Qt5 Tutorial FFmpeg Converter using QProcess - 2020

In this tutorial, we will learn how to issue system command to run external executables using QProcess.

To get quicker understanding, I'll explain the FFmpeg dialog first with pictures, step by step.

When the code runs, this is the initial FFmpeg dialog.

Filling in input and output, then click convert.

Internally, it will execute ffmpgeg -i input output using QProcess.

It may pop-up QMessage if output file is already:

Otherwise, it will start to convert:

By the readyRead() signal, it starts putting standard output from ffmpeg into QTestEdit.

When the conversion finished, the "Transcoding Status" label shows "Successful" or "Failed". Also, the Play Input button will be enabled if it's successful.

The two buttons are for playing using ffplay.

The native default media player will be used for those play buttons.

This tutorial's main focus is how to run external executable from Qt application.

In this example, we used QProcess on Windows 8.

The external executables such as ffmpeg.exe and ffplay.exe

The first step is to creating a QProcess object. Then, setup signals and slots in the constructor of FFmpeg dialog.

    mTranscodingProcess = new QProcess(this);
    connect(mTranscodingProcess, SIGNAL(started()), this, SLOT(processStarted()));
    connect(mTranscodingProcess,SIGNAL(readyReadStandardOutput()),this,SLOT(readyReadStandardOutput()));
    connect(mTranscodingProcess, SIGNAL(finished(int)), this, SLOT(encodingFinished()));

When we click the "Convert" button, after checking the input and output, in the on_startButton_clicked(), the strat() method of QProcess will be called as shown below:

mTranscodingProcess->start(program, arguments);

As soon as it starts, it signal started() will be emitted. When there is a standard output from the ffmpeg command, the readyReadStandardOutput() signal will be triggered and the readyReadStandardOutput() will get called. Then the output will be appended to QString which eventually put into the QEditText:

mOutputString.append(mTranscodingProcess->readAllStandardOutput());
ui->textEdit->setText(mOutputString);

When it's done, at the finished(int) signal, encodingFinished() slot will setup some final things such as resetting the "Transcoding Status" label and enabling "Play Input" button.

The ffmpeg binaries are all external and the player as well.

In this version of FFmpeg dialog is using statically built ffmpeg.exe.

The process of setting it up on Windows is very simple, and it's using already statically built version:
http://www.bogotobogo.com/VideoStreaming/ffmpeg_on_Windows.php.

For linux, I built it from source on Fedora 18, and the steps of builing is also in my another page:
http://www.bogotobogo.com/VideoStreaming/ffmpeg.php.

When we click the Play button, it'll use a default media player on a platform.

In the other tutorial, I'll post the Qt ffmpeg with its own player.

Extension of this tutorial is available:
Video Player with HTML5 QWebView and FFmpeg Converter.

The video recording below is the player built using HTML5 video tag with QWebView as its canvas.

Here are the source codes:

1. dialog.cpp
2. dialog.h
3. dialog.ui
4. main.cpp
5. ffmpeg.pro

dialog.h:

// dialog.h

#ifndef DIALOG_H
#define DIALOG_H

#include <QDialog>
#include <QProcess>
#include <QFile>
#include <QTextEdit>

namespace Ui {
class Dialog;
}

class Dialog : public QDialog
{
    Q_OBJECT

public:
    explicit Dialog(QWidget *parent = 0);
    ~Dialog();

public slots:

public:

private slots:
     void on_startButton_clicked();
     void readyReadStandardOutput();
     void processStarted();
     void encodingFinished();
     void on_fileOpenButton_clicked();
     void on_playInputButton_clicked();
     void on_playOutputButton_clicked();

private:
    Ui::Dialog *ui;
    QProcess *mTranscodingProcess;
    QProcess *mInputPlayProcess;
    QProcess *mOutputPlayProcess;
    QString mOutputString;
};

#endif // DIALOG_H

dialog.cpp:

// dialog.cpp

#include "dialog.h"
#include "ui_dialog.h"
#include <QDebug>
#include <QString>
#include <QProcess>
#include <QScrollBar>
#include <QMessageBox>
#include <QFileInfo>
#include <QFileDialog>


Dialog::Dialog(QWidget *parent) :
    QDialog(parent),
    ui(new Ui::Dialog)
{
    ui->setupUi(this);

    // Play button for output - initially disabled
    ui->playOutputButton->setEnabled(false);

    // Create three processes
    // 1.transcoding, 2.input play 3.output play
    mTranscodingProcess = new QProcess(this);
    mInputPlayProcess = new QProcess(this);
    mOutputPlayProcess = new QProcess(this);

    connect(mTranscodingProcess, SIGNAL(started()), this, SLOT(processStarted()));

    connect(mTranscodingProcess,SIGNAL(readyReadStandardOutput()),this,SLOT(readyReadStandardOutput()));
    connect(mTranscodingProcess, SIGNAL(finished(int)), this, SLOT(encodingFinished()));
}

Dialog::~Dialog()
{
    delete ui;
}

void Dialog::processStarted()
{
    qDebug() << "processStarted()";
}

// conversion start
void Dialog::on_startButton_clicked()
{
    QString program = "C:/FFmpeg/bin/ffmpeg";

    QStringList arguments;
    QString input = ui->fromLineEdit->text();
    if(input.isEmpty()) {
        qDebug() << "No input";
        QMessageBox::information(this,
                     tr("ffmpeg"),tr("Input file not specified"));
        return;
    }
    QString output = ui->toLineEdit->text();
    if(output.isEmpty()) {
        qDebug() << "No output";
        QMessageBox::information(this,
                     tr("ffmpeg"),tr("Output file not specified"));
        return;
    }

    QString fileName = ui->toLineEdit->text();
    qDebug() << "output file check " << fileName;
    qDebug() << "QFile::exists(fileName) = " << QFile::exists(fileName);
    if (QFile::exists(fileName)) {
         if (QMessageBox::question(this, tr("ffmpeg"),
                    tr("There already exists a file called %1 in "
                    "the current directory. Overwrite?").arg(fileName),
                    QMessageBox::Yes|QMessageBox::No, QMessageBox::No)
             == QMessageBox::No)
             return;
         QFile::remove(fileName);
         while(QFile::exists(fileName)) {
             qDebug() << "output file still there";
         }
     }

    arguments << "-i" << input << output;

    qDebug() << arguments;

    mTranscodingProcess->setProcessChannelMode(QProcess::MergedChannels);
    mTranscodingProcess->start(program, arguments);
}

void Dialog::readyReadStandardOutput()
{
    mOutputString.append(mTranscodingProcess->readAllStandardOutput());
    ui->textEdit->setText(mOutputString);

    // put the slider at the bottom
    ui->textEdit->verticalScrollBar()
            ->setSliderPosition(
                ui->textEdit->verticalScrollBar()->maximum());
}

void Dialog::encodingFinished()
{
    // Set the encoding status by checking output file's existence
    QString fileName = ui->toLineEdit->text();

    if (QFile::exists(fileName)) {
        ui->transcodingStatusLabel
                ->setText("Transcoding Status: Successful!");
        ui->playOutputButton->setEnabled(true);
    }
    else {
        ui->transcodingStatusLabel
                ->setText("Transcoding Status: Failed!");
    }
}

// Browse... button clicked - this is for input file
void Dialog::on_fileOpenButton_clicked()
{
    QString fileName =
        QFileDialog::getOpenFileName(
                this,
                tr("Open File"),
                "C:/TEST",
                tr("videoss (*.mp4 *.mov *.avi)"));
    if (!fileName.isEmpty()) {
        ui->fromLineEdit->setText(fileName);
    }
}

void Dialog::on_playInputButton_clicked()
{
    QString program = "C:/FFmpeg/bin/ffplay";
    QStringList arguments;
    QString input = ui->fromLineEdit->text();
    arguments << input;
    mInputPlayProcess->start(program, arguments);
}

void Dialog::on_playOutputButton_clicked()
{
    QString program = "C:/FFmpeg/bin/ffplay";
    QStringList arguments;
    QString output = ui->toLineEdit->text();
    arguments << output;
    mInputPlayProcess->start(program, arguments);
}