager::GetMetricC( CString sClassName, CString sValueName )