Use face bounding quad to check if it may be partially inside the frustum, and force tessellation if it is.

I really don't like the additional CPU load required to do this, but it is necessary to reduce the amount of near-camera polygon clipping.

ps1bsp.h

@@ -106,10 +106,14 @@ typedef struct ps1bsp_face_s
 	unsigned short planeId;
 	unsigned char side;
 
+	// TODO: not sure if we should make separate lists for triangles and non-triangle polygons here
+	// Could potentially improve drawing performance but at the cost of more memory use per face
+
 	// Used for high-quality tesselated textured drawing
 	unsigned short firstPolygon;
 	unsigned char numPolygons;
-	unsigned char totalQuads;
+
+	unsigned char totalPrimitives;	// Used for early primitive buffer checks
 
 	// Used for low-quality untextured vertex colored drawing
 	unsigned short firstFaceVertex;
@@ -120,6 +124,7 @@ typedef struct ps1bsp_face_s
 
 	// Used for backface culling
 	SVECTOR center;
+	unsigned short bounds[4];
 
 	// Run-time data
 	const struct ps1bsp_face_s* nextFace;	// For chaining faces in drawing order

world.c

@@ -85,7 +85,21 @@ static INLINE short world_cull_backface(const world_t *world, const ps1bsp_face_
     // UPDATE: tessellation does solve this, to an extent. However we must also make sure not to discard faces partially inside the frustum, and force tessellation on those.
     int camDot = m_dot12(&cam_vec, &cam_dir);
     if (camDot < 0)
+    {
+        // Check if the face may be partially inside the frustum. If so, we draw it with a high amount of tessellation.
+        for (int i = 0; i < 4; ++i)
+        {
+            const ps1bsp_vertex_t *vert = &world->vertices[face->bounds[i]];
+            if (frustum_pointInside((SVECTOR*)vert))
+            {
+                // TODO: use a simplified formula with only face area to determine tessellation LOD
+                return 1;
+            }
+        }
+
+        // Face really isn't visible
         return 0;
+    }
 
     // Flip angle for faces that are on the opposite side of their plane
     char flip = 2 * face->side - 1;
@@ -191,7 +205,7 @@ static void world_drawFaces(const world_t *world, const ps1bsp_face_t *firstFace
     for (const ps1bsp_face_t *face = firstFace; face != NULL; face = face->nextFace)
     {
         // Early primitive buffer check
-        if (!mem_checkprim(sizeof(POLY_GT4), face->totalQuads))
+        if (!mem_checkprim(sizeof(POLY_GT4), face->totalPrimitives))
             break;
 
         world_drawface(world, face, curOT);