Mostly deal with problems caused by polygon offset (#15867)

src/client/tile.cpp

@@ -21,11 +21,16 @@ void TileLayer::applyMaterialOptions(video::SMaterial &material, int layer) cons
 	/*
 	 * The second layer is for overlays, but uses the same vertex positions
 	 * as the first, which easily leads to Z-fighting.
-	 * To fix this we can offset the polygons in the direction of the camera.
+	 * To fix this we offset the polygons of the *first layer* away from the camera.
 	 * This only affects the depth buffer and leads to no visual gaps in geometry.
+	 *
+	 * However, doing so intrudes the "Z space" of the overlay of the next node
+	 * so that leads to inconsistent Z-sorting again. :(
+	 * HACK: For lack of a better approach we restrict this to cases where
+	 * an overlay is actually present.
 	 */
-	if (layer == 1) {
-		material.PolygonOffsetSlopeScale = -1;
-		material.PolygonOffsetDepthBias = -1;
+	if (need_polygon_offset) {
+		material.PolygonOffsetSlopeScale = 1;
+		material.PolygonOffsetDepthBias = 1;
 	}
 }

src/client/tile.h

@@ -73,7 +73,8 @@ struct TileLayer
 			material_flags == other.material_flags &&
 			has_color == other.has_color &&
 			color == other.color &&
-			scale == other.scale;
+			scale == other.scale &&
+			need_polygon_offset == other.need_polygon_offset;
 	}
 
 	/*!
@@ -92,6 +93,12 @@ struct TileLayer
 	 */
 	void applyMaterialOptions(video::SMaterial &material, int layer) const;
 
+	/// @return is this layer uninitalized?
+	bool empty() const
+	{
+		return !shader_id && !texture_id;
+	}
+
 	/// @return is this layer semi-transparent?
 	bool isTransparent() const
 	{
@@ -125,6 +132,12 @@ struct TileLayer
 		MATERIAL_FLAG_TILEABLE_HORIZONTAL|
 		MATERIAL_FLAG_TILEABLE_VERTICAL;
 
+	u8 scale = 1;
+
+	/// does this tile need to have a positive polygon offset set?
+	/// @see TileLayer::applyMaterialOptions
+	bool need_polygon_offset = false;
+
 	/// @note not owned by this struct
 	std::vector<FrameSpec> *frames = nullptr;
 
@@ -136,8 +149,6 @@ struct TileLayer
 
 	//! If true, the tile has its own color.
 	bool has_color = false;
-
-	u8 scale = 1;
 };
 
 enum class TileRotation: u8 {

src/nodedef.cpp

@@ -919,6 +919,8 @@ void ContentFeatures::updateTextures(ITextureSource *tsrc, IShaderSource *shdsrc
 			fillTileAttribs(tsrc, &tiles[j].layers[1], tiles[j], tdef_overlay[j],
 					color, overlay_material, overlay_shader,
 					tdef[j].backface_culling, tsettings);
+
+		tiles[j].layers[0].need_polygon_offset = !tiles[j].layers[1].empty();
 	}
 
 	MaterialType special_material = material_type;