Stream video frames to directory

Decode video frames directly from a raw stream using PyAV — bypasses server-side metadata requirements.

Supervisely SDK version ≥ v6.73.578

Introduction

Use stream_video_frames_to_dir to decode frames directly from the raw video stream using PyAV without requiring server-side metadata.

The standard api.video.frame.download_path requires video metadata (frame count, timestamps) to be pre-calculated on the server. If that metadata has not been computed yet, the call will fail. stream_video_frames_to_dir bypasses this limitation by opening the raw video stream directly with PyAV and demuxing it locally.

Requires the video-av extra:

pip install 'supervisely[video-av]'

Performance depends on the video and the requested frame range. Use this function when server-side metadata is unavailable, not as a general-purpose speed optimization.

How to debug this tutorial

Step 1. Prepare ~/supervisely.env file with credentials. Learn more here.

Step 2. Set the required environment variables in local.env:

SERVER_ADDRESS=https://app.supervisely.com  # ⬅️ change value
API_TOKEN=your-token-here                   # ⬅️ change value
VIDEO_ID=12345                              # ⬅️ change value

Step 3. Run the script below.

Import libraries

import os
from dotenv import load_dotenv
import supervisely as sly
from supervisely.video.sampling import stream_video_frames_to_dir

Init API client

if sly.is_development():
    load_dotenv("local.env")
    load_dotenv(os.path.expanduser("~/supervisely.env"))

api = sly.Api.from_env()

Save frames to directory

Frame range (PNG)

Download frames 0–9 (first 10 frames) and save as PNG files:

video_id = int(os.environ["VIDEO_ID"])
output_dir = "/tmp/frames"

paths = stream_video_frames_to_dir(
    api=api,
    video_id=video_id,
    output_dir=output_dir,
    start=0,
    end=9,
)

print(f"Saved {len(paths)} frames:")
for p in paths:
    print(p)

Output:

Saved 10 frames:
/tmp/frames/frame_000000.png
/tmp/frames/frame_000001.png
...
/tmp/frames/frame_000009.png

Files are named frame_<index:06d>.<ext>. The output directory is created automatically if it does not exist.

All frames

JPEG output

Progress bar

Pass a progress_cb callable — it is called with 1 after each frame is saved:

with sly.tqdm_sly(total=50, message="Streaming frames") as pbar:
    paths = stream_video_frames_to_dir(
        api=api,
        video_id=video_id,
        output_dir="/tmp/frames",
        start=0,
        end=49,
        progress_cb=pbar.update,
    )

Decoding progress

Pass show_decoding_progress=True to display low-level tqdm progress bars for the demuxing phase (building the PTS map) and, for B-frame videos, the full-decode phase. Both bars count packets and are independent of progress_cb:

paths = stream_video_frames_to_dir(
    api=api,
    video_id=video_id,
    output_dir="/tmp/frames",
    start=0,
    end=49,
    show_decoding_progress=True,
)

Tuning parallelism and memory

Two parameters control the internal pipeline:

• max_write_workers (default 4) — number of threads writing frames to disk in parallel. Increase on fast NVMe storage.

• queue_maxsize (default 32) — size of the decode prefetch buffer in frames. Peak RAM ≈ (queue_maxsize + max_write_workers + 1) × frame_bytes. Reduce if you hit OOM errors.

paths = stream_video_frames_to_dir(
    api=api,
    video_id=video_id,
    output_dir="/tmp/frames",
    max_write_workers=8,  # more disk parallelism
    queue_maxsize=16,     # smaller decode buffer
)

Async version

Save to directory

Use async_stream_video_frames_to_dir inside an async function:

import asyncio
from supervisely.video.sampling import async_stream_video_frames_to_dir

async def main():
    paths = await async_stream_video_frames_to_dir(
        api=api,
        video_id=video_id,
        output_dir="/tmp/frames_async",
        start=0,
        end=9,
    )
    print(f"Saved {len(paths)} frames")

asyncio.run(main())

Frame-by-frame generator

Function reference

stream_video_frames_to_dir

stream_video_frames_to_dir(
    api,
    video_id,
    output_dir,
    start=None,                # first frame index (inclusive, 0-based); default: 0
    end=None,                  # last frame index (inclusive, 0-based); default: last frame
    ext="png",                 # image extension: "png", "jpg", etc.
    progress_cb=None,          # callable(1) called after each saved frame
    image_writer=None,         # custom writer fn(path, np_array); default: sly.image.write
    max_write_workers=4,       # parallel write threads; increase on fast storage
    queue_maxsize=32,          # decode prefetch buffer in frames; peak RAM ≈ (queue_maxsize + max_write_workers + 1) × frame_bytes
    show_decoding_progress=False,  # show tqdm bars for demux / B-frame decode phases
)

Returns a List[str] of absolute paths to saved frame files.

async_stream_video_frames_to_dir

Same signature as stream_video_frames_to_dir (including max_write_workers, queue_maxsize, and show_decoding_progress), but async. Returns List[str].

async_stream_video_frames

async_stream_video_frames(
    api,
    video_id,
    start=None,                    # first frame index (inclusive, 0-based); default: 0
    end=None,                      # last frame index (inclusive, 0-based); default: last frame
    queue_maxsize=32,              # decode prefetch buffer in frames; peak RAM ≈ (queue_maxsize + 1) × frame_bytes
    show_decoding_progress=False,  # show tqdm bars for demux / B-frame decode phases
)

Async generator. Yields (frame_index: int, image: np.ndarray) tuples. Image is in RGB format.