RENDERER: Add option to filter out collinear vertices.

help_variables.json

@@ -15627,6 +15627,23 @@
       "remarks": "See also: r_nearclip.",
       "type": "float"
     },
+    "r_remove_collinear_vertices": {
+        "default": "0",
+        "desc": "Filter out vertices causing triangles to have zero area. These types of triangles are a sore spot in some GPUs that risk introducing glitches. An expected side effect is a risk of seeing a sparkling pixel.",
+        "group-id": "35",
+        "remarks": "For macOS arm64 this is force-enabled to prevent cheating. This filtering is a temporary workaround before adding code to avoid generating such triangles in the first place.",
+        "type": "boolean",
+        "values": [
+            {
+                "description": "Disable filtering of collinear vertices.",
+                "name": "false"
+            },
+            {
+                "description": "Enable filtering of collinear vertices.",
+                "name": "true"
+            }
+        ]
+    },
     "r_fastsky": {
       "default": "0",
       "group-id": "51",

src/r_lightmaps.c

@@ -590,9 +590,56 @@ static int LightmapAllocBlock(int w, int h, int *x, int *y)
 	return LightmapAllocBlock(w, h, x, y);
 }
 
+#define EPSILON 1e-6
+
+// Check if triangle has a ~zero area
+// https://en.wikipedia.org/wiki/Collinearity
+static qbool R_ArePointsColinear(const vec3_t v1, const vec3_t v2, const vec3_t v3)
+{
+	vec3_t d0, d1, cross;
+
+	VectorSubtract(v2, v1, d0);
+	VectorSubtract(v3, v2, d1);
+
+	CrossProduct(d0, d1, cross);
+
+	return DotProduct(cross, cross) < EPSILON;
+}
+
+static void R_RemoveColinearVertices(glpoly_t *poly, float new_verts[][VERTEXSIZE])
+{
+	int i, v1_index, v2_index, v3_index, new_numverts = 0;
+	int numverts = poly->numverts;
+
+	v1_index = numverts - 1;
+	v2_index = 0;
+	v3_index = 1;
+
+	for (i = 0; i < numverts; i++) {
+		float *v1 = poly->verts[v1_index];
+		float *v2 = poly->verts[v2_index];
+		float *v3 = poly->verts[v3_index];
+
+		if (!R_ArePointsColinear(v1, v2, v3)) {
+			memcpy(new_verts[new_numverts], v2, sizeof(float) * VERTEXSIZE);
+			new_numverts++;
+		}
+
+		v1_index = v2_index;
+		v2_index = v3_index;
+		v3_index = (v3_index + 1) % numverts;
+	}
+
+	if (new_numverts > 0) {
+		memcpy(poly->verts, new_verts, new_numverts * sizeof(float) * VERTEXSIZE);
+		poly->numverts = new_numverts;
+	}
+}
+
 // 
 static void R_BuildSurfaceDisplayList(model_t* currentmodel, msurface_t *fa)
 {
+	extern cvar_t r_remove_collinear_vertices;
 	int i, lindex, lnumverts;
 	medge_t *pedges, *r_pedge;
 	float *vec, s, t;
@@ -684,8 +731,19 @@ static void R_BuildSurfaceDisplayList(model_t* currentmodel, msurface_t *fa)
 	}
 	poly->numverts = lnumverts;
 
+	// Some GPUs misbehave if fed triangles of empty size.
+	if (r_remove_collinear_vertices.value) {
+		if (poly->numverts > 4) {
+			float (*new_verts)[VERTEXSIZE] = Q_malloc(poly->numverts * sizeof(float[VERTEXSIZE]));
+			R_RemoveColinearVertices(poly, new_verts);
+			Q_free(new_verts);
+		} else {
+			float new_verts[4][VERTEXSIZE];
+			R_RemoveColinearVertices(poly, new_verts);
+		}
+	}
+
 	R_BrushModelPolygonToTriangleStrip(poly);
-	return;
 }
 
 static void R_BuildLightmapData(msurface_t* surf, int surfnum)

src/r_rmain.c

@@ -114,6 +114,15 @@ static void OnDynamicLightingChange(cvar_t* var, char* value, qbool* cancel)
 	}
 }
 
+static void OnRemoveCollinearVerticesChanged(cvar_t* var, char* value, qbool* cancel)
+{
+#if defined(__APPLE__) && defined(__aarch64__)
+	// At least arm64 based MacBook laptops are known to require this workaround.
+	Cvar_SetIgnoreCallback(var, "1");
+	*cancel = true;
+#endif
+}
+
 cvar_t cl_multiview                        = {"cl_multiview", "0" };
 cvar_t cl_mvdisplayhud                     = {"cl_mvdisplayhud", "1"};
 cvar_t cl_mvhudvertical                    = {"cl_mvhudvertical", "0"};
@@ -238,6 +247,8 @@ cvar_t gl_smoothmodels                     = {"gl_smoothmodels", "1"};
 
 cvar_t gl_vbo_clientmemory                 = {"gl_vbo_clientmemory", "0", CVAR_LATCH_GFX };
 
+cvar_t r_remove_collinear_vertices         = {"r_remove_collinear_vertices", "0", 0, OnRemoveCollinearVerticesChanged};
+
 //Returns true if the box is completely outside the frustom
 qbool R_CullBox(vec3_t mins, vec3_t maxs)
 {
@@ -750,6 +761,8 @@ void R_Init(void)
 	Cvar_Register(&cl_mvinset_top);
 	Cvar_Register(&cl_mvinset_right);
 
+	Cvar_Register(&r_remove_collinear_vertices);
+
 	Cvar_ResetCurrentGroup();
 
 	if (!hud_netgraph) {