Layered with mixins

- Render netherite diving gear with both layers properly
- Mixin to HumanoidArmorLayer to render both layers for all slots

src/main/java/com/simibubi/create/AllItems.java

@@ -252,32 +252,32 @@ public class AllItems {
 		.model((c, p) -> p.withExistingParent(c.getName(), p.mcLoc("item/barrier")))
 		.register();
 
-	public static final ItemEntry<BacktankItem>
+	public static final ItemEntry<? extends BacktankItem>
 
 		COPPER_BACKTANK = REGISTRATE.item("copper_backtank", p -> new BacktankItem(AllArmorMaterials.COPPER, p, Create.asResource("copper_diving"), COPPER_BACKTANK_PLACEABLE))
 			.model(AssetLookup.customGenericItemModel("_", "item"))
 			.tag(AllItemTags.PRESSURIZED_AIR_SOURCES.tag)
 			.register(),
 
-		NETHERITE_BACKTANK = REGISTRATE.item("netherite_backtank", p -> new BacktankItem(ArmorMaterials.NETHERITE, p, Create.asResource("netherite_diving"), NETHERITE_BACKTANK_PLACEABLE))
+		NETHERITE_BACKTANK = REGISTRATE.item("netherite_backtank", p -> new BacktankItem.MultiLayered(ArmorMaterials.NETHERITE, p, Create.asResource("netherite_diving"), NETHERITE_BACKTANK_PLACEABLE))
 			.model(AssetLookup.customGenericItemModel("_", "item"))
 			.tag(AllItemTags.PRESSURIZED_AIR_SOURCES.tag)
 			.register();
 
-	public static final ItemEntry<DivingHelmetItem>
+	public static final ItemEntry<? extends DivingHelmetItem>
 
 		COPPER_DIVING_HELMET = REGISTRATE.item("copper_diving_helmet", p -> new DivingHelmetItem(AllArmorMaterials.COPPER, p, Create.asResource("copper_diving")))
 			.register(),
 
-		NETHERITE_DIVING_HELMET = REGISTRATE.item("netherite_diving_helmet", p -> new DivingHelmetItem(ArmorMaterials.NETHERITE, p, Create.asResource("netherite_diving")))
+		NETHERITE_DIVING_HELMET = REGISTRATE.item("netherite_diving_helmet", p -> new DivingHelmetItem.MultiLayered(ArmorMaterials.NETHERITE, p, Create.asResource("netherite_diving")))
 			.register();
 
-	public static final ItemEntry<DivingBootsItem>
+	public static final ItemEntry<? extends DivingBootsItem>
 
 		COPPER_DIVING_BOOTS = REGISTRATE.item("copper_diving_boots", p -> new DivingBootsItem(AllArmorMaterials.COPPER, p, Create.asResource("copper_diving")))
 			.register(),
 
-		NETHERITE_DIVING_BOOTS = REGISTRATE.item("netherite_diving_boots", p -> new DivingBootsItem(ArmorMaterials.NETHERITE, p, Create.asResource("netherite_diving")))
+		NETHERITE_DIVING_BOOTS = REGISTRATE.item("netherite_diving_boots", p -> new DivingBootsItem.MultiLayered(ArmorMaterials.NETHERITE, p, Create.asResource("netherite_diving")))
 			.register();
 
 	public static final ItemEntry<SandPaperItem> SAND_PAPER = REGISTRATE.item("sand_paper", SandPaperItem::new)

src/main/java/com/simibubi/create/content/curiosities/armor/BacktankItem.java

@@ -1,10 +1,12 @@
 package com.simibubi.create.content.curiosities.armor;
 
+import java.util.Locale;
 import java.util.function.Supplier;
 
 import org.jetbrains.annotations.Nullable;
 
 import com.simibubi.create.content.curiosities.armor.CapacityEnchantment.ICapacityEnchantable;
+import com.simibubi.create.foundation.item.MultiLayeredArmorItem;
 
 import net.minecraft.core.NonNullList;
 import net.minecraft.nbt.CompoundTag;
@@ -27,9 +29,9 @@ public class BacktankItem extends BaseArmorItem implements ICapacityEnchantable
 	
 	private final Supplier<BacktankBlockItem> blockItem;
 
-	public BacktankItem(ArmorMaterial material, Properties properties, ResourceLocation textureLoc, Supplier<BacktankBlockItem> copperBacktankPlaceable) {
+	public BacktankItem(ArmorMaterial material, Properties properties, ResourceLocation textureLoc, Supplier<BacktankBlockItem> placeable) {
 		super(material, SLOT, properties, textureLoc);
-		this.blockItem = copperBacktankPlaceable;
+		this.blockItem = placeable;
 	}
 
 	@Nullable
@@ -108,4 +110,15 @@ public class BacktankItem extends BaseArmorItem implements ICapacityEnchantable
 			return this.getOrCreateDescriptionId();
 		}
 	}
+
+	public static class MultiLayered extends BacktankItem implements MultiLayeredArmorItem {
+		public MultiLayered(ArmorMaterial material, Properties properties, ResourceLocation textureLoc, Supplier<BacktankBlockItem> placeable) {
+			super(material, properties, textureLoc, placeable);
+		}
+
+		@Override
+		public String getArmorTexture(ItemStack stack, Entity entity, EquipmentSlot slot, String layer) {
+			return String.format(Locale.ROOT, "%s:textures/models/armor/%s_layer_%s.png", textureLoc.getNamespace(), textureLoc.getPath(), layer);
+		}
+	}
 }

src/main/java/com/simibubi/create/content/curiosities/armor/DivingBootsItem.java

@@ -1,5 +1,8 @@
 package com.simibubi.create.content.curiosities.armor;
 
+import java.util.Locale;
+
+import com.simibubi.create.foundation.item.MultiLayeredArmorItem;
 import com.simibubi.create.foundation.utility.NBTHelper;
 
 import net.minecraft.resources.ResourceLocation;
@@ -9,6 +12,7 @@ import net.minecraft.world.entity.LivingEntity;
 import net.minecraft.world.entity.Pose;
 import net.minecraft.world.entity.player.Player;
 import net.minecraft.world.item.ArmorMaterial;
+import net.minecraft.world.item.ItemStack;
 import net.minecraft.world.phys.Vec3;
 import net.minecraftforge.event.entity.living.LivingEvent.LivingUpdateEvent;
 import net.minecraftforge.eventbus.api.SubscribeEvent;
@@ -72,4 +76,15 @@ public class DivingBootsItem extends BaseArmorItem {
 		}
 		return true;
 	}
+
+	public static class MultiLayered extends DivingBootsItem implements MultiLayeredArmorItem {
+		public MultiLayered(ArmorMaterial material, Properties properties, ResourceLocation textureLoc) {
+			super(material, properties, textureLoc);
+		}
+
+		@Override
+		public String getArmorTexture(ItemStack stack, Entity entity, EquipmentSlot slot, String layer) {
+			return String.format(Locale.ROOT, "%s:textures/models/armor/%s_layer_%s.png", textureLoc.getNamespace(), textureLoc.getPath(), layer);
+		}
+	}
 }

src/main/java/com/simibubi/create/content/curiosities/armor/DivingHelmetItem.java

@@ -1,6 +1,9 @@
 package com.simibubi.create.content.curiosities.armor;
 
+import java.util.Locale;
+
 import com.simibubi.create.foundation.advancement.AllAdvancements;
+import com.simibubi.create.foundation.item.MultiLayeredArmorItem;
 
 import net.minecraft.resources.ResourceLocation;
 import net.minecraft.server.level.ServerPlayer;
@@ -82,4 +85,15 @@ public class DivingHelmetItem extends BaseArmorItem {
 		entity.addEffect(new MobEffectInstance(MobEffects.WATER_BREATHING, 30, 0, true, false, true));
 		BacktankUtil.consumeAir(entity, backtank, 1);
 	}
+
+	public static class MultiLayered extends DivingHelmetItem implements MultiLayeredArmorItem {
+		public MultiLayered(ArmorMaterial material, Properties properties, ResourceLocation textureLoc) {
+			super(material, properties, textureLoc);
+		}
+
+		@Override
+		public String getArmorTexture(ItemStack stack, Entity entity, EquipmentSlot slot, String layer) {
+			return String.format(Locale.ROOT, "%s:textures/models/armor/%s_layer_%s.png", textureLoc.getNamespace(), textureLoc.getPath(), layer);
+		}
+	}
 }

src/main/java/com/simibubi/create/foundation/item/MultiLayeredArmorItem.java

@@ -0,0 +1,20 @@
+package com.simibubi.create.foundation.item;
+
+import net.minecraft.world.entity.Entity;
+import net.minecraft.world.entity.EquipmentSlot;
+import net.minecraft.world.item.ArmorItem;
+import net.minecraft.world.item.DyeableLeatherItem;
+import net.minecraft.world.item.ItemStack;
+import net.minecraftforge.common.extensions.IForgeItem;
+
+/**
+ * This interface is meant to be implemented on {@link ArmorItem}s, which will allow them to be rendered on both the inner model and outer model.
+ *
+ * <p>Classes implementing this interface <b>must not</b> also implement {@link DyeableLeatherItem}.
+ *
+ * <p>Classes that implement this interface and override {@link IForgeItem#getArmorTexture(ItemStack, Entity, EquipmentSlot, String) getArmorTexture}
+ * must note that the {@code String} argument will be used for layer context instead of the type.
+ * This string will always be {@code "1"} when querying the location for the outer model or {@code "2"} when querying the location for the inner model.
+ */
+public interface MultiLayeredArmorItem {
+}

src/main/java/com/simibubi/create/foundation/mixin/HumanoidArmorLayerMixin.java

@@ -0,0 +1,87 @@
+package com.simibubi.create.foundation.mixin;
+
+import javax.annotation.Nullable;
+
+import org.spongepowered.asm.mixin.Final;
+import org.spongepowered.asm.mixin.Mixin;
+import org.spongepowered.asm.mixin.Shadow;
+import org.spongepowered.asm.mixin.Unique;
+import org.spongepowered.asm.mixin.injection.At;
+import org.spongepowered.asm.mixin.injection.Inject;
+import org.spongepowered.asm.mixin.injection.ModifyVariable;
+import org.spongepowered.asm.mixin.injection.At.Shift;
+import org.spongepowered.asm.mixin.injection.callback.CallbackInfo;
+import org.spongepowered.asm.mixin.injection.callback.CallbackInfoReturnable;
+import org.spongepowered.asm.mixin.injection.callback.LocalCapture;
+
+import com.mojang.blaze3d.vertex.PoseStack;
+import com.simibubi.create.foundation.item.MultiLayeredArmorItem;
+
+import net.minecraft.client.model.HumanoidModel;
+import net.minecraft.client.renderer.MultiBufferSource;
+import net.minecraft.client.renderer.entity.layers.HumanoidArmorLayer;
+import net.minecraft.world.entity.Entity;
+import net.minecraft.world.entity.EquipmentSlot;
+import net.minecraft.world.entity.LivingEntity;
+import net.minecraft.world.item.ArmorItem;
+import net.minecraft.world.item.ItemStack;
+
+@Mixin(HumanoidArmorLayer.class)
+public class HumanoidArmorLayerMixin {
+	@Shadow
+	@Final
+	private HumanoidModel<?> innerModel;
+	@Shadow
+	@Final
+	private HumanoidModel<?> outerModel;
+
+	@Unique
+	private boolean intercepted;
+	@Unique
+	private Boolean useInnerTexture;
+
+	@Shadow
+	private void renderArmorPiece(PoseStack poseStack, MultiBufferSource buffer, LivingEntity livingEntity, EquipmentSlot slot, int packedLight, HumanoidModel<?> model) {
+	}
+
+	@Shadow
+	private boolean usesInnerModel(EquipmentSlot slot) {
+		return false;
+	}
+
+	@Inject(method = "renderArmorPiece", at = @At(value = "INVOKE", target = "Lnet/minecraft/client/renderer/entity/layers/HumanoidArmorLayer;getParentModel()Lnet/minecraft/client/model/EntityModel;"), locals = LocalCapture.CAPTURE_FAILHARD, cancellable = true)
+	private void onRenderArmorPiece(PoseStack poseStack, MultiBufferSource buffer, LivingEntity livingEntity, EquipmentSlot slot, int packedLight, HumanoidModel<?> model, CallbackInfo ci, ItemStack stack, ArmorItem armorItem) {
+		if (intercepted) {
+			return;
+		}
+
+		if (armorItem instanceof MultiLayeredArmorItem) {
+			intercepted = true;
+
+			useInnerTexture = true;
+			renderArmorPiece(poseStack, buffer, livingEntity, slot, packedLight, innerModel);
+			useInnerTexture = false;
+			renderArmorPiece(poseStack, buffer, livingEntity, slot, packedLight, outerModel);
+
+			useInnerTexture = null;
+			intercepted = false;
+			ci.cancel();
+		}
+	}
+
+	@Inject(method = "usesInnerModel", at = @At("HEAD"), cancellable = true)
+	private void onUsesInnerModel(EquipmentSlot slot, CallbackInfoReturnable<Boolean> cir) {
+		if (useInnerTexture != null) {
+			cir.setReturnValue(useInnerTexture);
+		}
+	}
+
+	@ModifyVariable(method = "getArmorResource", at = @At(value = "INVOKE", target = "Lnet/minecraftforge/client/ForgeHooksClient;getArmorTexture(Lnet/minecraft/world/entity/Entity;Lnet/minecraft/world/item/ItemStack;Ljava/lang/String;Lnet/minecraft/world/entity/EquipmentSlot;Ljava/lang/String;)Ljava/lang/String;", shift = Shift.BEFORE), ordinal = 0)
+	private String modifyType(@Nullable String type, Entity entity, ItemStack stack, EquipmentSlot slot, @Nullable String typeArg) {
+		if (stack.getItem() instanceof MultiLayeredArmorItem) {
+			return usesInnerModel(slot) ? "2" : "1";
+		}
+
+		return type;
+	}
+}

src/main/resources/create.mixins.json

@@ -7,6 +7,7 @@
   "mixins": [
     "CustomItemUseEffectsMixin",
     "EntityMixin",
+    "HumanoidArmorLayerMixin",
     "MapItemSavedDataMixin",
     "ContraptionDriverInteractMixin",
     "accessor.AbstractProjectileDispenseBehaviorAccessor",