AVC / H.264 Annex B Decoder (Run)
Contents
AVC / H.264 Annex B Decoder (Run)#
This article explains how you can use Transcoder::run to decode an AVC / H.264 Annex B elementary stream to a raw YUV video file.
The code snippets in this article are from the dec_avc_file macOS 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 Annex B 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.
int main(int argc, char* argv[])
{
Options opt;
switch(prepareOptions(opt, argc, argv))
{
case Command: return 0;
case Error: return 1;
case Parsed: break;
}
Library::initialize();
bool decodeResult = decodeAvcFile(opt);
Library::shutdown();
return decodeResult ? 0 : 1;
}
Configure Input Socket#
The input socket defines the source H.264 Annex B elementary stream. We use MediaInfo to automatically detect the video parameters from the input file. 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.
bool decodeAvcFile(Options& opt)
{
// transcoder will fail if output exists (by design)
deleteFile(primo::ustring(opt.outputFile));
auto mediaInfo = primo::make_ref(Library::createMediaInfo());
mediaInfo->inputs()->at(0)->setFile(primo::ustring(opt.inputFile));
if(!mediaInfo->open())
{
printError("MediaInfo open:", mediaInfo->error());
return false;
}
// create input socket
auto inputSocket = primo::make_ref(Library::createMediaSocket(mediaInfo.get()));
// ...existing code...
}
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.
primo::ref<MediaSocket> createOutputSocket(Options& opt)
{
auto socket = primo::make_ref(Library::createMediaSocket());
socket->setFile(primo::ustring(opt.outputFile));
socket->setStreamType(StreamType::UncompressedVideo);
auto pin = primo::make_ref(Library::createMediaPin());
socket->pins()->add(pin.get());
auto vsi = primo::make_ref(Library::createVideoStreamInfo());
pin->setStreamInfo(vsi.get());
vsi->setStreamType(StreamType::UncompressedVideo);
vsi->setColorFormat(ColorFormat::YUV420);
return socket;
}
Configure Transcoder and Decode#
This is the main decoding function that ties everything together. After setting up the input socket with MediaInfo, we create the output socket using our helper function. 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 setAllowDemoMode(true) call 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. If any step fails, we print the error and return false.
bool decodeAvcFile(Options& opt)
{
// transcoder will fail if output exists (by design)
deleteFile(primo::ustring(opt.outputFile));
auto mediaInfo = primo::make_ref(Library::createMediaInfo());
mediaInfo->inputs()->at(0)->setFile(primo::ustring(opt.inputFile));
if(!mediaInfo->open())
{
printError("MediaInfo open:", mediaInfo->error());
return false;
}
// create input socket
auto inputSocket = primo::make_ref(Library::createMediaSocket(mediaInfo.get()));
// create output socket
auto outputSocket = createOutputSocket(opt);
// create transcoder
auto transcoder = primo::make_ref(Library::createTranscoder());
transcoder->setAllowDemoMode(true);
transcoder->inputs()->add(inputSocket.get());
transcoder->outputs()->add(outputSocket.get());
if(!transcoder->open())
{
printError("Transcoder open", transcoder->error());
return false;
}
if(!transcoder->run())
{
printError("Transcoder run", transcoder->error());
return false;
}
transcoder->close();
cout << "Output: " << opt.outputFile << endl;
return true;
}
Complete C++ 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 compiled and run. The main function handles command-line argument parsing, initializes AVBlocks, performs the decoding operation, and properly shuts down the library before exiting.
#include <cstdio>
#include <memory.h>
#include <iostream>
#include <iomanip>
#include <fstream>
#include <string>
#include <primo/avblocks/avb.h>
#include <primo/platform/reference++.h>
#include <primo/platform/error_facility.h>
#include <primo/platform/ustring.h>
#include "util.h"
#include "options.h"
using namespace primo::codecs;
using namespace primo::avblocks;
using namespace std;
primo::ref<MediaSocket> createOutputSocket(Options& opt)
{
auto socket = primo::make_ref(Library::createMediaSocket());
socket->setFile(primo::ustring(opt.outputFile));
socket->setStreamType(StreamType::UncompressedVideo);
auto pin = primo::make_ref(Library::createMediaPin());
socket->pins()->add(pin.get());
auto vsi = primo::make_ref(Library::createVideoStreamInfo());
pin->setStreamInfo(vsi.get());
vsi->setStreamType(StreamType::UncompressedVideo);
vsi->setColorFormat(ColorFormat::YUV420);
return socket;
}
bool decodeAvcFile(Options& opt)
{
// transcoder will fail if output exists (by design)
deleteFile(primo::ustring(opt.outputFile));
auto mediaInfo = primo::make_ref(Library::createMediaInfo());
mediaInfo->inputs()->at(0)->setFile(primo::ustring(opt.inputFile));
if(!mediaInfo->open())
{
printError("MediaInfo open:", mediaInfo->error());
return false;
}
// create input socket
auto inputSocket = primo::make_ref(Library::createMediaSocket(mediaInfo.get()));
// create output socket
auto outputSocket = createOutputSocket(opt);
// create transcoder
auto transcoder = primo::make_ref(Library::createTranscoder());
transcoder->setAllowDemoMode(true);
transcoder->inputs()->add(inputSocket.get());
transcoder->outputs()->add(outputSocket.get());
if(!transcoder->open())
{
printError("Transcoder open", transcoder->error());
return false;
}
if(!transcoder->run())
{
printError("Transcoder run", transcoder->error());
return false;
}
transcoder->close();
cout << "Output: " << opt.outputFile << endl;
return true;
}
int main(int argc, char* argv[])
{
Options opt;
switch(prepareOptions(opt, argc, argv))
{
case Command: return 0;
case Error: return 1;
case Parsed: break;
}
Library::initialize();
bool decodeResult = decodeAvcFile(opt);
Library::shutdown();
return decodeResult ? 0 : 1;
}
How to Run#
See the build instructions for macOS and the dec_avc_file example for details.
Command Line#
./dec_avc_file --input <file.h264> --output <file.yuv>
Examples#
List options:
./bin/x64/dec_avc_file --help
Usage: dec_avc_file -i <h264 file> -o <yuv file>
-h, --help
--input H264 input file.
--output YUV output file.
The following example decodes input file ./assets/vid/foreman_qcif.h264 into output file foreman_qcif.yuv:
mkdir -p ./output/dec_avc_file
./bin/x64/dec_avc_file \
--input ./assets/vid/foreman_qcif.h264 \
--output ./output/dec_avc_file/foreman_qcif.yuv