[doc] chore: update doc on how to redownload and rebuild dataset

README.md

@@ -525,6 +525,156 @@ We only share the annotations (https://huggingface.co/datasets/YongchengYAO/MedV
   ```
 </details>
 
+
+## Dataset Building Workflow
+
+### Workflow
+
+<details>
+<summary> MedVision Dataset Building Workflow (Black) </summary>
+<img src="fig/medvision-dataset-flow-b.png" alt="MedVision Dataset Building Workflow (Black)" /><br>
+</details>
+
+<details>
+<summary> MedVision Dataset Building Workflow (White) </summary>
+<img src="fig/medvision-dataset-flow-w.png" alt="MedVision Dataset Building Workflow (White)" /><br>
+</details>
+
+</br>
+
+There are a few venues to control the dataset loading and building behavior:
+
+- **Rebuild Dataset (Arrow files)**: Use the `download_mode` argument in `load_dataset()` ([docs](https://huggingface.co/docs/datasets/v3.6.0/en/package_reference/builder_classes#datasets.DownloadMode)).
+  - **[1]** Set `download_mode="force_redownload"` to ignore the cached Arrow files and trigger the data loading script `MedVision.py` to rebuild the dataset.
+- **Redownload Raw Data**:
+  - **[2]** `MedVision_FORCE_DOWNLOAD_DATA`: Set this environment variable to `True` to force re-downloading raw images and annotations.
+  - **[3]** `.downloaded_datasets.json`: This tracker file records downloaded status. Removing a dataset's entry here will trigger a re-download of the raw data for that dataset.
+  
+> [!Note] ⚠️ 
+> **How to properly update/redownload raw data?**
+>
+> If you need to update raw data (images, masks, landmarks) using [2] or [3], you **MUST ALSO** use [1] (`download_mode="force_redownload"`).
+>
+> Why? Because if Hugging Face finds a valid cached dataset (Arrow files), it will load it directly and **skip running the script entirely**. Without running the script, the environment variable [2] or tracker file [3] will never be checked.
+>
+> **Summary:**
+> - Update Arrow/Fields only: Use [1].
+> - Update Raw Data: Use [1] **AND** ([2] or [3]).
+>
+> 🔥 We will maintain a [change log](https://huggingface.co/datasets/YongchengYAO/MedVision/blob/main/doc/changelog.md) for essential updates.
+
+
+### Examples
+
+<details>
+<summary> Run this for the first time will download the raw data and build the dataset </summary>
+
+```python
+import os
+from datasets import load_dataset
+
+# Set data folder
+wd = os.path.join(os.getcwd(), "Data-testing")
+os.makedirs(wd, exist_ok=True)
+os.environ["MedVision_DATA_DIR"] = wd
+
+# Pick a dataset config name and split
+config = "OAIZIB-CM_BoxSize_Task01_Axial_Test"
+split_name = "test" # use "test" for testing set config; use "train" for training set config 
+
+# Get dataset
+ds = load_dataset(
+        "YongchengYAO/MedVision",
+        name=config,
+        trust_remote_code=True,
+        split=split_name,
+    )
+```
+</details>
+
+<details>
+<summary> Run the same script again will use the cached dataset </summary>
+
+```python
+import os
+from datasets import load_dataset
+
+# Set data folder
+wd = os.path.join(os.getcwd(), "Data-testing")
+os.makedirs(wd, exist_ok=True)
+os.environ["MedVision_DATA_DIR"] = wd
+
+# Pick a dataset config name and split
+config = "OAIZIB-CM_BoxSize_Task01_Axial_Test"
+split_name = "test" # use "test" for testing set config; use "train" for training set config 
+
+# Get dataset
+ds = load_dataset(
+        "YongchengYAO/MedVision",
+        name=config,
+        trust_remote_code=True,
+        split=split_name,
+    )
+```
+</details>
+
+<details>
+<summary> Adding `download_mode="force_redownload"` will skip raw data downloading and rebuild the dataset </summary>
+
+```python
+import os
+from datasets import load_dataset
+
+# Set data folder
+wd = os.path.join(os.getcwd(), "Data-testing")
+os.makedirs(wd, exist_ok=True)
+os.environ["MedVision_DATA_DIR"] = wd
+
+# Pick a dataset config name and split
+config = "OAIZIB-CM_BoxSize_Task01_Axial_Test"
+split_name = "test" # use "test" for testing set config; use "train" for training set config 
+
+# Get dataset
+ds = load_dataset(
+        "YongchengYAO/MedVision",
+        name=config,
+        trust_remote_code=True,
+        split=split_name,
+        download_mode="force_redownload",
+    )
+```
+</details>
+
+<details>
+<summary> Adding `download_mode="force_redownload"` and `os.environ["MedVision_FORCE_DOWNLOAD_DATA"] = "True"` will redownload raw data and rebuild the dataset </summary>
+
+```python
+import os
+from datasets import load_dataset
+
+# Set data folder
+wd = os.path.join(os.getcwd(), "Data-testing")
+os.makedirs(wd, exist_ok=True)
+os.environ["MedVision_DATA_DIR"] = wd
+
+# Pick a dataset config name and split
+config = "OAIZIB-CM_BoxSize_Task01_Axial_Test"
+split_name = "test" # use "test" for testing set config; use "train" for training set config 
+
+# Force redownload
+os.environ["MedVision_FORCE_DOWNLOAD_DATA"] = "True"
+
+# Get dataset
+ds = load_dataset(
+        "YongchengYAO/MedVision",
+        name=config,
+        trust_remote_code=True,
+        split=split_name,
+        download_mode="force_redownload",
+    )
+```
+</details>
+
 <br/>
 
 # Advanced Usage