Add dataset card

README.md

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+---
+pretty_name: NC4K
+task_categories:
+- image-segmentation
+- mask-generation
+size_categories:
+- 1K<n<10K
+tags:
+- camouflaged-object-detection
+- background-removal
+- salient-object-detection
+configs:
+- config_name: default
+  data_files:
+  - path: data/test-*
+    split: test
+dataset_info:
+  features:
+  - dtype: image
+    name: image
+  - dtype: image
+    name: mask
+  splits:
+  - name: test
+    num_examples: 4121
+---
+
+# NC4K
+
+The **NC4K** camouflaged-object test set — 4121 images with binary masks, as
+one `test` split, which is how the benchmark is published and scored.
+
+```python
+from datasets import load_dataset
+
+ds = load_dataset("nobg/NC4K", split="test")   # 4121 rows
+ds[0]["image"]   # PIL, original resolution
+ds[0]["mask"]    # PIL, the binary mask
+```
+
+## Why this mirror exists
+
+NC4K is the largest camouflaged-object *test* set and is published **test-only**: every
+paper that reports it reports it over all 4121 images. The upstream mirror
+[`PassbyGrocer/NC4K`](https://huggingface.co/datasets/PassbyGrocer/NC4K) had split it
+2884 / 618 / 619 into train / validation / test — a partition invented by the mirror, not
+present in the benchmark. Scoring its `test` split yields a number over 619 images that
+**no published result is comparable to**, while looking like a valid NC4K score.
+
+This mirror concatenates the three back into one `test` split and drops the invented
+boundary. Image and mask bytes are **bit-identical** to the source (verified by SHA-256 on
+all 4121 rows of both columns, in order — nothing is decoded or re-encoded);
+`gt` is renamed to `mask` for uniformity with the other `nobg` sets, and the per-object
+`instance` column is dropped.
+
+The source also ships a fourth, single-row `valid-00000-of-00001.parquet` that its own
+dataset config does not reference. It is **not** a 4122th image: the row is
+a 352×352 resize — the standard COD training resolution — whereas all 4121 real
+rows are at original resolution across 1 878 distinct sizes, none of them 352×352. It is a
+preprocessing artifact and is excluded.
+
+## Licensing
+
+**No license is declared** — not by NC4K's authors and not by the upstream mirror. It is
+left unset here rather than guessed; check with the original authors before any use beyond
+research. Same posture as [`nobg/COD10K`](https://huggingface.co/datasets/nobg/COD10K).
+
+## Citation
+
+```bibtex
+@inproceedings{lv2021simultaneously,
+  title={Simultaneously Localize, Segment and Rank the Camouflaged Objects},
+  author={Lv, Yunqiu and Zhang, Jing and Dai, Yuchao and Li, Aixuan and Liu, Bowen and Barnes, Nick and Fan, Deng-Ping},
+  booktitle={CVPR},
+  year={2021}
+}
+```