Index: tr_backend.c
===================================================================
--- tr_backend.c	(revision 2260)
+++ tr_backend.c	(working copy)
@@ -509,7 +509,44 @@
 	}
 }
 
+void
+interlace_stencil(int gliWindowWidth, int gliWindowHeight)
+{
+	GLint gliY;
+	// 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_ALWAYS,1,1); // to avoid interaction with stencil content
+	
+	// drawing stencil pattern
+	//qglColor4f(1,1,1,0);	// alfa is 0 not to interfere with alpha tests
+	for (gliY=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
+	qglFlush();
+}
 
+
 #define	MAC_EVENT_PUMP_MSEC		5
 
 /*
@@ -683,6 +720,9 @@
 		qglDepthRange (0, 1);
 	}
 
+	if (r_stereoEnabled->integer == 2)
+		interlace_stencil(glConfig.vidWidth, glConfig.vidHeight);
+
 #if 0
 	RB_DrawSun();
 #endif
@@ -961,9 +1001,17 @@
 	const drawBufferCommand_t	*cmd;
 
 	cmd = (const drawBufferCommand_t *)data;
+	
+	if (r_stereoEnabled->integer == 2) {
+		qglDrawBuffer( GL_BACK );
+		if (cmd->buffer == GL_BACK_LEFT)
+			glStencilFunc(GL_NOTEQUAL,1,1);
+		else
+			glStencilFunc(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 );