I have an item with three sizes. Is it possible to use just one texture for all three, and specify which part of the texture file to use? So the large would use the entire texture, the medium would use a 10x10 portion of the texture, and the small would use 6x6. I know the item model supports the use of elements, and you can set uv coordinates for the elements, but I'm not sure how exactly I would use that in this case, or if it's even possible. I can't think of any examples to look at. TIA.

6 hours ago, Daeruin said:

I know the item model supports the use of elements, and you can set uv coordinates for the elements

This is what you would do. You can use an editor like BlockBench to make the model. Torch block models use UVs IIRC

5 hours ago, Cadiboo said:

Torch block models use UVs IIRC

Yes, it does. Fairly good example of it's use too. (template_torch.json)

 

{
    "ambientocclusion": false,
    "textures": {
        "particle": "#torch"
    },
    "elements": [
        {   "from": [ 7, 0, 7 ],
            "to": [ 9, 10, 9 ],
            "shade": false,
            "faces": {
                "down": { "uv": [ 7, 13, 9, 15 ], "texture": "#torch" },
                "up":   { "uv": [ 7,  6, 9,  8 ], "texture": "#torch" }
            }
        },
        ...
    ]
}

I'm talking about an item, not a block. I didn't want to have to make a whole 3D item model. That wouldn't save me any time. I'm wanting to use the built-in item rendering, but just using a portion of an existing texture image.

All items have 3d models. Some of them just happen to have a single flat element with a full texture UV. You can even find the model at item/generated.json (I think). You can either split your texture into multiple textures or have a non-default model. I would recommend the former as it’s more future-proof and will still be compatible with whatever extra stuff minecraft does to get the 1-pixel-wide sides to the default models.