Other posts on this subject seem to be outdated, as mentioned methods no longer exist.

 

I feel like this should be very simple but I need a background for a slot in a custom GUI. I just can't seem to understand how to add an image file from my mod resources to a sprite location. I know sprites have to be within atlas textures and I elected to stitch it to the atlas blocks PNG which I know is publicly referenced in PlayerContainer. I've gone into a TextureStitchEvent.Pre subscriber method and run

if(!event.getMap().getTextureLocation().equals(PlayerContainer.LOCATION_BLOCKS_TEXTURE)) return;

event.addSprite(new ResourceLocation(MOD_ID,"item/empty_item_slot_sword"));

but, unfortunately, this doesn't seem to add the sprite to the map and I think this is because I never give it the proper file location of the image. Very hard to be sure it doesn't work though but it doesn't appear as a background to the slots in my custom GUI both when I use this.setBackground or override getBackground. I'm not sure where I'm supposed to assign a sprite an image file or even if I need to. Vanilla's way of doing this is a bit tricky to understand and I don't see any of the empty_armor_slot PNGs being referenced anywhere except the LegacyPackWrapper (which maps new IDs to old IDs, probably to help with resource packs).

 

Any help would be very cool

The method still revolves around TextureStitchEvent$Pre and overriding #getBackground. Please show your slot implementation.

private static final ResourceLocation EMPTY_SWORD_TEXTURE = new ResourceLocation("item/empty_item_slot_sword");

@Override
public Pair<ResourceLocation, ResourceLocation> getBackground() {
    return Pair.of(PlayerContainer.LOCATION_BLOCKS_TEXTURE, EMPTY_SWORD_TEXTURE);
}

This is the relevant code for the background. The class itself extends SlotItemHandler which is a subclass of Slot. Not sure what else I have to do to make it work 

Noticed that right after posting and yep that fixed it. Cheers, sorry for wasting your time.