# Bounding boxes in FLUX 3 Image Klein

> Updated 2026-10-16 · https://imageklein.com/flux-3-image-klein-bounding-boxes

In FLUX 3 Image Klein you control layout by ending the prompt with a JSON list of boxes. Each box has coordinates [y0, x0, y1, x1] on a 0–1000 grid and an id you mention in the text as <id>; boxes can add a new element, move an existing one, or anchor what must stay unchanged.

## The format

A prompt is your instruction, a space, then a JSON array. Each item:

| Key | Meaning |
|---|---|
| id | Name you use in the instruction as <id> |
| from | "ref_image_0" for something already in the input image, null for a new element |
| src_bbox | Where it is now — null for new elements |
| tgt_bbox | Where it should be: [y0, x0, y1, x1], 0–1000 from the top-left |
| desc | A clear description of the element |

Three kinds of box: **new** (generated at the target), **move** (source → target), and **anchor** (source = target, keep it as is). The input image is referred to as `<ref_image_0>`.

## Example: place new elements

```
A cozy reading corner with a green armchair <chair_1> on the left and a tall floor lamp <lamp_1> on the right, warm evening light. [{"id":"chair_1","from":null,"src_bbox":null,"tgt_bbox":[400,50,950,500],"desc":"A green velvet armchair."},{"id":"lamp_1","from":null,"src_bbox":null,"tgt_bbox":[100,700,950,850],"desc":"A tall brass floor lamp with a cream shade."}]
```

## Example: move one thing, keep the rest

```
In <ref_image_0>, move the coffee cup <cup_1> to the right side of the table and keep the laptop <laptop_1> exactly where it is. [{"id":"cup_1","from":"ref_image_0","src_bbox":[520,120,700,280],"tgt_bbox":[520,700,700,860],"desc":"A white ceramic coffee cup."},{"id":"laptop_1","from":"ref_image_0","src_bbox":[300,300,750,680],"tgt_bbox":[300,300,750,680],"desc":"An open silver laptop."}]
```

## Tips

- Give a box to everything that must not change (anchors) — not only to what you edit.
- Write specific `desc` text: material, color, size.
- Use a move box only when you know both where the element is and where it should go.
- Boxes guide placement and scale; they are not hard masks. Elements can spill over, very small ones may not appear, and edits can touch nearby pixels.
- Drawing boxes by hand is easier in the official Gradio demo in BFL's GitHub repo or the Hugging Face Space.

The format is documented by BFL for FLUX 3 Image, and FLUX 3 Image Klein keeps the same bounding-box feature. Our examples are illustrations, not BFL's.

## FAQ

**What coordinate system do FLUX 3 bounding boxes use?**
[y0, x0, y1, x1] on a normalized 0–1000 grid measured from the top-left corner; [600, 0, 1000, 1000] is the bottom 40% of the image.

**Are FLUX 3 Image Klein bounding boxes masks?**
No. They guide where and how big elements are; content can extend past a box and edits can affect nearby areas.


## Sources

- [BFL docs — FLUX 3 Image bounding boxes](https://docs.bfl.ai/flux_3/flux3_image_bounding_boxes)
- [Hugging Face — black-forest-labs/flux-3-image-klein (model card)](https://huggingface.co/black-forest-labs/flux-3-image-klein)
- [Black Forest Labs — FLUX 3 Image Klein (blog, Oct 15, 2026)](https://bfl.ai/blog/flux-3-image-klein)
- [GitHub — black-forest-labs/flux-3-image-klein (reference code)](https://github.com/black-forest-labs/flux-3-image-klein)