Item polys

The default poly generation for items works really well, so I don't know why you'd want to make a custom item poly. But I'm not here to judge.

One basic thing you might want to do is to use a different base item for your modded item. This is easy enough:

public void registerPolys(PolyRegistry registry) {
    registry.registerItemPoly(moddedItem, new CustomModelDataPoly(registery.getCMDManager(), moddedItem, Items.DIAMOND_SWORD))
}

Note

If your item is damageable, use DamageableItemPoly instead of CustomModelDataPoly.

If you want to create your own item poly, you should implement the ItemPoly interface. Never modify the input itemstack directly, always make a copy before doing so.

Global item polys

Global item polys are applied to all items before they are sent to the client. Global item polys implement ItemTransformer and can be registered using PolyRegistry#registerGlobalItemPoly. Keep in mind that, like normal item polys, global item polys should never modify the input itemstack directly.

The CustomModelDataManager

The CMD manager is made to prevent polys from using the same CMD values on the same items. At the simplest level, you can request a CMD value to use for a specific item. You can also request a number of values, the number returned is the first value you may use, see the javadoc for more detail. The CMD manager can also round-robin from a list of items. This is the preferred way of doing things as this prevents recipe conflicts (the recipe book doesn’t take the CMD value into account, this isn’t a huge deal but is a minor annoyance to clients).

To get the CustomModelDataManager, use PolyRegistry.getSharedValues(CustomModelDataManager.KEY).