AVC / H.264 Annex B Decoder (Run)

This article explains how you can use Transcoder.run to decode an AVC / H.264 elementary stream to a raw YUV video file.

The code snippets in this article are from the dec_avc_file sample.

Source Video

For source we use the foreman_qcif.h264 file from the AVBlocks Assets repository. After downloading and unzipping you will find foreman_qcif.h264 in the vid subdirectory.

Code

This code takes a compressed H.264 elementary stream file and decodes it to raw YUV video format.

Initialize AVBlocks

The first step in any AVBlocks application is to initialize the library. This must be done before using any other AVBlocks functionality. The Library.initialize() method sets up the internal state and loads necessary codecs. Always remember to call Library.shutdown() at the end of your program to properly clean up resources and release any allocated memory.

Library.initialize()

# Set license information. Without this AVBlocks runs in Demo mode.
# Library.set_license("<license-string>")

result = decode(input_file, output_file)

Library.shutdown()

sys.exit(0 if result else 2)

Configure Output Socket

The output socket defines where and how the decoded raw YUV video data will be written. We configure it to output an uncompressed YUV 4:2:0 video file. The socket represents the output destination, while the pin represents the specific video stream within that destination. By setting ColorFormat.YUV420, we ensure the output uses the standard YUV 4:2:0 planar format, which is the most common format for raw video and provides a good balance between quality and file size.

def create_output_socket(output_file: str) -> MediaSocket:
    """Create output socket for YUV file."""
    socket = MediaSocket()
    socket.file = output_file
    socket.stream_type = StreamType.UncompressedVideo

    pin = MediaPin()
    socket.pins.add(pin)

    vsi = VideoStreamInfo()
    pin.stream_info = vsi

    vsi.stream_type = StreamType.UncompressedVideo
    vsi.color_format = ColorFormat.YUV420

    return socket

Configure Transcoder and Decode

This is the main decoding function that ties everything together. First, we create a MediaInfo object and open it against the H.264 file to detect the input stream format, then build the matching input socket with MediaSocket.from_media_info. This is more convenient than manually specifying parameters, as MediaInfo reads the stream headers and extracts all necessary information about the encoded video, including resolution, frame rate, and codec settings. Then we create the output socket using our helper method. The transcoder is the core component that performs the actual decoding - it takes the compressed H.264 data from the input and converts it to uncompressed YUV data for the output.

The allow_demo_mode = True setting allows the transcoder to work even without a valid license (useful for testing, but not recommended for production). We then add our input and output sockets to the transcoder, open it to prepare for processing, run the actual transcoding operation, and finally close it to clean up.

def decode(input_file: str, output_file: str) -> bool:
    """Decode H.264/AVC file to YUV."""
    # Transcoder will fail if output exists (by design)
    delete_file(output_file)

    # Create output directory if it doesn't exist
    output_dir = os.path.dirname(output_file)
    if output_dir and not os.path.exists(output_dir):
        os.makedirs(output_dir)

    # Open input file with MediaInfo
    media_info = MediaInfo()
    media_info.inputs[0].file = input_file

    if not media_info.open():
        print_error("MediaInfo.open()", media_info.error)
        return False

    in_socket = MediaSocket.from_media_info(media_info)
    out_socket = create_output_socket(output_file)

    # Create transcoder
    with Transcoder() as transcoder:
        transcoder.allow_demo_mode = True
        transcoder.inputs.add(in_socket)
        transcoder.outputs.add(out_socket)

        res = transcoder.open()
        print_error("Transcoder open", transcoder.error)
        if not res:
            return False

        res = transcoder.run()
        print_error("Transcoder run", transcoder.error)
        if not res:
            return False

        transcoder.close()
        print_error("Transcoder close", transcoder.error)

    return True

Complete Python Code

Here’s the complete working example that demonstrates H.264 decoding using AVBlocks. This code combines all the previous snippets into a functional program that can be run. The main function handles command-line argument parsing, initializes AVBlocks, performs the decoding operation, and properly shuts down the library before exiting.

"""
Decode H.264/AVC file to YUV.

This sample decodes an H.264/AVC video file to raw YUV format using AVBlocks.
"""

import os
import sys
import click

from avblocks import (
    Library, Transcoder, MediaInfo, MediaSocket, MediaPin,
    VideoStreamInfo, StreamType, ColorFormat, ErrorFacility
)


def delete_file(filename: str):
    """Delete a file if it exists."""
    try:
        if os.path.exists(filename):
            os.remove(filename)
    except OSError:
        pass


def print_error(action: str, error) -> None:
    """Print error information."""
    if action:
        print(f"{action}: ", end="")

    if error.facility == ErrorFacility.Success:
        print("Success")
        return

    print(f"{error.message or ''}, facility:{error.facility} code:{error.code} hint:{error.hint or ''}")


def create_output_socket(output_file: str) -> MediaSocket:
    """Create output socket for YUV file."""
    socket = MediaSocket()
    socket.file = output_file
    socket.stream_type = StreamType.UncompressedVideo

    pin = MediaPin()
    socket.pins.add(pin)

    vsi = VideoStreamInfo()
    pin.stream_info = vsi

    vsi.stream_type = StreamType.UncompressedVideo
    vsi.color_format = ColorFormat.YUV420

    return socket


def decode(input_file: str, output_file: str) -> bool:
    """Decode H.264/AVC file to YUV."""
    # Transcoder will fail if output exists (by design)
    delete_file(output_file)

    # Create output directory if it doesn't exist
    output_dir = os.path.dirname(output_file)
    if output_dir and not os.path.exists(output_dir):
        os.makedirs(output_dir)

    # Open input file with MediaInfo
    media_info = MediaInfo()
    media_info.inputs[0].file = input_file

    if not media_info.open():
        print_error("MediaInfo.open()", media_info.error)
        return False

    in_socket = MediaSocket.from_media_info(media_info)
    out_socket = create_output_socket(output_file)

    # Create transcoder
    with Transcoder() as transcoder:
        transcoder.allow_demo_mode = True
        transcoder.inputs.add(in_socket)
        transcoder.outputs.add(out_socket)

        res = transcoder.open()
        print_error("Transcoder open", transcoder.error)
        if not res:
            return False

        res = transcoder.run()
        print_error("Transcoder run", transcoder.error)
        if not res:
            return False

        transcoder.close()
        print_error("Transcoder close", transcoder.error)

    return True


@click.command()
@click.option('-i', '--input', 'input_file',
              help='Input H.264/AVC file',
              type=click.Path(exists=True))
@click.option('-o', '--output', 'output_file',
              help='Output YUV file',
              type=click.Path())
def main(input_file: str, output_file: str):
    """Decode H.264/AVC file to YUV."""

    # Set default options if not provided
    if not input_file or not output_file:
        script_dir = os.path.dirname(os.path.abspath(__file__))

        if not input_file:
            input_file = os.path.join(script_dir, "../../assets/vid/foreman_qcif.h264")
        if not output_file:
            output_file = os.path.join(script_dir, "../../output/dec_avc_file/foreman_qcif.yuv")

        print("Using default options:")
        print(f"  --input {input_file}")
        print(f"  --output {output_file}")
        print()

    # Validate options
    print(f"--input: {input_file}")
    print(f"--output: {output_file}")

    Library.initialize()

    # Set license information. Without this AVBlocks runs in Demo mode.
    # Library.set_license("<license-string>")

    result = decode(input_file, output_file)

    Library.shutdown()

    sys.exit(0 if result else 2)


if __name__ == '__main__':
    main()

How to Run

See the Python sample for details. The samples are installed as command-line tools when you install the package from the repository root with uv pip install --editable . (see Installation).

Command Line

dec-avc-file --input <file.h264> --output <file.yuv>

Examples

List options:

dec-avc-file --help
Usage: dec-avc-file [OPTIONS]

  Decode H.264/AVC file to YUV.

Options:
  -i, --input PATH   Input H.264/AVC file
  -o, --output PATH  Output YUV file
  --help             Show this message and exit.

Decode the input file ./assets/vid/foreman_qcif.h264 into output file ./output/dec_avc_file/foreman_qcif.yuv:

# Linux and macOS
mkdir -p ./output/dec_avc_file

dec-avc-file \
    --input ./assets/vid/foreman_qcif.h264 \
    --output ./output/dec_avc_file/foreman_qcif.yuv

# Windows
New-Item -ItemType Directory -Force -Path ./output/dec_avc_file

dec-avc-file `
    --input ./assets/vid/foreman_qcif.h264 `
    --output ./output/dec_avc_file/foreman_qcif.yuv