Load turtle overlays from a registry

- Add a new computercraft:turtle_overlay dynamic registry, which stores
   turtle overlays. Turtle overlays are just a model id and an
   (optional) boolean flag, which specifies whether this overlay is
   compatible with the elf/christmas model.

 - Change the computercraft:overlay component to accept a
   Holder<TurtleOverlay> (instead of just a model ID). This accepts both
   an overlay ID or an inline overlay object (e.g. you can do
   cc:turtle_normal[computercraft:overlay={model:"foo"}].

 - Update turtle model and BE rendering code to render both the overlay
   and elf (if compatible). Fixes #1663.

 - Ideally we'd automatically load all models listed in the overlay
   registry. However, resource loading happens separately to datapacks,
   so we can't link the two.

   Instead, we add a new assets/computercraft/extra_models.json file
   that lists any additional models that should be loaded and baked.

   This file includes all built-in overlay models, but external resource
   packs and/or mods can easily extend it.

projects/common/src/client/java/dan200/computercraft/client/ClientRegistry.java

@@ -25,6 +25,7 @@ import dan200.computercraft.shared.computer.core.ComputerState;
 import dan200.computercraft.shared.computer.core.ServerContext;
 import dan200.computercraft.shared.computer.inventory.AbstractComputerMenu;
 import dan200.computercraft.shared.media.items.DiskItem;
+import dan200.computercraft.shared.turtle.TurtleOverlay;
 import net.minecraft.Util;
 import net.minecraft.client.Minecraft;
 import net.minecraft.client.color.item.ItemColor;
@@ -53,6 +54,7 @@ import net.minecraft.world.level.ItemLike;
 import javax.annotation.Nullable;
 import java.io.File;
 import java.io.IOException;
+import java.util.Collection;
 import java.util.function.BiConsumer;
 import java.util.function.Consumer;
 import java.util.function.Supplier;
@@ -143,15 +145,14 @@ public final class ClientRegistry {
         register.accept(GuiSprites.initialise(minecraft.getTextureManager()));
     }
 
-    private static final String[] EXTRA_MODELS = new String[]{
+    private static final ResourceLocation[] EXTRA_MODELS = {
-        "block/turtle_colour",
+        TurtleOverlay.ELF_MODEL,
-        "block/turtle_elf_overlay",
+        TurtleBlockEntityRenderer.COLOUR_TURTLE_MODEL,
-        "block/turtle_rainbow_overlay",
-        "block/turtle_trans_overlay",
     };
 
-    public static void registerExtraModels(Consumer<ResourceLocation> register) {
+    public static void registerExtraModels(Consumer<ResourceLocation> register, Collection<ResourceLocation> extraModels) {
-        for (var model : EXTRA_MODELS) register.accept(ResourceLocation.fromNamespaceAndPath(ComputerCraftAPI.MOD_ID, model));
+        for (var model : EXTRA_MODELS) register.accept(model);
+        extraModels.forEach(register);
         TurtleUpgradeModellers.getDependencies().forEach(register);
     }
 

projects/common/src/client/java/dan200/computercraft/client/model/ExtraModels.java

@@ -0,0 +1,68 @@
+// SPDX-FileCopyrightText: 2024 The CC: Tweaked Developers
+//
+// SPDX-License-Identifier: MPL-2.0
+
+package dan200.computercraft.client.model;
+
+import com.google.gson.Gson;
+import com.google.gson.JsonElement;
+import com.google.gson.JsonParseException;
+import com.mojang.serialization.Codec;
+import com.mojang.serialization.JsonOps;
+import dan200.computercraft.api.ComputerCraftAPI;
+import dan200.computercraft.shared.turtle.TurtleOverlay;
+import net.minecraft.resources.ResourceLocation;
+import net.minecraft.server.packs.resources.ResourceManager;
+import org.slf4j.Logger;
+import org.slf4j.LoggerFactory;
+
+import java.io.IOException;
+import java.util.*;
+
+/**
+ * A list of extra models to load on the client.
+ * <p>
+ * This is largely intended for use with {@linkplain TurtleOverlay turtle overlays}. As overlays are stored in a dynamic
+ * registry, they are not available when resources are loaded, and so we need a way to request the overlays' models be
+ * loaded.
+ *
+ * @param models The models to load.
+ */
+public record ExtraModels(List<ResourceLocation> models) {
+    private static final Logger LOG = LoggerFactory.getLogger(ExtraModels.class);
+    private static final Gson GSON = new Gson();
+
+    /**
+     * The path where the extra models are listed.
+     */
+    public static final ResourceLocation PATH = ResourceLocation.fromNamespaceAndPath(ComputerCraftAPI.MOD_ID, "extra_models.json");
+
+    /**
+     * The coded used to store the extra model file.
+     */
+    public static final Codec<ExtraModels> CODEC = ResourceLocation.CODEC.listOf().xmap(ExtraModels::new, ExtraModels::models);
+
+    /**
+     * Get the list of all extra models to load.
+     *
+     * @param resources The current resource manager.
+     * @return A set of all resources to load.
+     */
+    public static Collection<ResourceLocation> loadAll(ResourceManager resources) {
+        Set<ResourceLocation> out = new HashSet<>();
+
+        for (var path : resources.getResourceStack(PATH)) {
+            ExtraModels models;
+            try (var stream = path.openAsReader()) {
+                models = ExtraModels.CODEC.parse(JsonOps.INSTANCE, GSON.fromJson(stream, JsonElement.class)).getOrThrow(JsonParseException::new);
+            } catch (IOException | RuntimeException e) {
+                LOG.error("Failed to load extra models from {}", path.sourcePackId());
+                continue;
+            }
+
+            out.addAll(models.models());
+        }
+
+        return Collections.unmodifiableCollection(out);
+    }
+}

projects/common/src/client/java/dan200/computercraft/client/model/turtle/TurtleModelParts.java

@@ -12,13 +12,14 @@ import dan200.computercraft.api.turtle.ITurtleUpgrade;
 import dan200.computercraft.api.turtle.TurtleSide;
 import dan200.computercraft.api.upgrades.UpgradeData;
 import dan200.computercraft.client.platform.ClientPlatformHelper;
-import dan200.computercraft.client.render.TurtleBlockEntityRenderer;
 import dan200.computercraft.client.turtle.TurtleUpgradeModellers;
+import dan200.computercraft.shared.turtle.TurtleOverlay;
 import dan200.computercraft.shared.turtle.items.TurtleItem;
 import dan200.computercraft.shared.util.DataComponentUtil;
 import dan200.computercraft.shared.util.Holiday;
 import net.minecraft.client.Minecraft;
 import net.minecraft.client.resources.model.BakedModel;
+import net.minecraft.client.resources.model.ModelManager;
 import net.minecraft.core.component.DataComponents;
 import net.minecraft.resources.ResourceLocation;
 import net.minecraft.world.item.ItemStack;
@@ -52,7 +53,7 @@ public final class TurtleModelParts<T> {
         boolean colour,
         @Nullable UpgradeData<ITurtleUpgrade> leftUpgrade,
         @Nullable UpgradeData<ITurtleUpgrade> rightUpgrade,
-        @Nullable ResourceLocation overlay,
+        @Nullable TurtleOverlay overlay,
         boolean christmas,
         boolean flip
     ) {
@@ -113,10 +114,10 @@ public final class TurtleModelParts<T> {
         var parts = new ArrayList<BakedModel>(4);
         parts.add(transform(combo.colour() ? colourModel : familyModel, transformation));
 
-        var overlayModelLocation = TurtleBlockEntityRenderer.getTurtleOverlayModel(combo.overlay(), combo.christmas());
+        if (combo.overlay() != null) addPart(parts, modelManager, transformation, combo.overlay().model());
-        if (overlayModelLocation != null) {
+
-            parts.add(transform(ClientPlatformHelper.get().getModel(modelManager, overlayModelLocation), transformation));
+        var showChristmas = TurtleOverlay.showElfOverlay(combo.overlay(), combo.christmas());
-        }
+        if (showChristmas) addPart(parts, modelManager, transformation, TurtleOverlay.ELF_MODEL);
 
         addUpgrade(parts, transformation, TurtleSide.LEFT, combo.leftUpgrade());
         addUpgrade(parts, transformation, TurtleSide.RIGHT, combo.rightUpgrade());
@@ -124,6 +125,10 @@ public final class TurtleModelParts<T> {
         return parts;
     }
 
+    private void addPart(List<BakedModel> parts, ModelManager modelManager, Transformation transformation, ResourceLocation model) {
+        parts.add(transform(ClientPlatformHelper.get().getModel(modelManager, model), transformation));
+    }
+
     private void addUpgrade(List<BakedModel> parts, Transformation transformation, TurtleSide side, @Nullable UpgradeData<ITurtleUpgrade> upgrade) {
         if (upgrade == null) return;
         var model = TurtleUpgradeModellers.getModel(upgrade.upgrade(), upgrade.data(), side);

projects/common/src/client/java/dan200/computercraft/client/render/TurtleBlockEntityRenderer.java

@@ -11,6 +11,7 @@ import dan200.computercraft.api.ComputerCraftAPI;
 import dan200.computercraft.api.turtle.TurtleSide;
 import dan200.computercraft.client.platform.ClientPlatformHelper;
 import dan200.computercraft.client.turtle.TurtleUpgradeModellers;
+import dan200.computercraft.shared.turtle.TurtleOverlay;
 import dan200.computercraft.shared.turtle.blocks.TurtleBlockEntity;
 import dan200.computercraft.shared.util.Holiday;
 import net.minecraft.client.Minecraft;
@@ -28,8 +29,7 @@ import net.minecraft.world.phys.HitResult;
 import javax.annotation.Nullable;
 
 public class TurtleBlockEntityRenderer implements BlockEntityRenderer<TurtleBlockEntity> {
-    private static final ResourceLocation COLOUR_TURTLE_MODEL = ResourceLocation.fromNamespaceAndPath(ComputerCraftAPI.MOD_ID, "block/turtle_colour");
+    public static final ResourceLocation COLOUR_TURTLE_MODEL = ResourceLocation.fromNamespaceAndPath(ComputerCraftAPI.MOD_ID, "block/turtle_colour");
-    private static final ResourceLocation ELF_OVERLAY_MODEL = ResourceLocation.fromNamespaceAndPath(ComputerCraftAPI.MOD_ID, "block/turtle_elf_overlay");
 
     private final BlockEntityRenderDispatcher renderer;
     private final Font font;
@@ -39,12 +39,6 @@ public class TurtleBlockEntityRenderer implements BlockEntityRenderer<TurtleBloc
         font = context.getFont();
     }
 
-    public static @Nullable ResourceLocation getTurtleOverlayModel(@Nullable ResourceLocation overlay, boolean christmas) {
-        if (overlay != null) return overlay;
-        if (christmas) return ELF_OVERLAY_MODEL;
-        return null;
-    }
-
     @Override
     public void render(TurtleBlockEntity turtle, float partialTicks, PoseStack transform, MultiBufferSource buffers, int lightmapCoord, int overlayLight) {
         transform.pushPose();
@@ -99,10 +93,11 @@ public class TurtleBlockEntityRenderer implements BlockEntityRenderer<TurtleBloc
         }
 
         // Render the overlay
-        var overlayModel = getTurtleOverlayModel(overlay, Holiday.getCurrent() == Holiday.CHRISTMAS);
+        if (overlay != null) renderModel(transform, buffers, lightmapCoord, overlayLight, overlay.model(), null);
-        if (overlayModel != null) {
+
-            renderModel(transform, buffers, lightmapCoord, overlayLight, overlayModel, null);
+        // And the Christmas overlay.
-        }
+        var showChristmas = TurtleOverlay.showElfOverlay(overlay, Holiday.getCurrent() == Holiday.CHRISTMAS);
+        if (showChristmas) renderModel(transform, buffers, lightmapCoord, overlayLight, TurtleOverlay.ELF_MODEL, null);
 
         // Render the upgrades
         renderUpgrade(transform, buffers, lightmapCoord, overlayLight, turtle, TurtleSide.LEFT, partialTicks);

projects/common/src/client/java/dan200/computercraft/data/client/ClientDataProviders.java

@@ -6,15 +6,18 @@ package dan200.computercraft.data.client;
 
 import dan200.computercraft.client.gui.GuiSprites;
 import dan200.computercraft.data.DataProviders;
+import dan200.computercraft.shared.turtle.TurtleOverlay;
 import dan200.computercraft.shared.turtle.inventory.UpgradeSlot;
 import net.minecraft.client.renderer.texture.atlas.SpriteSource;
 import net.minecraft.client.renderer.texture.atlas.SpriteSources;
 import net.minecraft.client.renderer.texture.atlas.sources.SingleFile;
+import net.minecraft.core.HolderLookup;
 import net.minecraft.resources.ResourceLocation;
 import net.minecraft.server.packs.PackType;
 
 import java.util.List;
 import java.util.Optional;
+import java.util.concurrent.CompletableFuture;
 import java.util.stream.Stream;
 
 /**
@@ -26,7 +29,7 @@ public final class ClientDataProviders {
     private ClientDataProviders() {
     }
 
-    public static void add(DataProviders.GeneratorSink generator) {
+    public static void add(DataProviders.GeneratorSink generator, CompletableFuture<HolderLookup.Provider> registries) {
         generator.addFromCodec("Block atlases", PackType.CLIENT_RESOURCES, "atlases", SpriteSources.FILE_CODEC, out -> {
             out.accept(ResourceLocation.withDefaultNamespace("blocks"), List.of(
                 new SingleFile(UpgradeSlot.LEFT_UPGRADE, Optional.empty()),
@@ -44,5 +47,12 @@ public final class ClientDataProviders {
                 GuiSprites.COMPUTER_COLOUR.textures()
             ).flatMap(x -> x).<SpriteSource>map(x -> new SingleFile(x, Optional.empty())).toList());
         });
+
+        generator.add(pack -> new ExtraModelsProvider(pack, registries) {
+            @Override
+            public Stream<ResourceLocation> getModels(HolderLookup.Provider registries) {
+                return registries.lookupOrThrow(TurtleOverlay.REGISTRY).listElements().map(x -> x.value().model());
+            }
+        });
     }
 }

projects/common/src/client/java/dan200/computercraft/data/client/ExtraModelsProvider.java

@@ -0,0 +1,52 @@
+// SPDX-FileCopyrightText: 2024 The CC: Tweaked Developers
+//
+// SPDX-License-Identifier: MPL-2.0
+
+package dan200.computercraft.data.client;
+
+import com.mojang.serialization.JsonOps;
+import dan200.computercraft.client.model.ExtraModels;
+import net.minecraft.core.HolderLookup;
+import net.minecraft.data.CachedOutput;
+import net.minecraft.data.DataProvider;
+import net.minecraft.data.PackOutput;
+import net.minecraft.resources.ResourceLocation;
+
+import java.nio.file.Path;
+import java.util.concurrent.CompletableFuture;
+import java.util.stream.Stream;
+
+/**
+ * A data provider to generate {@link ExtraModels}.
+ */
+abstract class ExtraModelsProvider implements DataProvider {
+    private final Path path;
+    private final CompletableFuture<HolderLookup.Provider> registries;
+
+    ExtraModelsProvider(PackOutput output, CompletableFuture<HolderLookup.Provider> registries) {
+        path = output.getOutputFolder(PackOutput.Target.RESOURCE_PACK).resolve(ExtraModels.PATH.getNamespace()).resolve(ExtraModels.PATH.getPath());
+        this.registries = registries;
+    }
+
+    /**
+     * Return a stream of models to load.
+     *
+     * @param registries The current registries.
+     * @return The collection of extra models to load.
+     */
+    public abstract Stream<ResourceLocation> getModels(HolderLookup.Provider registries);
+
+    @Override
+    public final CompletableFuture<?> run(CachedOutput output) {
+        return registries.thenCompose(registries -> {
+            var models = new ExtraModels(getModels(registries).sorted().toList());
+            var json = ExtraModels.CODEC.encodeStart(JsonOps.INSTANCE, models).getOrThrow(IllegalStateException::new);
+            return DataProvider.saveStable(output, json, path);
+        });
+    }
+
+    @Override
+    public final String getName() {
+        return "Extra Models";
+    }
+}

projects/common/src/generated/resources/assets/computercraft/extra_models.json

@@ -0,0 +1 @@
+["computercraft:block/turtle_rainbow_overlay", "computercraft:block/turtle_trans_overlay"]

projects/common/src/generated/resources/data/computercraft/advancement/recipes/redstone/turtle_advanced_overlays/rainbow_flag.json

@@ -3,7 +3,7 @@
   "criteria": {
     "has_dye": {"conditions": {"items": [{"items": "#c:dyes"}]}, "trigger": "minecraft:inventory_changed"},
     "has_the_recipe": {
-      "conditions": {"recipe": "computercraft:turtle_advanced_overlays/turtle_trans_overlay"},
+      "conditions": {"recipe": "computercraft:turtle_advanced_overlays/rainbow_flag"},
       "trigger": "minecraft:recipe_unlocked"
     },
     "has_turtle": {
@@ -12,5 +12,5 @@
     }
   },
   "requirements": [["has_the_recipe", "has_turtle", "has_dye"]],
-  "rewards": {"recipes": ["computercraft:turtle_advanced_overlays/turtle_trans_overlay"]}
+  "rewards": {"recipes": ["computercraft:turtle_advanced_overlays/rainbow_flag"]}
 }

projects/common/src/generated/resources/data/computercraft/advancement/recipes/redstone/turtle_advanced_overlays/trans_flag.json

@@ -3,7 +3,7 @@
   "criteria": {
     "has_dye": {"conditions": {"items": [{"items": "#c:dyes"}]}, "trigger": "minecraft:inventory_changed"},
     "has_the_recipe": {
-      "conditions": {"recipe": "computercraft:turtle_advanced_overlays/turtle_rainbow_overlay"},
+      "conditions": {"recipe": "computercraft:turtle_advanced_overlays/trans_flag"},
       "trigger": "minecraft:recipe_unlocked"
     },
     "has_turtle": {
@@ -12,5 +12,5 @@
     }
   },
   "requirements": [["has_the_recipe", "has_turtle", "has_dye"]],
-  "rewards": {"recipes": ["computercraft:turtle_advanced_overlays/turtle_rainbow_overlay"]}
+  "rewards": {"recipes": ["computercraft:turtle_advanced_overlays/trans_flag"]}
 }

projects/common/src/generated/resources/data/computercraft/advancement/recipes/redstone/turtle_normal_overlays/rainbow_flag.json

@@ -3,7 +3,7 @@
   "criteria": {
     "has_dye": {"conditions": {"items": [{"items": "#c:dyes"}]}, "trigger": "minecraft:inventory_changed"},
     "has_the_recipe": {
-      "conditions": {"recipe": "computercraft:turtle_normal_overlays/turtle_trans_overlay"},
+      "conditions": {"recipe": "computercraft:turtle_normal_overlays/rainbow_flag"},
       "trigger": "minecraft:recipe_unlocked"
     },
     "has_turtle": {
@@ -12,5 +12,5 @@
     }
   },
   "requirements": [["has_the_recipe", "has_turtle", "has_dye"]],
-  "rewards": {"recipes": ["computercraft:turtle_normal_overlays/turtle_trans_overlay"]}
+  "rewards": {"recipes": ["computercraft:turtle_normal_overlays/rainbow_flag"]}
 }

projects/common/src/generated/resources/data/computercraft/advancement/recipes/redstone/turtle_normal_overlays/trans_flag.json

@@ -3,7 +3,7 @@
   "criteria": {
     "has_dye": {"conditions": {"items": [{"items": "#c:dyes"}]}, "trigger": "minecraft:inventory_changed"},
     "has_the_recipe": {
-      "conditions": {"recipe": "computercraft:turtle_normal_overlays/turtle_rainbow_overlay"},
+      "conditions": {"recipe": "computercraft:turtle_normal_overlays/trans_flag"},
       "trigger": "minecraft:recipe_unlocked"
     },
     "has_turtle": {
@@ -12,5 +12,5 @@
     }
   },
   "requirements": [["has_the_recipe", "has_turtle", "has_dye"]],
-  "rewards": {"recipes": ["computercraft:turtle_normal_overlays/turtle_rainbow_overlay"]}
+  "rewards": {"recipes": ["computercraft:turtle_normal_overlays/trans_flag"]}
 }

projects/common/src/generated/resources/data/computercraft/computercraft/turtle_overlay/rainbow_flag.json

@@ -0,0 +1 @@
+{"model": "computercraft:block/turtle_rainbow_overlay", "show_elf_overlay": true}

projects/common/src/generated/resources/data/computercraft/computercraft/turtle_overlay/trans_flag.json

@@ -0,0 +1 @@
+{"model": "computercraft:block/turtle_trans_overlay", "show_elf_overlay": true}

projects/common/src/generated/resources/data/computercraft/recipe/turtle_advanced_overlays/rainbow_flag.json

@@ -20,7 +20,7 @@
     {"item": "computercraft:turtle_advanced"}
   ],
   "result": {
-    "components": {"computercraft:overlay": "computercraft:block/turtle_rainbow_overlay"},
+    "components": {"computercraft:overlay": "computercraft:rainbow_flag"},
     "count": 1,
     "id": "computercraft:turtle_advanced"
   }

projects/common/src/generated/resources/data/computercraft/recipe/turtle_advanced_overlays/trans_flag.json

@@ -17,7 +17,7 @@
     {"item": "computercraft:turtle_advanced"}
   ],
   "result": {
-    "components": {"computercraft:overlay": "computercraft:block/turtle_trans_overlay"},
+    "components": {"computercraft:overlay": "computercraft:trans_flag"},
     "count": 1,
     "id": "computercraft:turtle_advanced"
   }

projects/common/src/generated/resources/data/computercraft/recipe/turtle_normal_overlays/rainbow_flag.json

@@ -20,7 +20,7 @@
     {"item": "computercraft:turtle_normal"}
   ],
   "result": {
-    "components": {"computercraft:overlay": "computercraft:block/turtle_rainbow_overlay"},
+    "components": {"computercraft:overlay": "computercraft:rainbow_flag"},
     "count": 1,
     "id": "computercraft:turtle_normal"
   }

projects/common/src/generated/resources/data/computercraft/recipe/turtle_normal_overlays/trans_flag.json

@@ -17,7 +17,7 @@
     {"item": "computercraft:turtle_normal"}
   ],
   "result": {
-    "components": {"computercraft:overlay": "computercraft:block/turtle_trans_overlay"},
+    "components": {"computercraft:overlay": "computercraft:trans_flag"},
     "count": 1,
     "id": "computercraft:turtle_normal"
   }

projects/common/src/main/java/dan200/computercraft/data/DataProviders.java

@@ -7,6 +7,7 @@ package dan200.computercraft.data;
 import com.mojang.serialization.Codec;
 import dan200.computercraft.api.pocket.IPocketUpgrade;
 import dan200.computercraft.api.turtle.ITurtleUpgrade;
+import dan200.computercraft.shared.turtle.TurtleOverlay;
 import net.minecraft.Util;
 import net.minecraft.core.HolderLookup;
 import net.minecraft.core.RegistrySetBuilder;
@@ -38,6 +39,7 @@ public final class DataProviders {
             Util.make(new RegistrySetBuilder(), builder -> {
                 builder.add(ITurtleUpgrade.REGISTRY, TurtleUpgradeProvider::addUpgrades);
                 builder.add(IPocketUpgrade.REGISTRY, PocketUpgradeProvider::addUpgrades);
+                builder.add(TurtleOverlay.REGISTRY, TurtleOverlays::register);
             }));
         var fullRegistries = fullRegistryPatch.thenApply(RegistrySetBuilder.PatchedRegistries::full);
 
@@ -57,7 +59,8 @@ public final class DataProviders {
         // and invoke that.
         try {
             Class.forName("dan200.computercraft.data.client.ClientDataProviders")
-                .getMethod("add", GeneratorSink.class).invoke(null, generator);
+                .getMethod("add", GeneratorSink.class, CompletableFuture.class)
+                .invoke(null, generator, fullRegistries);
         } catch (ReflectiveOperationException e) {
             throw new RuntimeException(e);
         }

projects/common/src/main/java/dan200/computercraft/data/RecipeProvider.java

@@ -28,6 +28,7 @@ import dan200.computercraft.shared.recipe.ImpostorShapelessRecipe;
 import dan200.computercraft.shared.recipe.TransformShapedRecipe;
 import dan200.computercraft.shared.recipe.TransformShapelessRecipe;
 import dan200.computercraft.shared.recipe.function.CopyComponents;
+import dan200.computercraft.shared.turtle.TurtleOverlay;
 import dan200.computercraft.shared.turtle.items.TurtleItem;
 import dan200.computercraft.shared.turtle.recipes.TurtleUpgradeRecipe;
 import dan200.computercraft.shared.util.ColourUtils;
@@ -45,6 +46,7 @@ import net.minecraft.data.recipes.RecipeCategory;
 import net.minecraft.data.recipes.RecipeOutput;
 import net.minecraft.data.recipes.ShapedRecipeBuilder;
 import net.minecraft.data.recipes.ShapelessRecipeBuilder;
+import net.minecraft.resources.ResourceKey;
 import net.minecraft.resources.ResourceLocation;
 import net.minecraft.tags.ItemTags;
 import net.minecraft.tags.TagKey;
@@ -96,7 +98,7 @@ final class RecipeProvider extends net.minecraft.data.recipes.RecipeProvider {
         diskColours(add);
         pocketUpgrades(add, registries);
         turtleUpgrades(add, registries);
-        turtleOverlays(add);
+        turtleOverlays(add, registries);
 
         addSpecial(add, new DiskRecipe(CraftingBookCategory.MISC));
         addSpecial(add, new ColourableRecipe(CraftingBookCategory.MISC));
@@ -189,8 +191,8 @@ final class RecipeProvider extends net.minecraft.data.recipes.RecipeProvider {
         }
     }
 
-    private void turtleOverlays(RecipeOutput add) {
+    private void turtleOverlays(RecipeOutput add, HolderLookup.Provider registries) {
-        turtleOverlay(add, "turtle_trans_overlay", x -> x
+        turtleOverlay(add, registries, TurtleOverlays.TRANS_FLAG, x -> x
             .unlockedBy("has_dye", inventoryChange(itemPredicate(ingredients.dye())))
             .requires(ColourUtils.getDyeTag(DyeColor.LIGHT_BLUE))
             .requires(ColourUtils.getDyeTag(DyeColor.PINK))
@@ -198,7 +200,7 @@ final class RecipeProvider extends net.minecraft.data.recipes.RecipeProvider {
             .requires(Items.STICK)
         );
 
-        turtleOverlay(add, "turtle_rainbow_overlay", x -> x
+        turtleOverlay(add, registries, TurtleOverlays.RAINBOW_FLAG, x -> x
             .unlockedBy("has_dye", inventoryChange(itemPredicate(ingredients.dye())))
             .requires(ColourUtils.getDyeTag(DyeColor.RED))
             .requires(ColourUtils.getDyeTag(DyeColor.ORANGE))
@@ -210,14 +212,14 @@ final class RecipeProvider extends net.minecraft.data.recipes.RecipeProvider {
         );
     }
 
-    private void turtleOverlay(RecipeOutput add, String overlay, Consumer<ShapelessSpecBuilder> build) {
+    private void turtleOverlay(RecipeOutput add, HolderLookup.Provider registries, ResourceKey<TurtleOverlay> overlay, Consumer<ShapelessSpecBuilder> build) {
-        var overlayId = ResourceLocation.fromNamespaceAndPath(ComputerCraftAPI.MOD_ID, "block/" + overlay);
+        var holder = registries.lookupOrThrow(overlay.registryKey()).getOrThrow(overlay);
 
         for (var turtleItem : turtleItems()) {
             var name = RegistryHelper.getKeyOrThrow(BuiltInRegistries.ITEM, turtleItem);
 
             var builder = ShapelessSpecBuilder
-                .shapeless(RecipeCategory.REDSTONE, DataComponentUtil.createStack(turtleItem, ModRegistry.DataComponents.OVERLAY.get(), overlayId))
+                .shapeless(RecipeCategory.REDSTONE, DataComponentUtil.createStack(turtleItem, ModRegistry.DataComponents.OVERLAY.get(), holder))
                 .group(name.withSuffix("_overlay").toString())
                 .unlockedBy("has_turtle", inventoryChange(turtleItem));
             build.accept(builder);
@@ -226,7 +228,7 @@ final class RecipeProvider extends net.minecraft.data.recipes.RecipeProvider {
                 .build(s -> new TransformShapelessRecipe(s, List.of(
                     CopyComponents.builder(turtleItem).exclude(ModRegistry.DataComponents.OVERLAY.get()).build()
                 )))
-                .save(add, name.withSuffix("_overlays/" + overlay));
+                .save(add, name.withSuffix("_overlays/" + overlay.location().getPath()));
         }
     }
 

projects/common/src/main/java/dan200/computercraft/data/TurtleOverlays.java

@@ -0,0 +1,38 @@
+// SPDX-FileCopyrightText: 2024 The CC: Tweaked Developers
+//
+// SPDX-License-Identifier: MPL-2.0
+
+package dan200.computercraft.data;
+
+import dan200.computercraft.api.ComputerCraftAPI;
+import dan200.computercraft.shared.turtle.TurtleOverlay;
+import net.minecraft.data.worldgen.BootstrapContext;
+import net.minecraft.resources.ResourceKey;
+import net.minecraft.resources.ResourceLocation;
+
+/**
+ * Built-in turtle overlays.
+ */
+final class TurtleOverlays {
+    public static final ResourceKey<TurtleOverlay> RAINBOW_FLAG = create("rainbow_flag");
+    public static final ResourceKey<TurtleOverlay> TRANS_FLAG = create("trans_flag");
+
+    private static ResourceKey<TurtleOverlay> create(String name) {
+        return ResourceKey.create(TurtleOverlay.REGISTRY, ResourceLocation.fromNamespaceAndPath(ComputerCraftAPI.MOD_ID, name));
+    }
+
+    private TurtleOverlays() {
+    }
+
+    public static void register(BootstrapContext<TurtleOverlay> registry) {
+        registry.register(RAINBOW_FLAG, new TurtleOverlay(
+            ResourceLocation.fromNamespaceAndPath(ComputerCraftAPI.MOD_ID, "block/turtle_rainbow_overlay"),
+            true
+        ));
+
+        registry.register(TRANS_FLAG, new TurtleOverlay(
+            ResourceLocation.fromNamespaceAndPath(ComputerCraftAPI.MOD_ID, "block/turtle_trans_overlay"),
+            true
+        ));
+    }
+}

projects/common/src/main/java/dan200/computercraft/shared/ModRegistry.java

@@ -72,6 +72,7 @@ import dan200.computercraft.shared.recipe.*;
 import dan200.computercraft.shared.recipe.function.CopyComponents;
 import dan200.computercraft.shared.recipe.function.RecipeFunction;
 import dan200.computercraft.shared.turtle.FurnaceRefuelHandler;
+import dan200.computercraft.shared.turtle.TurtleOverlay;
 import dan200.computercraft.shared.turtle.blocks.TurtleBlock;
 import dan200.computercraft.shared.turtle.blocks.TurtleBlockEntity;
 import dan200.computercraft.shared.turtle.inventory.TurtleMenu;
@@ -95,7 +96,6 @@ import net.minecraft.network.RegistryFriendlyByteBuf;
 import net.minecraft.network.chat.Component;
 import net.minecraft.network.codec.ByteBufCodecs;
 import net.minecraft.network.codec.StreamCodec;
-import net.minecraft.resources.ResourceLocation;
 import net.minecraft.world.flag.FeatureFlags;
 import net.minecraft.world.inventory.MenuType;
 import net.minecraft.world.item.BlockItem;
@@ -320,8 +320,8 @@ public final class ModRegistry {
         /**
          * The overlay on a turtle.
          */
-        public static final RegistryEntry<DataComponentType<ResourceLocation>> OVERLAY = register("overlay", b -> b
+        public static final RegistryEntry<DataComponentType<Holder<TurtleOverlay>>> OVERLAY = register("overlay", b -> b
-            .persistent(ResourceLocation.CODEC).networkSynchronized(ResourceLocation.STREAM_CODEC)
+            .persistent(TurtleOverlay.CODEC).networkSynchronized(TurtleOverlay.STREAM_CODEC)
         );
 
         /**

projects/common/src/main/java/dan200/computercraft/shared/turtle/TurtleOverlay.java

@@ -0,0 +1,78 @@
+// SPDX-FileCopyrightText: 2024 The CC: Tweaked Developers
+//
+// SPDX-License-Identifier: MPL-2.0
+
+package dan200.computercraft.shared.turtle;
+
+import com.mojang.serialization.Codec;
+import com.mojang.serialization.codecs.RecordCodecBuilder;
+import dan200.computercraft.api.ComputerCraftAPI;
+import dan200.computercraft.shared.ModRegistry;
+import dan200.computercraft.shared.util.Holiday;
+import net.minecraft.core.Holder;
+import net.minecraft.core.Registry;
+import net.minecraft.network.RegistryFriendlyByteBuf;
+import net.minecraft.network.codec.ByteBufCodecs;
+import net.minecraft.network.codec.StreamCodec;
+import net.minecraft.resources.RegistryFileCodec;
+import net.minecraft.resources.ResourceKey;
+import net.minecraft.resources.ResourceLocation;
+
+import javax.annotation.Nullable;
+
+/**
+ * A cosmetic overlay on a turtle.
+ *
+ * @param model          The path to the overlay's model.
+ * @param showElfOverlay Whether this overlay is compatible with the {@linkplain #ELF_MODEL Christmas elf model}.
+ * @see ModRegistry.DataComponents#OVERLAY
+ */
+public record TurtleOverlay(ResourceLocation model, boolean showElfOverlay) {
+    /**
+     * The registry turtle overlays are stored in.
+     */
+    public static final ResourceKey<Registry<TurtleOverlay>> REGISTRY = ResourceKey.createRegistryKey(ResourceLocation.fromNamespaceAndPath(ComputerCraftAPI.MOD_ID, "turtle_overlay"));
+
+    /**
+     * The codec used to read/write turtle overlay definitions from datapacks.
+     */
+    public static final Codec<TurtleOverlay> DIRECT_CODEC = RecordCodecBuilder.create(instance -> instance.group(
+        ResourceLocation.CODEC.fieldOf("model").forGetter(TurtleOverlay::model),
+        Codec.BOOL.optionalFieldOf("show_elf_overlay", false).forGetter(TurtleOverlay::showElfOverlay)
+    ).apply(instance, TurtleOverlay::new));
+
+    /**
+     * The codec used for {@link TurtleOverlay} instances.
+     *
+     * @see ModRegistry.DataComponents#OVERLAY
+     */
+    public static final Codec<Holder<TurtleOverlay>> CODEC = RegistryFileCodec.create(REGISTRY, DIRECT_CODEC);
+
+    /**
+     * The stream codec used for {@link TurtleOverlay} instances.
+     */
+    public static final StreamCodec<RegistryFriendlyByteBuf, Holder<TurtleOverlay>> STREAM_CODEC = ByteBufCodecs.holder(REGISTRY, StreamCodec.composite(
+        ResourceLocation.STREAM_CODEC, TurtleOverlay::model,
+        ByteBufCodecs.BOOL, TurtleOverlay::showElfOverlay,
+        TurtleOverlay::new
+    ));
+
+    /**
+     * An additional overlay that is rendered on all turtles at {@linkplain Holiday#CHRISTMAS Christmas}.
+     *
+     * @see #showElfOverlay()
+     * @see #showElfOverlay(TurtleOverlay, boolean)
+     */
+    public static final ResourceLocation ELF_MODEL = ResourceLocation.fromNamespaceAndPath(ComputerCraftAPI.MOD_ID, "block/turtle_elf_overlay");
+
+    /**
+     * Determine whether we should show the {@linkplain #ELF_MODEL elf overlay}.
+     *
+     * @param overlay   The current {@link TurtleOverlay}.
+     * @param christmas Whether it is Christmas.
+     * @return Whether we should show the elf overlay.
+     */
+    public static boolean showElfOverlay(@Nullable TurtleOverlay overlay, boolean christmas) {
+        return christmas && (overlay == null || overlay.showElfOverlay());
+    }
+}

projects/common/src/main/java/dan200/computercraft/shared/turtle/blocks/TurtleBlockEntity.java

@@ -20,6 +20,7 @@ import dan200.computercraft.shared.computer.core.ServerComputer;
 import dan200.computercraft.shared.config.Config;
 import dan200.computercraft.shared.container.BasicContainer;
 import dan200.computercraft.shared.platform.PlatformHelper;
+import dan200.computercraft.shared.turtle.TurtleOverlay;
 import dan200.computercraft.shared.turtle.apis.TurtleAPI;
 import dan200.computercraft.shared.turtle.core.TurtleBrain;
 import dan200.computercraft.shared.turtle.inventory.TurtleMenu;
@@ -30,7 +31,6 @@ import net.minecraft.core.NonNullList;
 import net.minecraft.core.component.DataComponentMap;
 import net.minecraft.core.component.DataComponents;
 import net.minecraft.nbt.CompoundTag;
-import net.minecraft.resources.ResourceLocation;
 import net.minecraft.server.level.ServerLevel;
 import net.minecraft.world.Container;
 import net.minecraft.world.ContainerHelper;
@@ -222,8 +222,9 @@ public class TurtleBlockEntity extends AbstractComputerBlockEntity implements Ba
         return brain.getColour();
     }
 
-    public @Nullable ResourceLocation getOverlay() {
+    public @Nullable TurtleOverlay getOverlay() {
-        return brain.getOverlay();
+        var overlay = brain.getOverlay();
+        return overlay == null ? null : overlay.value();
     }
 
     public ITurtleAccess getAccess() {

projects/common/src/main/java/dan200/computercraft/shared/turtle/core/TurtleBrain.java

@@ -20,6 +20,7 @@ import dan200.computercraft.shared.computer.core.ComputerFamily;
 import dan200.computercraft.shared.computer.core.ServerComputer;
 import dan200.computercraft.shared.config.Config;
 import dan200.computercraft.shared.container.InventoryDelegate;
+import dan200.computercraft.shared.turtle.TurtleOverlay;
 import dan200.computercraft.shared.turtle.blocks.TurtleBlockEntity;
 import dan200.computercraft.shared.util.BlockEntityHelpers;
 import dan200.computercraft.shared.util.Holiday;
@@ -32,7 +33,6 @@ import net.minecraft.core.component.DataComponentPatch;
 import net.minecraft.core.particles.ParticleTypes;
 import net.minecraft.nbt.CompoundTag;
 import net.minecraft.nbt.Tag;
-import net.minecraft.resources.ResourceLocation;
 import net.minecraft.server.level.ServerLevel;
 import net.minecraft.tags.FluidTags;
 import net.minecraft.world.Container;
@@ -74,7 +74,7 @@ public class TurtleBrain implements TurtleAccessInternal {
     private int selectedSlot = 0;
     private int fuelLevel = 0;
     private int colourHex = -1;
-    private @Nullable ResourceLocation overlay = null;
+    private @Nullable Holder<TurtleOverlay> overlay = null;
 
     private TurtleAnimation animation = TurtleAnimation.NONE;
     private int animationProgress = 0;
@@ -134,7 +134,7 @@ public class TurtleBrain implements TurtleAccessInternal {
         // Read fields
         colourHex = nbt.contains(NBT_COLOUR) ? nbt.getInt(NBT_COLOUR) : -1;
         fuelLevel = nbt.contains(NBT_FUEL) ? nbt.getInt(NBT_FUEL) : 0;
-        overlay = nbt.contains(NBT_OVERLAY) ? ResourceLocation.parse(nbt.getString(NBT_OVERLAY)) : null;
+        overlay = nbt.contains(NBT_OVERLAY) ? NBTUtil.decodeFrom(TurtleOverlay.CODEC, registries, nbt, NBT_OVERLAY) : null;
 
         // Read upgrades
         setUpgradeDirect(TurtleSide.LEFT, NBTUtil.decodeFrom(TurtleUpgrades.instance().upgradeDataCodec(), registries, nbt, NBT_LEFT_UPGRADE));
@@ -144,7 +144,7 @@ public class TurtleBrain implements TurtleAccessInternal {
     private void writeCommon(CompoundTag nbt, HolderLookup.Provider registries) {
         nbt.putInt(NBT_FUEL, fuelLevel);
         if (colourHex != -1) nbt.putInt(NBT_COLOUR, colourHex);
-        if (overlay != null) nbt.putString(NBT_OVERLAY, overlay.toString());
+        if (overlay != null) NBTUtil.encodeTo(TurtleOverlay.CODEC, registries, nbt, NBT_OVERLAY, overlay);
 
         // Write upgrades
         NBTUtil.encodeTo(TurtleUpgrades.instance().upgradeDataCodec(), registries, nbt, NBT_LEFT_UPGRADE, getUpgradeWithData(TurtleSide.LEFT));
@@ -418,11 +418,11 @@ public class TurtleBrain implements TurtleAccessInternal {
         BlockEntityHelpers.updateBlock(owner);
     }
 
-    public @Nullable ResourceLocation getOverlay() {
+    public @Nullable Holder<TurtleOverlay> getOverlay() {
         return overlay;
     }
 
-    public void setOverlay(@Nullable ResourceLocation overlay) {
+    public void setOverlay(@Nullable Holder<TurtleOverlay> overlay) {
         if (!Objects.equals(this.overlay, overlay)) {
             this.overlay = overlay;
             BlockEntityHelpers.updateBlock(owner);

projects/common/src/main/java/dan200/computercraft/shared/turtle/items/TurtleItem.java

@@ -12,11 +12,11 @@ import dan200.computercraft.api.upgrades.UpgradeData;
 import dan200.computercraft.impl.TurtleUpgrades;
 import dan200.computercraft.shared.ModRegistry;
 import dan200.computercraft.shared.computer.items.AbstractComputerItem;
+import dan200.computercraft.shared.turtle.TurtleOverlay;
 import dan200.computercraft.shared.turtle.blocks.TurtleBlock;
 import net.minecraft.core.cauldron.CauldronInteraction;
 import net.minecraft.core.component.DataComponents;
 import net.minecraft.network.chat.Component;
-import net.minecraft.resources.ResourceLocation;
 import net.minecraft.world.ItemInteractionResult;
 import net.minecraft.world.item.ItemStack;
 import net.minecraft.world.level.block.LayeredCauldronBlock;
@@ -74,8 +74,9 @@ public class TurtleItem extends AbstractComputerItem {
         return stack.get(side == TurtleSide.LEFT ? ModRegistry.DataComponents.LEFT_TURTLE_UPGRADE.get() : ModRegistry.DataComponents.RIGHT_TURTLE_UPGRADE.get());
     }
 
-    public static @Nullable ResourceLocation getOverlay(ItemStack stack) {
+    public static @Nullable TurtleOverlay getOverlay(ItemStack stack) {
-        return stack.get(ModRegistry.DataComponents.OVERLAY.get());
+        var overlay = stack.get(ModRegistry.DataComponents.OVERLAY.get());
+        return overlay == null ? null : overlay.value();
     }
 
     public static int getFuelLevel(ItemStack stack) {

projects/common/src/main/java/dan200/computercraft/shared/util/ComponentizationFixers.java

@@ -82,7 +82,8 @@ public class ComponentizationFixers {
 
         if (item.is(TURTLES)) {
             item.moveTagToComponent("Fuel", "computercraft:fuel");
-            item.moveTagToComponent("Overlay", "computercraft:overlay");
+
+            item.removeTag("Overlay").result().ifPresent(x -> item.setComponent("computercraft:overlay", fixOverlay(x)));
 
             moveUpgradeToComponent(item, ops, "LeftUpgrade", "LeftUpgradeNbt", "computercraft:left_turtle_upgrade");
             moveUpgradeToComponent(item, ops, "RightUpgrade", "RightUpgradeNbt", "computercraft:right_turtle_upgrade");
@@ -152,6 +153,7 @@ public class ComponentizationFixers {
         return typed -> typed.updateTyped(input, output, typed2 -> typed2.update(DSL.remainderFinder(), x -> {
             x = moveUpgradeData(x, "LeftUpgrade", "LeftUpgradeNbt");
             x = moveUpgradeData(x, "RightUpgrade", "RightUpgradeNbt");
+            x = x.update("Overlay", ComponentizationFixers::fixOverlay);
             return x;
         }));
     }
@@ -164,6 +166,20 @@ public class ComponentizationFixers {
         return ops.set(key, createUpgradeData(ops, upgradeId, ops.get(dataKey))).remove(dataKey);
     }
 
+    private static Dynamic<?> fixOverlay(Dynamic<?> overlay) {
+        // Rewrite known overlays to their new ids.
+        var overlayId = overlay.asString(null);
+        if (overlayId == null) return overlay;
+
+        return switch (overlayId) {
+            // Map known overlays to their new id.
+            case "computercraft:block/turtle_trans_overlay" -> overlay.createString("computercraft:trans_flag");
+            case "computercraft:block/turtle_rainbow_overlay" -> overlay.createString("computercraft:rainbow_flag");
+            // And unknown overlays to a direct entry.
+            default -> overlay.emptyMap().set("model", overlay);
+        };
+    }
+
     /**
      * Create a new upgrade data.
      *

projects/common/src/testMod/kotlin/dan200/computercraft/gametest/Turtle_Test.kt

@@ -4,6 +4,7 @@
 
 package dan200.computercraft.gametest
 
+import dan200.computercraft.api.ComputerCraftAPI
 import dan200.computercraft.api.detail.BasicItemDetailProvider
 import dan200.computercraft.api.detail.VanillaDetailRegistries
 import dan200.computercraft.api.lua.ObjectArguments
@@ -21,7 +22,9 @@ import dan200.computercraft.shared.peripheral.modem.wired.CableBlock
 import dan200.computercraft.shared.peripheral.modem.wired.CableModemVariant
 import dan200.computercraft.shared.peripheral.monitor.MonitorBlock
 import dan200.computercraft.shared.peripheral.monitor.MonitorEdgeState
+import dan200.computercraft.shared.turtle.TurtleOverlay
 import dan200.computercraft.shared.turtle.apis.TurtleAPI
+import dan200.computercraft.shared.turtle.items.TurtleItem
 import dan200.computercraft.shared.util.WaterloggableHelpers
 import dan200.computercraft.test.core.assertArrayEquals
 import dan200.computercraft.test.core.computer.LuaTaskContext
@@ -654,6 +657,27 @@ class Turtle_Test {
         }
     }
 
+    /**
+     * Loads a structure created on an older version of the game, and checks that data fixers have been applied.
+     */
+    @GameTest
+    fun Data_fixers(helper: GameTestHelper) = helper.sequence {
+        thenExecute {
+            val overlay = helper.level.registryAccess().registryOrThrow(TurtleOverlay.REGISTRY)
+                .get(ResourceLocation.fromNamespaceAndPath(ComputerCraftAPI.MOD_ID, "trans_flag"))!!
+            val upgrade = helper.level.registryAccess().registryOrThrow(ITurtleUpgrade.REGISTRY)
+                .get(ResourceLocation.withDefaultNamespace("diamond_pickaxe"))!!
+
+            val turtleBe = helper.getBlockEntity(BlockPos(1, 2, 1), ModRegistry.BlockEntities.TURTLE_NORMAL.get())
+            assertEquals(overlay, turtleBe.overlay)
+            assertEquals(upgrade, turtleBe.getUpgrade(TurtleSide.LEFT))
+
+            val turtleItem = turtleBe.getItem(0)
+            assertEquals(overlay, TurtleItem.getOverlay(turtleItem))
+            assertEquals(upgrade, TurtleItem.getUpgrade(turtleItem, TurtleSide.LEFT))
+        }
+    }
+
     /**
      * `turtle.suck` only pulls for the current side.
      */

projects/common/src/testMod/resources/data/cctest/structures/turtle_test.data_fixers.snbt

@@ -0,0 +1,39 @@
+{
+    DataVersion: 3465,
+    size: [3, 3, 3],
+    data: [
+        {pos: [0, 0, 0], state: "minecraft:polished_andesite"},
+        {pos: [0, 0, 1], state: "minecraft:polished_andesite"},
+        {pos: [0, 0, 2], state: "minecraft:polished_andesite"},
+        {pos: [1, 0, 0], state: "minecraft:polished_andesite"},
+        {pos: [1, 0, 1], state: "minecraft:polished_andesite"},
+        {pos: [1, 0, 2], state: "minecraft:polished_andesite"},
+        {pos: [2, 0, 0], state: "minecraft:polished_andesite"},
+        {pos: [2, 0, 1], state: "minecraft:polished_andesite"},
+        {pos: [2, 0, 2], state: "minecraft:polished_andesite"},
+        {pos: [0, 1, 0], state: "minecraft:air"},
+        {pos: [0, 1, 1], state: "minecraft:air"},
+        {pos: [0, 1, 2], state: "minecraft:air"},
+        {pos: [1, 1, 0], state: "minecraft:air"},
+        {pos: [1, 1, 1], state: "computercraft:turtle_normal{facing:south,waterlogged:false}", nbt: {ComputerId: 123, Fuel: 0, Items: [{Count: 1b, Slot: 0b, id: "computercraft:turtle_normal", tag: {ComputerId: 123, LeftUpgrade: "minecraft:diamond_pickaxe", Overlay: "computercraft:block/turtle_trans_overlay"}}], LeftUpgrade: "minecraft:diamond_pickaxe", LeftUpgradeNbt: {}, On: 0b, Overlay: "computercraft:block/turtle_trans_overlay", Owner: {LowerId: -4770154215762454977L, Name: "Player436", UpperId: 114963728012426084L}, Slot: 0, id: "computercraft:turtle_normal"}},
+        {pos: [1, 1, 2], state: "minecraft:air"},
+        {pos: [2, 1, 0], state: "minecraft:air"},
+        {pos: [2, 1, 1], state: "minecraft:air"},
+        {pos: [2, 1, 2], state: "minecraft:air"},
+        {pos: [0, 2, 0], state: "minecraft:air"},
+        {pos: [0, 2, 1], state: "minecraft:air"},
+        {pos: [0, 2, 2], state: "minecraft:air"},
+        {pos: [1, 2, 0], state: "minecraft:air"},
+        {pos: [1, 2, 1], state: "minecraft:air"},
+        {pos: [1, 2, 2], state: "minecraft:air"},
+        {pos: [2, 2, 0], state: "minecraft:air"},
+        {pos: [2, 2, 1], state: "minecraft:air"},
+        {pos: [2, 2, 2], state: "minecraft:air"}
+    ],
+    entities: [],
+    palette: [
+        "minecraft:polished_andesite",
+        "minecraft:air",
+        "computercraft:turtle_normal{facing:south,waterlogged:false}"
+    ]
+}

projects/fabric/src/client/java/dan200/computercraft/client/ComputerCraftClient.java

@@ -53,7 +53,7 @@ public class ComputerCraftClient {
         ClientRegistry.registerMainThread(ItemProperties::register);
 
         PreparableModelLoadingPlugin.register(CustomModelLoader::prepare, (state, context) -> {
-            ClientRegistry.registerExtraModels(context::addModels);
+            ClientRegistry.registerExtraModels(context::addModels, state.getExtraModels());
             context.resolveModel().register(ctx -> state.loadModel(ctx.id()));
             context.modifyModelAfterBake().register((model, ctx) -> model == null ? null : state.wrapModel(ctx, model));
         });

projects/fabric/src/client/java/dan200/computercraft/client/model/CustomModelLoader.java

@@ -25,6 +25,7 @@ import org.slf4j.LoggerFactory;
 import javax.annotation.Nullable;
 import java.io.IOException;
 import java.io.Reader;
+import java.util.Collection;
 import java.util.HashMap;
 import java.util.Map;
 import java.util.concurrent.CompletableFuture;
@@ -46,13 +47,15 @@ public final class CustomModelLoader {
 
     private final Map<ResourceLocation, UnbakedModel> models = new HashMap<>();
     private final Map<ResourceLocation, String> emissiveModels = new HashMap<>();
+    private final Collection<ResourceLocation> extraModels;
 
-    private CustomModelLoader() {
+    private CustomModelLoader(Collection<ResourceLocation> extraModels) {
+        this.extraModels = extraModels;
     }
 
     public static CompletableFuture<CustomModelLoader> prepare(ResourceManager resources, Executor executor) {
         return CompletableFuture.supplyAsync(() -> {
-            var loader = new CustomModelLoader();
+            var loader = new CustomModelLoader(ExtraModels.loadAll(resources));
             for (var resource : resources.listResources("models", x -> x.getNamespace().equals(ComputerCraftAPI.MOD_ID) && x.getPath().endsWith(".json")).entrySet()) {
                 loader.loadModel(resource.getKey(), resource.getValue());
             }
@@ -85,6 +88,10 @@ public final class CustomModelLoader {
         }
     }
 
+    public Collection<ResourceLocation> getExtraModels() {
+        return extraModels;
+    }
+
     /**
      * Load a custom model. This searches for CC models with a custom {@code loader} field.
      *

projects/fabric/src/main/java/dan200/computercraft/shared/ComputerCraft.java

@@ -25,6 +25,7 @@ import dan200.computercraft.shared.peripheral.modem.wired.WiredModemFullBlockEnt
 import dan200.computercraft.shared.peripheral.modem.wireless.WirelessModemBlockEntity;
 import dan200.computercraft.shared.platform.FabricConfigFile;
 import dan200.computercraft.shared.recipe.function.RecipeFunction;
+import dan200.computercraft.shared.turtle.TurtleOverlay;
 import net.fabricmc.fabric.api.command.v2.CommandRegistrationCallback;
 import net.fabricmc.fabric.api.event.lifecycle.v1.ServerChunkEvents;
 import net.fabricmc.fabric.api.event.lifecycle.v1.ServerLifecycleEvents;
@@ -70,6 +71,7 @@ public class ComputerCraft {
 
         DynamicRegistries.registerSynced(ITurtleUpgrade.REGISTRY, TurtleUpgrades.instance().upgradeCodec());
         DynamicRegistries.registerSynced(IPocketUpgrade.REGISTRY, PocketUpgrades.instance().upgradeCodec());
+        DynamicRegistries.registerSynced(TurtleOverlay.REGISTRY, TurtleOverlay.DIRECT_CODEC);
 
         ModRegistry.register();
         ModRegistry.registerMainThread();

projects/forge/src/client/java/dan200/computercraft/client/ForgeClientRegistry.java

@@ -6,6 +6,7 @@ package dan200.computercraft.client;
 
 import dan200.computercraft.api.ComputerCraftAPI;
 import dan200.computercraft.api.client.turtle.RegisterTurtleModellersEvent;
+import dan200.computercraft.client.model.ExtraModels;
 import dan200.computercraft.client.model.turtle.TurtleModelLoader;
 import dan200.computercraft.client.turtle.TurtleUpgradeModellers;
 import net.minecraft.client.Minecraft;
@@ -57,7 +58,8 @@ public final class ForgeClientRegistry {
     @SubscribeEvent
     public static void registerModels(ModelEvent.RegisterAdditional event) {
         gatherModellers();
-        ClientRegistry.registerExtraModels(x -> event.register(ModelResourceLocation.standalone(x)));
+        var extraModels = ExtraModels.loadAll(Minecraft.getInstance().getResourceManager());
+        ClientRegistry.registerExtraModels(x -> event.register(ModelResourceLocation.standalone(x)), extraModels);
     }
 
     @SubscribeEvent

projects/forge/src/main/java/dan200/computercraft/ComputerCraft.java

@@ -33,6 +33,7 @@ import dan200.computercraft.shared.peripheral.modem.wired.WiredModemFullBlockEnt
 import dan200.computercraft.shared.peripheral.modem.wireless.WirelessModemBlockEntity;
 import dan200.computercraft.shared.platform.ForgeConfigFile;
 import dan200.computercraft.shared.recipe.function.RecipeFunction;
+import dan200.computercraft.shared.turtle.TurtleOverlay;
 import net.minecraft.network.RegistryFriendlyByteBuf;
 import net.minecraft.network.protocol.common.custom.CustomPacketPayload;
 import net.minecraft.server.level.ServerPlayer;
@@ -92,6 +93,7 @@ public final class ComputerCraft {
     public static void registerDynamicRegistries(DataPackRegistryEvent.NewRegistry event) {
         event.dataPackRegistry(ITurtleUpgrade.REGISTRY, TurtleUpgrades.instance().upgradeCodec(), TurtleUpgrades.instance().upgradeCodec());
         event.dataPackRegistry(IPocketUpgrade.REGISTRY, PocketUpgrades.instance().upgradeCodec(), PocketUpgrades.instance().upgradeCodec());
+        event.dataPackRegistry(TurtleOverlay.REGISTRY, TurtleOverlay.DIRECT_CODEC, TurtleOverlay.DIRECT_CODEC);
     }
 
     @SubscribeEvent