Index: tr_backend.c
===================================================================
--- tr_backend.c	(revision 2260)
+++ tr_backend.c	(working copy)
@@ -507,9 +507,54 @@
 	} else {
 		qglDisable (GL_CLIP_PLANE0);
 	}
+
+	backEnd.doneStereoInterlace = qfalse;
 }
 
+void
+RB_InterlaceStencil(int gliWindowWidth, int gliWindowHeight)
+{
+	GLint gliY;
 
+	if (backEnd.doneStereoInterlace == qtrue || r_stereoEnabled->integer < 2) return;
+	// r_stereoEnabled 2 : left-right interlaced output
+	// r_stereoEnabled 3 : right-left interlaced output
+
+	backEnd.doneStereoInterlace = qtrue;
+
+	// seting screen-corresponding geometry
+	qglViewport(0,0,gliWindowWidth,gliWindowHeight);
+	qglMatrixMode(GL_MODELVIEW);
+	qglLoadIdentity();
+	qglMatrixMode (GL_PROJECTION);
+	qglLoadIdentity();
+	qglOrtho(0.0,gliWindowWidth-1,0.0,gliWindowHeight-1,-1,1);
+	qglMatrixMode(GL_MODELVIEW);
+	qglLoadIdentity();
+		
+	// clearing and configuring stencil drawing
+	qglDrawBuffer(GL_BACK);
+	qglEnable(GL_STENCIL_TEST);
+	qglClearStencil(0);
+	qglClear(GL_STENCIL_BUFFER_BIT);
+	qglStencilOp (GL_REPLACE, GL_REPLACE, GL_REPLACE); // colorbuffer is copied to stencil
+	qglDisable(GL_DEPTH_TEST);
+	qglStencilFunc(GL_NEVER,1,1); // to avoid interaction with stencil content
+	
+	// drawing stencil pattern
+	qglColor4f(1,1,1,0);	// alpha is 0 to not interfere with alpha tests
+	for (gliY=(r_stereoEnabled->integer == 3 ? 1 : 0); gliY<gliWindowHeight; gliY+=2)
+	{
+		qglLineWidth(1);
+		qglBegin(GL_LINES);
+			qglVertex2f(0,gliY);
+			qglVertex2f(gliWindowWidth,gliY);
+		qglEnd();	
+	}
+	qglStencilOp (GL_KEEP, GL_KEEP, GL_KEEP); // disabling changes in stencil buffer
+}
+
+
 #define	MAC_EVENT_PUMP_MSEC		5
 
 /*
@@ -691,6 +737,9 @@
 
 	// add light flares on lights that aren't obscured
 	RB_RenderFlares();
+
+	// Interlace screen
+	RB_InterlaceStencil(glConfig.vidWidth, glConfig.vidHeight);
 }
 
 
@@ -711,6 +760,9 @@
 void	RB_SetGL2D (void) {
 	backEnd.projection2D = qtrue;
 
+	if (r_stereoEnabled->integer >= 2) // 2d must be drawn on both eyes
+		qglStencilFunc(GL_ALWAYS, 1, 1);
+
 	// set 2D virtual screen size
 	qglViewport( 0, 0, glConfig.vidWidth, glConfig.vidHeight );
 	qglScissor( 0, 0, glConfig.vidWidth, glConfig.vidHeight );
@@ -961,9 +1013,17 @@
 	const drawBufferCommand_t	*cmd;
 
 	cmd = (const drawBufferCommand_t *)data;
+	
+	if (r_stereoEnabled->integer >= 2) {
+		qglDrawBuffer( GL_BACK );
+		if (cmd->buffer == GL_BACK_LEFT)
+			qglStencilFunc(GL_NOTEQUAL,1,1);
+		else
+			qglStencilFunc(GL_EQUAL,1,1);
+	}
+	else
+		qglDrawBuffer( cmd->buffer );
 
-	qglDrawBuffer( cmd->buffer );
-
 	// clear screen for debugging
 	if ( r_clear->integer ) {
 		qglClearColor( 1, 0, 0.5, 1 );
@@ -1079,6 +1139,9 @@
 const void	*RB_SwapBuffers( const void *data ) {
 	const swapBuffersCommand_t	*cmd;
 
+	if (r_stereoEnabled->integer >= 2)
+		qglStencilFunc(GL_ALWAYS, 1, 1);
+
 	// finish any 2D drawing if needed
 	if ( tess.numIndexes ) {
 		RB_EndSurface();
@@ -1120,6 +1183,8 @@
 
 	backEnd.projection2D = qfalse;
 
+	backEnd.doneStereoInterlace = qfalse;
+
 	return (const void *)(cmd + 1);
 }
 
Index: tr_cmds.c
===================================================================
--- tr_cmds.c	(revision 2260)
+++ tr_cmds.c	(working copy)
@@ -441,14 +441,17 @@
 		{
 			ri.Printf( PRINT_ALL, "Warning: not enough stencil bits to measure overdraw: %d\n", glConfig.stencilBits );
 			ri.Cvar_Set( "r_measureOverdraw", "0" );
-			r_measureOverdraw->modified = qfalse;
 		}
 		else if ( r_shadows->integer == 2 )
 		{
 			ri.Printf( PRINT_ALL, "Warning: stencil shadows and overdraw measurement are mutually exclusive\n" );
 			ri.Cvar_Set( "r_measureOverdraw", "0" );
-			r_measureOverdraw->modified = qfalse;
 		}
+		else if ( r_stereoEnabled->integer >= 2 )
+		{
+			ri.Printf( PRINT_ALL, "Warning: interlaced stereo and overdraw measurement are mutually exclusive\n" );
+			ri.Cvar_Set( "r_measureOverdraw", "0" );
+		}
 		else
 		{
 			R_SyncRenderThread();
Index: tr_local.h
===================================================================
--- tr_local.h	(revision 2260)
+++ tr_local.h	(working copy)
@@ -872,6 +872,8 @@
 	byte		color2D[4];
 	qboolean	vertexes2D;		// shader needs to be finished
 	trRefEntity_t	entity2D;	// currentEntity will point at this when doing 2D rendering
+
+	qboolean doneStereoInterlace;
 } backEndState_t;
 
 /*
@@ -1707,5 +1710,7 @@
 void R_DoneFreeType( void );
 void RE_RegisterFont(const char *fontName, int pointSize, fontInfo_t *font);
 
+// Stereo Interlace
+void RB_InterlaceStencil( int gliWindowWidth, int gliWindowHeight );
 
 #endif //TR_LOCAL_H