Convert Quakespasm to use interlaced R-L video for passive 3D monitors. See https://oldvcr.blogspot.com/2020/10/stereoscopic-computing-converting-quake.html

quakespasm3d.diff

--- gl_rmain.c.OLD	2020-08-02 16:35:29.798497930 -0700
+++ gl_rmain.c	2020-10-03 23:32:00.309704966 -0700
@@ -77,16 +77,17 @@
 cvar_t	gl_affinemodels = {"gl_affinemodels","0",CVAR_NONE};
 cvar_t	gl_polyblend = {"gl_polyblend","1",CVAR_NONE};
 cvar_t	gl_flashblend = {"gl_flashblend","0",CVAR_ARCHIVE};
 cvar_t	gl_playermip = {"gl_playermip","0",CVAR_NONE};
 cvar_t	gl_nocolors = {"gl_nocolors","0",CVAR_NONE};
 
 //johnfitz -- new cvars
 cvar_t	r_stereo = {"r_stereo","0",CVAR_NONE};
+cvar_t	r_stereomode = {"r_stereomode","3",CVAR_NONE};
 cvar_t	r_stereodepth = {"r_stereodepth","128",CVAR_NONE};
 cvar_t	r_clearcolor = {"r_clearcolor","2",CVAR_ARCHIVE};
 cvar_t	r_drawflat = {"r_drawflat","0",CVAR_NONE};
 cvar_t	r_flatlightstyles = {"r_flatlightstyles", "0", CVAR_NONE};
 cvar_t	gl_fullbrights = {"gl_fullbrights", "1", CVAR_ARCHIVE};
 cvar_t	gl_farclip = {"gl_farclip", "16384", CVAR_ARCHIVE};
 cvar_t	gl_overbright = {"gl_overbright", "1", CVAR_ARCHIVE};
 cvar_t	gl_overbright_models = {"gl_overbright_models", "1", CVAR_ARCHIVE};
@@ -109,16 +110,65 @@
 cvar_t	r_slimealpha = {"r_slimealpha","0",CVAR_NONE};
 
 float	map_wateralpha, map_lavaalpha, map_telealpha, map_slimealpha;
 
 qboolean r_drawflat_cheatsafe, r_fullbright_cheatsafe, r_lightmap_cheatsafe, r_drawworld_cheatsafe; //johnfitz
 
 cvar_t	r_scale = {"r_scale", "1", CVAR_ARCHIVE};
 
+qboolean stenciled = false;
+
+// 3D interlace using stencil buffer
+void
+RB_InterlaceStencil()
+{
+	GLint gliY;
+	if (stenciled) return;
+ 
+	// r_stereomode 1: old anaglyph
+	// r_stereomode 2: left-right, r_stereo 3: right-left
+
+	// init draw coordinates for stencil buffer
+	glViewport(0,0,glwidth,glheight);
+	glMatrixMode(GL_MODELVIEW);
+	glLoadIdentity();
+	glMatrixMode (GL_PROJECTION);
+	glLoadIdentity();
+	glOrtho(0.0,glwidth-1,0.0,glheight-1,-1,1);
+	glMatrixMode(GL_MODELVIEW);
+	glLoadIdentity();
+		
+	// draw interlaced stencil
+	glEnable(GL_STENCIL_TEST);
+	glDisable(GL_DEPTH_TEST);
+	glClear(GL_STENCIL_BUFFER_BIT | GL_DEPTH_BUFFER_BIT);
+	glClearStencil(0);
+	glColorMask(GL_FALSE, GL_FALSE, GL_FALSE, GL_FALSE);
+	glDepthMask(GL_FALSE);
+	glStencilFunc(GL_NEVER, 1, 0xff);
+	glStencilOp(GL_REPLACE, GL_KEEP, GL_KEEP);
+	glStencilMask(0xff);
+	glClear(GL_STENCIL_BUFFER_BIT);
+
+	for (gliY=(r_stereo.value == 3 ? 1 : 0); gliY<glheight; gliY+=2)
+	{
+		glLineWidth(1);
+		glBegin(GL_LINES);
+			glVertex2f(0,gliY);
+			glVertex2f(glwidth,gliY);
+		glEnd();	
+	}
+
+	glColorMask(GL_TRUE, GL_TRUE, GL_TRUE, GL_TRUE);
+	glDepthMask(GL_TRUE);
+	glStencilMask(0x00);
+	stenciled = true;
+}
+
 //==============================================================================
 //
 // GLSL GAMMA CORRECTION
 //
 //==============================================================================
 
 static GLuint r_gamma_texture;
 static GLuint r_gamma_program;
@@ -518,19 +568,16 @@
 R_Clear -- johnfitz -- rewritten and gutted
 =============
 */
 void R_Clear (void)
 {
 	unsigned int clearbits;
 
 	clearbits = GL_DEPTH_BUFFER_BIT;
-	// from mh -- if we get a stencil buffer, we should clear it, even though we don't use it
-	if (gl_stencilbits)
-		clearbits |= GL_STENCIL_BUFFER_BIT;
 	if (gl_clear.value)
 		clearbits |= GL_COLOR_BUFFER_BIT;
 	glClear (clearbits);
 }
 
 /*
 ===============
 R_SetupScene -- johnfitz -- this is the stuff that needs to be done once per eye in stereo mode
@@ -870,42 +917,29 @@
 */
 void R_DrawShadows (void)
 {
 	int i;
 
 	if (!r_shadows.value || !r_drawentities.value || r_drawflat_cheatsafe || r_lightmap_cheatsafe)
 		return;
 
-	// Use stencil buffer to prevent self-intersecting shadows, from Baker (MarkV)
-	if (gl_stencilbits)
-	{
-		glClear(GL_STENCIL_BUFFER_BIT);
-		glStencilFunc(GL_EQUAL, 0, ~0);
-		glStencilOp(GL_KEEP, GL_KEEP, GL_INCR);
-		glEnable(GL_STENCIL_TEST);
-	}
-
 	for (i=0 ; i<cl_numvisedicts ; i++)
 	{
 		currententity = cl_visedicts[i];
 
 		if (currententity->model->type != mod_alias)
 			continue;
 
 		if (currententity == &cl.viewent)
 			return;
 
 		GL_DrawAliasShadow (currententity);
 	}
 
-	if (gl_stencilbits)
-	{
-		glDisable(GL_STENCIL_TEST);
-	}
 }
 
 /*
 ================
 R_RenderScene
 ================
 */
 void R_RenderScene (void)
@@ -1082,36 +1116,65 @@
 	if (r_stereo.value)
 	{
 		float eyesep = CLAMP(-8.0f, r_stereo.value, 8.0f);
 		float fdepth = CLAMP(32.0f, r_stereodepth.value, 1024.0f);
 
 		AngleVectors (r_refdef.viewangles, vpn, vright, vup);
 
 		//render left eye (red)
-		glColorMask(1, 0, 0, 1);
+		if (r_stereomode.value == 1)
+		{
+			glColorMask(1, 0, 0, 1);
+		}
+		else
+		{
+			// 0 = first line (2), 1 = second (3)
+			RB_InterlaceStencil();
+			glStencilFunc(GL_EQUAL,
+				(r_stereomode.value == 2) ? 0 : 1,
+				0xff);
+		}
 
 		VectorMA (r_refdef.vieworg, -0.5f * eyesep, vright, r_refdef.vieworg);
 		frustum_skew = 0.5 * eyesep * NEARCLIP / fdepth;
 		srand((int) (cl.time * 1000)); //sync random stuff between eyes
 
 		R_RenderScene ();
 
 		//render right eye (cyan)
 		glClear (GL_DEPTH_BUFFER_BIT);
-		glColorMask(0, 1, 1, 1);
+
+		if (r_stereomode.value == 1)
+		{
+			glColorMask(0, 1, 1, 1);
+		}
+		else
+		{
+			glStencilFunc(GL_EQUAL,
+				(r_stereomode.value == 2) ? 1 : 0,
+				0xff);
+		}
 
 		VectorMA (r_refdef.vieworg, 1.0f * eyesep, vright, r_refdef.vieworg);
 		frustum_skew = -frustum_skew;
 		srand((int) (cl.time * 1000)); //sync random stuff between eyes
 
 		R_RenderScene ();
 
 		//restore
-		glColorMask(1, 1, 1, 1);
+		if (r_stereomode.value == 1)
+		{
+			glColorMask(1, 1, 1, 1);
+		}
+		else
+		{
+			glStencilFunc(GL_ALWAYS, 1, 0xff);
+		}
+		
 		VectorMA (r_refdef.vieworg, -0.5f * eyesep, vright, r_refdef.vieworg);
 		frustum_skew = 0.0f;
 	}
 	else
 	{
 		R_RenderScene ();
 	}
 	//johnfitz
--- gl_rmisc.c.OLD	2020-10-04 18:40:48.710076545 -0700
+++ gl_rmisc.c	2020-10-03 23:34:15.643555455 -0700
@@ -21,16 +21,17 @@
 
 */
 // r_misc.c
 
 #include "quakedef.h"
 
 //johnfitz -- new cvars
 extern cvar_t r_stereo;
+extern cvar_t r_stereomode;
 extern cvar_t r_stereodepth;
 extern cvar_t r_clearcolor;
 extern cvar_t r_drawflat;
 extern cvar_t r_flatlightstyles;
 extern cvar_t gl_fullbrights;
 extern cvar_t gl_farclip;
 extern cvar_t gl_overbright;
 extern cvar_t gl_overbright_models;
@@ -200,16 +201,17 @@
 	Cvar_RegisterVariable (&gl_affinemodels);
 	Cvar_RegisterVariable (&gl_polyblend);
 	Cvar_RegisterVariable (&gl_flashblend);
 	Cvar_RegisterVariable (&gl_playermip);
 	Cvar_RegisterVariable (&gl_nocolors);
 
 	//johnfitz -- new cvars
 	Cvar_RegisterVariable (&r_stereo);
+	Cvar_RegisterVariable (&r_stereomode);
 	Cvar_RegisterVariable (&r_stereodepth);
 	Cvar_RegisterVariable (&r_clearcolor);
 	Cvar_SetCallback (&r_clearcolor, R_SetClearColor_f);
 	Cvar_RegisterVariable (&r_waterquality);
 	Cvar_RegisterVariable (&r_oldwater);
 	Cvar_RegisterVariable (&r_waterwarp);
 	Cvar_RegisterVariable (&r_drawflat);
 	Cvar_RegisterVariable (&r_flatlightstyles);