Feature: Slope-aware and roadtype-specific one-way sprites.

Per OpenTTD#10238, the baseset only provided one-way sprites for flat tiles,
 matching the default road type. These look bad on slopes or on
 some custom roadtypes.

Action5 type 0x09 is extended to accept a set of 18 sprites.
The first 6 are unchanged from before this patch, followed by
 similar groups for north-facing and then south-facing slopes.

For compatibility with older basesets and NewGRFs, if an Action5
 provides only sprites for flat tiles these will be used on slopes
 as in previous versions.

Action3 type 0x0B, for roadtypes only, may optionally provide a
 set of 18 sprites as above for use with that roadtype.

src/newgrf.cpp

@@ -6250,9 +6250,17 @@ static void GraphicsNew(ByteReader *buf)
 		if (offset <= depot_no_track_offset && offset + num > depot_no_track_offset) _loaded_newgrf_features.tram = TRAMWAY_REPLACE_DEPOT_NO_TRACK;
 	}
 
+	/* If the baseset or grf only provides sprites for flat tiles (pre #10282), duplicate those for use on slopes. */
+	bool dup_oneway_sprites = ((type == 0x09) && (offset + num) <= SPR_ONEWAY_SLOPE_N_OFFSET);
+
 	for (; num > 0; num--) {
 		_cur.nfo_line++;
-		LoadNextSprite(replace == 0 ? _cur.spriteid++ : replace++, *_cur.file, _cur.nfo_line);
+		int load_index = (replace == 0 ? _cur.spriteid++ : replace++);
+		LoadNextSprite(load_index, *_cur.file, _cur.nfo_line);
+		if (dup_oneway_sprites) {
+			DupSprite(load_index, load_index + SPR_ONEWAY_SLOPE_N_OFFSET);
+			DupSprite(load_index, load_index + SPR_ONEWAY_SLOPE_S_OFFSET);
+		}
 	}
 
 	_cur.skip_sprites = skip_num;

src/road.h

@@ -66,6 +66,7 @@ enum RoadTypeSpriteGroup {
 	ROTSG_DEPOT,          ///< Optional: Depot images
 	ROTSG_reserved3,      ///<           Placeholder, if we add road fences (for highways).
 	ROTSG_ROADSTOP,       ///< Required: Drive-in stop surface
+	ROTSG_ONEWAY,         ///< Optional: One-way indicator images
 	ROTSG_END,
 };
 

src/road_cmd.cpp

@@ -1600,7 +1600,17 @@ static void DrawRoadBits(TileInfo *ti)
 	if (road_rti != nullptr) {
 		DisallowedRoadDirections drd = GetDisallowedRoadDirections(ti->tile);
 		if (drd != DRD_NONE) {
-			DrawGroundSpriteAt(SPR_ONEWAY_BASE + drd - 1 + ((road == ROAD_X) ? 0 : 3), PAL_NONE, 8, 8, GetPartialPixelZ(8, 8, ti->tileh));
+			SpriteID oneway = GetCustomRoadSprite(road_rti, ti->tile, ROTSG_ONEWAY);
+
+			if (oneway == 0) oneway = SPR_ONEWAY_BASE;
+
+			if ((ti->tileh == SLOPE_NE) || (ti->tileh == SLOPE_NW)) {
+				oneway += SPR_ONEWAY_SLOPE_N_OFFSET;
+			} else if ((ti->tileh == SLOPE_SE) || (ti->tileh == SLOPE_SW)) {
+				oneway += SPR_ONEWAY_SLOPE_S_OFFSET;
+			}
+
+			DrawGroundSpriteAt(oneway + drd - 1 + ((road == ROAD_X) ? 0 : 3), PAL_NONE, 8, 8, GetPartialPixelZ(8, 8, ti->tileh));
 		}
 	}
 

src/table/sprites.h

@@ -290,8 +290,10 @@ static const SpriteID SPR_TRAMWAY_DEPOT_NO_TRACK       = SPR_TRAMWAY_BASE + 113;
 static const uint16 TRAMWAY_SPRITE_COUNT = 119;
 
 /** One way road sprites */
-static const SpriteID SPR_ONEWAY_BASE = SPR_TRAMWAY_BASE + TRAMWAY_SPRITE_COUNT;
-static const uint16 ONEWAY_SPRITE_COUNT = 6;
+static const SpriteID SPR_ONEWAY_BASE           = SPR_TRAMWAY_BASE + TRAMWAY_SPRITE_COUNT;
+static const SpriteID SPR_ONEWAY_SLOPE_N_OFFSET = 6;
+static const SpriteID SPR_ONEWAY_SLOPE_S_OFFSET = 12;
+static const uint16 ONEWAY_SPRITE_COUNT = 18;
 
 /** Flags sprites (in same order as enum NetworkLanguage) */
 static const SpriteID SPR_FLAGS_BASE = SPR_ONEWAY_BASE + ONEWAY_SPRITE_COUNT;