 b/docs/data/converters.md
+# Converters {: #converters }
+Data can be read from and writen to various sources, like JSON/BRAT/CSV files or dataframes, which expect a key-value representation and not Doc object.
+For that purpose, we document here a set of converters that can be used to convert between these representations and Doc objects.
+Converters can be configured in the `from_*` (or `read_*` in the case of files) and `to_*` (or `write_*` in the case of files) methods, depending on the chosen `converter` argument, which can be:
+- a function, in which case it will be interpreted as a custom converter
+- a string, in which case it will be interpreted as the name of a pre-defined converter
+## No converter (`converter=None`) {: #none }
+Except in `read_standoff` and `write_standoff`, the default converter is `None`. When `converter=None`, readers output the raw content of the input data (most often dictionaries) and writers expect dictionaries. This can actually be useful is you plan to use Streams without converting to Doc objects, for instance to parallelizing the execution of a function on raw Json, Parquet files or simple lists.
+```python
+import edsnlp.data
+def complex_func(n):
+    return n * n
+stream = edsnlp.data.from_iterable(range(20))
+stream = stream.map(complex_func)
+stream = stream.set_processing(num_cpu_workers=2)
+res = list(stream)
+```
+## Custom converter {: #custom }
+You can always define your own converter functions to convert between your data and Doc objects.
+### Reading from a custom schema
+```{ .python .no-check }
+import edsnlp, edsnlp.pipes as eds
+from spacy.tokens import Doc
+from edsnlp.data.converters import get_current_tokenizer
+from typing import Dict
+def convert_row_to_dict(row: Dict) -> Doc:
+    # Tokenizer will be inferred from the pipeline
+    doc = get_current_tokenizer()(row["custom_content"])
+    doc._.note_id = row["custom_id"]
+    doc._.note_datetime = row["custom_datetime"]
+    # ...
+    return doc
+nlp = edsnlp.blank("eds")
+nlp.add_pipe(eds.normalizer())
+nlp.add_pipe(eds.covid())
+# Any kind of reader (`edsnlp.data.read/from_...`) can be used here
+docs = edsnlp.data.from_pandas(
+    # Path to the file or directory
+    dataframe,
+    # How to convert JSON-like samples to Doc objects
+    converter=convert_row_to_dict,
+)
+docs = docs.map_pipeline(nlp)
+```
+### Writing to a custom schema
+```{ .python .no-check }
+def convert_doc_to_row(doc: Doc) -> Dict:
+    return {
+        "custom_id": doc._.id,
+        "custom_content": doc.text,
+        "custom_datetime": doc._.note_datetime,
+        # ...
+    }
+# Any kind of writer (`edsnlp.data.write/to_...`) can be used here
+docs.write_parquet(
+    "path/to/output_folder",
+    # How to convert Doc objects to JSON-like samples
+    converter=convert_doc_to_row,
+)
+```
+!!! note "One row per entity"
+    This function can also return a list of dicts, for instance one dict per detected entity, that will be treated as multiple rows in dataframe writers (e.g., `to_pandas`, `to_spark`, `write_parquet`).
+    ```{ .python .no-check }
+    def convert_ents_to_rows(doc: Doc) -> List[Dict]:
+        return [
+            {
+                "note_id": doc._.id,
+                "ent_text": ent.text,
+                "ent_label": ent.label_,
+                "custom_datetime": doc._.note_datetime,
+                # ...
+            }
+            for ent in doc.ents
+        ]
+    docs.write_parquet(
+        "path/to/output_folder",
+        # How to convert entities of Doc objects to JSON-like samples
+        converter=convert_ents_to_rows,
+    )
+    ```
+## OMOP (`converter="omop"`) {: #omop }
+OMOP is a schema that is used in the medical domain. It is based on the [OMOP Common Data Model](https://ohdsi.github.io/CommonDataModel/). We are mainly interested in the `note` table, which contains
+the clinical notes, and deviate from the original schema by adding an *optional* `entities` column that can be computed from the `note_nlp` table.
+Therefore, a complete OMOP-style document would look like this:
+```{ .json }
+{
+  "note_id": 0,
+  "note_text": "Le patient ...",
+  "entities": [
+    {
+      "note_nlp_id": 0,
+      "start_char": 3,
+      "end_char": 10,
+      "lexical_variant": "patient",
+      "note_nlp_source_value": "person",
+      # optional fields
+      "negated": False,
+      "certainty": "probable",
+      ...
+    },
+    ...
+  ],
+  # optional fields
+  "custom_doc_field": "..."
+  ...
+}
+```
+### Converting OMOP data to Doc objects {: #edsnlp.data.converters.OmopDict2DocConverter }
+::: edsnlp.data.converters.OmopDict2DocConverter
+    options:
+        heading_level: 4
+        show_source: false
+### Converting Doc objects to OMOP data {: #edsnlp.data.converters.OmopDoc2DictConverter }
+::: edsnlp.data.converters.OmopDoc2DictConverter
+    options:
+        heading_level: 4
+        show_source: false
+## Standoff (`converter="standoff"`) {: #standoff }
+Standoff refers mostly to the [BRAT standoff format](https://brat.nlplab.org/standoff.html), but doesn't indicate how
+the annotations should be stored in a JSON-like schema. We use the following schema:
+```{ .json }
+{
+  "doc_id": 0,
+  "text": "Le patient ...",
+  "entities": [
+    {
+      "entity_id": 0,
+      "label": "drug",
+      "fragments": [{
+        "start": 0,
+        "end": 10
+      }],
+      "attributes": {
+        "negated": True,
+        "certainty": "probable"
+      }
+    },
+    ...
+  ]
+}
+```
+### Converting Standoff data to Doc objects {: #edsnlp.data.converters.StandoffDict2DocConverter }
+::: edsnlp.data.converters.StandoffDict2DocConverter
+    options:
+        heading_level: 4
+        show_source: false
+### Converting Doc objects to Standoff data {: #edsnlp.data.converters.StandoffDoc2DictConverter }
+::: edsnlp.data.converters.StandoffDoc2DictConverter
+    options:
+        heading_level: 4
+        show_source: false
+## Entities (`converter="ents"`) {: #edsnlp.data.converters.EntsDoc2DictConverter }
+We also provide a simple one-way (export) converter to convert Doc into a list of dictionaries,
+one per entity, that can be used to write to a dataframe. The schema of each produced row is the following:
+```{ .json }
+{
+    "note_id": 0,
+    "start": 3,
+    "end": 10,
+    "label": "drug",
+    "lexical_variant": "patient",
+    # Optional fields
+    "negated": False,
+    "certainty": "probable"
+    ...
+}
+```
+::: edsnlp.data.converters.EntsDoc2DictConverter
+    options:
+        heading_level: 4
+        show_source: false