AVC / H.264 Access Unit Decoder

This article explains how you can use Transcoder.push to decode AVC / H.264 access units to raw YUV video frames.

The code snippets in this article are from the dec_avc_au Python sample.

Source Video

As video input we use the foreman_qcif.h264.au directory from the AVBlocks Assets archive. After downloading and unzipping you will find foreman_qcif.h264.au in the vid subdirectory. The directory contains AVC / H.264 access units named au_####.h264.

Code

This code reads a sequence of AVC / H.264 access-unit files, pushes each access unit into a Transcoder, and writes decoded raw YUV video frames to a file.

Initialize AVBlocks

The first step in any AVBlocks application is to initialize the library. This must be done before using any other AVBlocks functionality. Call Library.shutdown() before the program exits to clean up resources.

Library.initialize()

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

result = decode_aus(input_dir, output_file, COLOR_FORMATS[color_format.lower()])

Library.shutdown()

sys.exit(0 if result else 2)

Create Output Socket

The output socket is configured for uncompressed video. The sample uses the color format from the command line when provided. The output file path is specified on the command line.

def create_output_socket(output_file: str, color_format: int) -> 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 = color_format
    vsi.scan_type = ScanType.Progressive

    return socket

Configure Transcoder

The sample uses MediaInfo on the first access-unit file to detect the input stream properties. It then creates an input socket from that media information, clears the file and stream from the socket, creates the output socket, and opens the transcoder.

def build_au_path(input_dir: str, index: int) -> str:
    """Build the path to an access unit file."""
    pattern = "au_{:04d}.h264"
    return os.path.join(input_dir, pattern.format(index))


def configure_transcoder(transcoder: Transcoder, au_file: str, output_file: str, color_format: int) -> bool:
    """Configure transcoder using the first AU file."""
    media_info = MediaInfo()
    media_info.inputs[0].file = au_file

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

    # Create input socket from media info
    in_socket = MediaSocket.from_media_info(media_info)
    in_socket.file = None
    # Note: in_socket.stream = None is not needed in Python

    # Create output socket
    out_socket = create_output_socket(output_file, color_format)

    transcoder.inputs.add(in_socket)
    transcoder.outputs.add(out_socket)

    return True

Push Access Units

The main transcode loop reads files named au_####.h264 from the input directory. Each file is loaded into a MediaBuffer, attached to a MediaSample, and pushed into the transcoder. After all access units are pushed, the sample flushes the transcoder to write delayed decoded frames.

def decode_aus(input_dir: str, output_file: str, color_format: int) -> bool:
    """Decode H.264 access units to YUV."""
    # Delete output file if exists
    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)

    first_au_file = build_au_path(input_dir, 0)
    if not os.path.exists(first_au_file):
        print(f"First AU file not found: {first_au_file}")
        return False

    with Transcoder() as transcoder:
        transcoder.allow_demo_mode = True

        # Configure transcoder using first AU file
        if not configure_transcoder(transcoder, first_au_file, output_file, color_format):
            return False

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

        # Process all AU files
        i = 0
        while True:
            au_file = build_au_path(input_dir, i)
            if not os.path.exists(au_file):
                break

            with open(au_file, 'rb') as f:
                data = f.read()

            sample = MediaSample()
            sample.buffer = MediaBuffer(data)

            if not transcoder.push(0, sample):
                print_error("Transcoder push", transcoder.error)
                return False

            i += 1

        if not transcoder.flush():
            print_error("Transcoder flush", transcoder.error)
            return False

        transcoder.close()

    print(f"Output file: {output_file}")
    return True

Complete Code

Here’s the complete working example that demonstrates AVC / H.264 access-unit decoding using AVBlocks for Python.

"""
Decode H.264/AVC access units to YUV.

This sample decodes a sequence of H.264 access unit files to raw YUV format.
Each AU file contains a single NAL unit (e.g., au_0000.h264, au_0001.h264, etc.).
"""

import os
import sys
import click

from avblocks import (
    Library, Transcoder, MediaInfo, MediaSocket, MediaPin,
    VideoStreamInfo, MediaSample, MediaBuffer, StreamType,
    ColorFormat, ScanType, 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 build_au_path(input_dir: str, index: int) -> str:
    """Build the path to an access unit file."""
    pattern = "au_{:04d}.h264"
    return os.path.join(input_dir, pattern.format(index))


def create_output_socket(output_file: str, color_format: int) -> 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 = color_format
    vsi.scan_type = ScanType.Progressive

    return socket


def configure_transcoder(transcoder: Transcoder, au_file: str, output_file: str, color_format: int) -> bool:
    """Configure transcoder using the first AU file."""
    media_info = MediaInfo()
    media_info.inputs[0].file = au_file

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

    # Create input socket from media info
    in_socket = MediaSocket.from_media_info(media_info)
    in_socket.file = None
    # Note: in_socket.stream = None is not needed in Python

    # Create output socket
    out_socket = create_output_socket(output_file, color_format)

    transcoder.inputs.add(in_socket)
    transcoder.outputs.add(out_socket)

    return True


def decode_aus(input_dir: str, output_file: str, color_format: int) -> bool:
    """Decode H.264 access units to YUV."""
    # Delete output file if exists
    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)

    first_au_file = build_au_path(input_dir, 0)
    if not os.path.exists(first_au_file):
        print(f"First AU file not found: {first_au_file}")
        return False

    with Transcoder() as transcoder:
        transcoder.allow_demo_mode = True

        # Configure transcoder using first AU file
        if not configure_transcoder(transcoder, first_au_file, output_file, color_format):
            return False

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

        # Process all AU files
        i = 0
        while True:
            au_file = build_au_path(input_dir, i)
            if not os.path.exists(au_file):
                break

            with open(au_file, 'rb') as f:
                data = f.read()

            sample = MediaSample()
            sample.buffer = MediaBuffer(data)

            if not transcoder.push(0, sample):
                print_error("Transcoder push", transcoder.error)
                return False

            i += 1

        if not transcoder.flush():
            print_error("Transcoder flush", transcoder.error)
            return False

        transcoder.close()

    print(f"Output file: {output_file}")
    return True


# Color format mapping
COLOR_FORMATS = {
    'yuv420': ColorFormat.YUV420,
    'yv12': ColorFormat.YV12,
    'nv12': ColorFormat.NV12,
    'yuv422': ColorFormat.YUV422,
    'bgra32': ColorFormat.BGRA32,
    'rgb24': ColorFormat.BGR24,
}


@click.command()
@click.option('-i', '--input', 'input_dir',
              help='Input directory containing AU files (au_0000.h264, au_0001.h264, ...)',
              type=click.Path(exists=True))
@click.option('-o', '--output', 'output_file',
              help='Output YUV file',
              type=click.Path())
@click.option('-c', '--color', 'color_format',
              help='Output color format (yuv420, yv12, nv12, yuv422, bgra32, rgb24)',
              default='yuv420',
              type=click.Choice(list(COLOR_FORMATS.keys()), case_sensitive=False))
def main(input_dir: str, output_file: str, color_format: str):
    """Decode H.264/AVC access units to YUV."""

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

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

        print("Using default options:")
        print(f"  --input {input_dir}")
        print(f"  --output {output_file}")
        print(f"  --color {color_format}")
        print()

    # Validate options
    print(f"--input: {input_dir}")
    print(f"--output: {output_file}")
    print(f"--color: {color_format}")

    Library.initialize()

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

    result = decode_aus(input_dir, output_file, COLOR_FORMATS[color_format.lower()])

    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-au --input <directory> --output <file.yuv> [--color <COLOR>]

Examples

List options:

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

  Decode H.264/AVC access units to YUV.

Options:
  -i, --input PATH                Input directory containing AU files
                                  (au_0000.h264, au_0001.h264, ...)
  -o, --output PATH               Output YUV file
  -c, --color [yuv420|yv12|nv12|yuv422|bgra32|rgb24]
                                  Output color format (yuv420, yv12, nv12,
                                  yuv422, bgra32, rgb24)
  --help                          Show this message and exit.

List supported color formats:

dec-avc-au --colors

Decode the H.264 Access Units from ./assets/vid/foreman_qcif.h264.au/ into output file ./output/dec_avc_au/foreman_qcif.yuv:

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

dec-avc-au \
    --input ./assets/vid/foreman_qcif.h264.au \
    --output ./output/dec_avc_au/foreman_qcif.yuv \
    --color yuv420

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

dec-avc-au `
    --input ./assets/vid/foreman_qcif.h264.au `
    --output ./output/dec_avc_au/foreman_qcif.yuv `
    --color yuv420