GLES1 renderer: discard framebuffer

Available only on GLES1, allows to use a "performance hint" to the
GPU to discard the contents of depth and stencil buffers after each
frame. Some hardware might want to reuse them, but Quake 2 doesn't
work that way.
Controlled by gl1_discardfb cvar, default `1`.

doc/040_cvarlist.md

@@ -492,6 +492,10 @@ it's `+set busywait 0` (setting the `busywait` cvar) and `-portable`
 * **gl1_stencilshadow**: If `gl_shadows` is set to `1`, this makes them
   look a bit better (no flickering) by using the stencil buffer.
 
+* **gl1_discardfb**: Only available in ES1. If set to `1` (default),
+  send a hint to discard framebuffers after finishing a frame. Useful
+  for GPUs that attempt to reuse them, something Quake 2 doesn't do.
+
 
 ## Graphics (OpenGL 3.2 and OpenGL ES3 only)
 

src/client/refresh/gl1/gl1_main.c

@@ -91,6 +91,7 @@ cvar_t *gl1_particle_square;
 cvar_t *gl1_palettedtexture;
 cvar_t *gl1_pointparameters;
 cvar_t *gl1_multitexture;
+cvar_t *gl1_discardfb;
 
 cvar_t *gl_drawbuffer;
 cvar_t *gl_lightmap;
@@ -1224,6 +1225,9 @@ R_Register(void)
 	gl1_palettedtexture = ri.Cvar_Get("r_palettedtextures", "0", CVAR_ARCHIVE);
 	gl1_pointparameters = ri.Cvar_Get("gl1_pointparameters", "1", CVAR_ARCHIVE);
 	gl1_multitexture = ri.Cvar_Get("gl1_multitexture", "1", CVAR_ARCHIVE);
+#ifdef YQ2_GL1_GLES
+	gl1_discardfb = ri.Cvar_Get("gl1_discardfb", "1", CVAR_ARCHIVE);
+#endif
 
 	gl_drawbuffer = ri.Cvar_Get("gl_drawbuffer", "GL_BACK", 0);
 	r_vsync = ri.Cvar_Get("r_vsync", "1", CVAR_ARCHIVE);
@@ -1641,6 +1645,43 @@ RI_Init(void)
 
 	// ----
 
+	/* Discard framebuffer: Available only on GLES1, enables the use of a "performance hint"
+	 * to the graphic driver, to get rid of the contents of the depth and stencil buffers.
+	 * Useful for some GPUs that may attempt to keep them and/or write them back to
+	 * external/uniform memory, actions that are useless for Quake 2 rendering path.
+	 * https://registry.khronos.org/OpenGL/extensions/EXT/EXT_discard_framebuffer.txt
+	 */
+	gl_config.discardfb = false;
+
+#ifdef YQ2_GL1_GLES
+	R_Printf(PRINT_ALL, " - Discard framebuffer: ");
+
+	if (strstr(gl_config.extensions_string, "GL_EXT_discard_framebuffer"))
+	{
+			qglDiscardFramebufferEXT = (void (APIENTRY *)(GLenum, GLsizei, const GLenum *))
+					RI_GetProcAddress ("glDiscardFramebufferEXT");
+	}
+
+	if (gl1_discardfb->value)
+	{
+		if (qglDiscardFramebufferEXT)
+		{
+			gl_config.discardfb = true;
+			R_Printf(PRINT_ALL, "Okay\n");
+		}
+		else
+		{
+			R_Printf(PRINT_ALL, "Failed\n");
+		}
+	}
+	else
+	{
+		R_Printf(PRINT_ALL, "Disabled\n");
+	}
+#endif
+
+	// ----
+
 	/* Big lightmaps: this used to be fast, but after the implementation of the "GL Buffer", it
 	 * became too evident that the bigger the texture, the slower the call to glTexSubImage2D() is.
 	 * Original logic remains, but it's preferable not to make it visible to the user.

src/client/refresh/gl1/gl1_sdl.c

@@ -47,6 +47,13 @@ void
 RI_EndFrame(void)
 {
 	R_ApplyGLBuffer();	// to draw buffered 2D text
+#ifdef YQ2_GL1_GLES
+	if (gl_config.discardfb)
+	{
+		static const GLenum attachments[] = { GL_DEPTH_EXT, GL_STENCIL_EXT };
+		qglDiscardFramebufferEXT(GL_FRAMEBUFFER_OES, 2, attachments);
+	}
+#endif
 	SDL_GL_SwapWindow(window);
 }
 

src/client/refresh/gl1/header/local.h

@@ -429,6 +429,7 @@ typedef struct
 	qboolean palettedtexture;
 	qboolean pointparameters;
 	qboolean multitexture;
+	qboolean discardfb;
 
 	// ----
 

src/client/refresh/gl1/header/qgl.h

@@ -100,5 +100,7 @@ extern void ( APIENTRY *qglColorTableEXT ) ( GLenum, GLenum, GLsizei, GLenum,
 		GLenum, const GLvoid * );
 extern void ( APIENTRY *qglActiveTexture ) ( GLenum texture );
 extern void ( APIENTRY *qglClientActiveTexture ) ( GLenum texture );
+extern void ( APIENTRY *qglDiscardFramebufferEXT ) ( GLenum target,
+		GLsizei numAttachments, const GLenum *attachments );
 
 #endif

src/client/refresh/gl1/qgl.c

@@ -44,6 +44,8 @@ void (APIENTRY *qglColorTableEXT)(GLenum, GLenum, GLsizei, GLenum, GLenum,
 		const GLvoid *);
 void (APIENTRY *qglActiveTexture) (GLenum texture);
 void (APIENTRY *qglClientActiveTexture) (GLenum texture);
+void (APIENTRY *qglDiscardFramebufferEXT) (GLenum target,
+		GLsizei numAttachments, const GLenum *attachments);
 
 /* ========================================================================= */
 
@@ -54,6 +56,7 @@ void QGL_EXT_Reset ( void )
 	qglColorTableEXT       = NULL;
 	qglActiveTexture       = NULL;
 	qglClientActiveTexture = NULL;
+	qglDiscardFramebufferEXT = NULL;
 }
 
 /* ========================================================================= */