add embed images in conversion to v3 script; add parquet writer in conversion script

src/lerobot/datasets/v30/convert_dataset_v21_to_v30.py

@@ -50,9 +50,9 @@ from typing import Any
 
 import jsonlines
 import pandas as pd
-import pyarrow as pa
+import pyarrow.parquet as pq
 import tqdm
-from datasets import Dataset, Features, Image
+from datasets import Dataset, concatenate_datasets
 from huggingface_hub import HfApi, snapshot_download
 from requests import HTTPError
 
@@ -68,6 +68,7 @@ from lerobot.datasets.utils import (
     LEGACY_EPISODES_STATS_PATH,
     LEGACY_TASKS_PATH,
     cast_stats_to_numpy,
+    embed_images,
     flatten_dict,
     get_file_size_in_mb,
     get_parquet_file_size_in_mb,
@@ -175,24 +176,34 @@ def convert_tasks(root, new_root):
 
 
 def concat_data_files(paths_to_cat, new_root, chunk_idx, file_idx, image_keys):
-    # TODO(rcadene): to save RAM use Dataset.from_parquet(file) and concatenate_datasets
-    dataframes = [pd.read_parquet(file) for file in paths_to_cat]
-    # Concatenate all DataFrames along rows
-    concatenated_df = pd.concat(dataframes, ignore_index=True)
+    """Concatenate multiple parquet data files into a single file.
+
+    This function uses HuggingFace datasets to properly handle image embedding,
+    ensuring the output has the same internal structure as datasets created through
+    live recording. This is critical for training performance.
+
+    Args:
+        paths_to_cat: List of parquet file paths to concatenate
+        new_root: Root directory for the new dataset
+        chunk_idx: Chunk index for the output file
+        file_idx: File index within the chunk
+        image_keys: List of feature keys that contain images
+    """
+
+    datasets_list: list[Dataset] = [Dataset.from_parquet(str(file)) for file in paths_to_cat]  # type: ignore[misc]
+    concatenated_ds: Dataset = concatenate_datasets(datasets_list)
+
+    if len(image_keys) > 0:
+        logging.debug(f"Embedding {len(image_keys)} image features for optimal training performance")
+        concatenated_ds = embed_images(concatenated_ds)
 
     path = new_root / DEFAULT_DATA_PATH.format(chunk_index=chunk_idx, file_index=file_idx)
     path.parent.mkdir(parents=True, exist_ok=True)
 
-    if len(image_keys) > 0:
-        schema = pa.Schema.from_pandas(concatenated_df)
-        features = Features.from_arrow_schema(schema)
-        for key in image_keys:
-            features[key] = Image()
-        schema = features.arrow_schema
-    else:
-        schema = None
-
-    concatenated_df.to_parquet(path, index=False, schema=schema)
+    table = concatenated_ds.with_format("arrow")[:]
+    writer = pq.ParquetWriter(path, schema=table.schema, compression="snappy", use_dictionary=True)
+    writer.write_table(table)
+    writer.close()
 
 
 def convert_data(root: Path, new_root: Path, data_file_size_in_mb: int):