How to Implement Barcode Scanner with Webcam Using Qt QCamera

Xiao Ling - Aug 23 '21 - - Dev Community

A few days ago, I wrote a blog post about how to implement a desktop barcode reader application using Qt and C++. The application only supports still image mode. In this article, I am going to add a webcam support to empower the application to scan barcodes in real time.

CMake Configuration for QCamera on Windows

To add webcam support in Qt application, we need to import the QCamera class from QtMultimedia library. In my Windows Qt environment, I only installed Qt 6.1.2, which does not include QtMultimedia library. According to the Qt documentation, we can find the QCamera class in Qt 5.

So, I need to run the Qt maintenance tool to install Qt 5.12.11.

Qt maintenance tool

Thereafter update the PATH environment variables to downgrade the build tools.

Qt Windows tool path

The QCamera header file is located in Qt/5.12.11/mingw73_64/include/QtMultimedia/qcamera.h.

In CMakeLists.txt file, I change Qt version to 5 and link the QtMultimedia library:

cmake_minimum_required(VERSION 3.5)

project(BarcodeReader VERSION 0.1 LANGUAGES CXX)

set(CMAKE_INCLUDE_CURRENT_DIR ON)

set(CMAKE_AUTOUIC ON)
set(CMAKE_AUTOMOC ON)
set(CMAKE_AUTORCC ON)

set(CMAKE_CXX_STANDARD 11)
set(CMAKE_CXX_STANDARD_REQUIRED ON)

if (CMAKE_HOST_WIN32)
    link_directories("${PROJECT_SOURCE_DIR}/platform/windows/lib/") 
elseif(CMAKE_HOST_UNIX)
    link_directories("${PROJECT_SOURCE_DIR}/platform/linux/")
endif()
include_directories("${PROJECT_SOURCE_DIR}/include/")

find_package(Qt5 COMPONENTS Widgets REQUIRED)
find_package(Qt5MultimediaWidgets REQUIRED)

set(PROJECT_SOURCES
        main.cpp
        mainwindow.cpp
        mainwindow.h
        mainwindow.ui
        myvideosurface.h
        myvideosurface.cpp
)

add_executable(${PROJECT_NAME} ${PROJECT_SOURCES})

if (CMAKE_HOST_WIN32)
    target_link_libraries(${PROJECT_NAME} PRIVATE Qt5::Widgets Qt5::MultimediaWidgets "DBRx64")    
elseif(CMAKE_HOST_UNIX)
    target_link_libraries(${PROJECT_NAME} PRIVATE Qt5::Widgets Qt5::MultimediaWidgets "DynamsoftBarcodeReader")
endif()

if(CMAKE_HOST_WIN32)
    add_custom_command(TARGET ${PROJECT_NAME} POST_BUILD 
        COMMAND ${CMAKE_COMMAND} -E copy_directory
        "${PROJECT_SOURCE_DIR}/platform/windows/bin/"      
        $<TARGET_FILE_DIR:${PROJECT_NAME}>)
endif()
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The myvideosurface.h file and myvideosurface.cpp file are used to display the webcam preview and recognize barcodes in real time. I will talk about the implementation in the next section.

Real-time Barcode Scanning

QCamera contains a setViewfinder() function that can be used to display the webcam preview. The supported parameters of setViewfinder() include QVideoWidget, QGraphicsVideoItem and QAbstractVideoSurface.

My goal is to get preview frames from the webcam for barcode detection. Therefore, I should subclass the QAbstractVideoSurface class and get the frame data in present() method:

I create a myvideosurface.h file:

#ifndef MYVIDEOSURFACE_H
#define MYVIDEOSURFACE_H

#include <QImage>
#include <QPixmap>
#include <QDebug>
#include <QCamera>
#include <QCameraInfo>
#include <QAbstractVideoSurface>
#include <QLabel>
#include <QDateTime>

#include "DynamsoftCommon.h"
#include "DynamsoftBarcodeReader.h"

QT_BEGIN_NAMESPACE
namespace Ui
{
    class MainWindow;
}
QT_END_NAMESPACE

class MyVideoSurface : public QAbstractVideoSurface
{
    Q_OBJECT
private:
    Ui::MainWindow *ui;
    void *reader;
    bool is_detecting;

public:
    MyVideoSurface(QObject *parent, Ui::MainWindow *ui, void *reader);
    ~MyVideoSurface();

    void reset();

    QList<QVideoFrame::PixelFormat>
    supportedPixelFormats(QAbstractVideoBuffer::HandleType type) const;

    bool present(const QVideoFrame &frame);
};
#endif // MYVIDEOSURFACE_H
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The constructor of MyVideoSurface class takes a QObject pointer, a Ui::MainWindow pointer and a void pointer. The Ui::MainWindow pointer is used to access all the UI widgets. The void *reader pointer is used to detect barcodes.

Next, I create a MyVideoSurface.cpp file to add the implementation of MyVideoSurface class.

The supportedPixelFormats() is to get the supported pixel formats of the webcam. Which format should I use? Honestly, I don't know. I use QVideoFrame::Format_RGB24 at first, but it seems that QVideoFrame::Format_RGB24 is not supported by my webcam. According to the error message, I use QVideoFrame::Format_ARGB32 instead:

QList<QVideoFrame::PixelFormat> MyVideoSurface::supportedPixelFormats(QAbstractVideoBuffer::HandleType type) const
{
    return QList<QVideoFrame::PixelFormat>()
               << QVideoFrame::Format_RGB32;
}
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In present() function, I get the frame data from the QVideoFrame object and convert it to QImage object:

bool MyVideoSurface::present(const QVideoFrame &frame)
{
    if (frame.isValid() && is_detecting)
    {
        QVideoFrame cloneFrame(frame);
        cloneFrame.map(QAbstractVideoBuffer::ReadOnly);
        const QImage img(cloneFrame.bits(),
                         cloneFrame.width(),
                         cloneFrame.height(),
                         QVideoFrame::imageFormatFromPixelFormat(cloneFrame.pixelFormat()));

        cloneFrame.unmap();

        return true;
    }
    return false;
}
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To display the preview frame, I need to make a copy of the frame data:

QImage cp = img.copy();
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In my case, the image I get from the QVideoFrame object is vertically inverted. I can flip the image with QImage::mirrored() function:

cp = cp.mirrored(false, true);
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Then, I call DBR_DecodeBuffer() to detect barcodes in the frame and draw the bounding boxes on the image:

int ret = DBR_DecodeBuffer(reader, (unsigned char *)cp.bits(), cp.width(), cp.height(), cp.bytesPerLine(), IPF_ARGB_8888, "");

DBR_GetAllTextResults(reader, &handler);

QPixmap pm = QPixmap::fromImage(cp);
QPainter painter(&pm);
painter.setPen(Qt::red);

QString out = "";
TextResult **results = handler->results;
for (int index = 0; index < handler->resultsCount; index++)
{
    LocalizationResult* localizationResult = results[index]->localizationResult;
    out += "Index: " + QString::number(index) + ", Elapsed time: " + QString::number(start.msecsTo(end)) + "ms\n";
    out += "Barcode format: " + QString(results[index]->barcodeFormatString) + "\n";
    out += "Barcode value: " + QString(results[index]->barcodeText) + "\n";
    out += "Bounding box: (" + QString::number(localizationResult->x1) + ", " + QString::number(localizationResult->y1) + ") "
    + "(" + QString::number(localizationResult->x2) + ", " + QString::number(localizationResult->y2) + ") "
    + "(" + QString::number(localizationResult->x3) + ", " + QString::number(localizationResult->y3) + ") "
    + "(" + QString::number(localizationResult->x4) + ", " + QString::number(localizationResult->y4) + ")\n";
    out += "----------------------------------------------------------------------------------------\n";

    painter.drawLine(localizationResult->x1, localizationResult->y1, localizationResult->x2, localizationResult->y2);
    painter.drawLine(localizationResult->x2, localizationResult->y2, localizationResult->x3, localizationResult->y3);
    painter.drawLine(localizationResult->x3, localizationResult->y3, localizationResult->x4, localizationResult->y4);
    painter.drawLine(localizationResult->x4, localizationResult->y4, localizationResult->x1, localizationResult->y1);
}

DBR_FreeTextResults(&handler);

painter.end();
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Finally, I display the image on the label and show the barcode recognition results in the text box:

ui->label->setPixmap(pm.scaled(ui->label->size(), Qt::KeepAspectRatio, Qt::SmoothTransformation));
ui->textEdit_results->setText(out);
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So far, the myvideosurface part is finished. I can go to the mainwindow.cpp to add code for the camera control logic.

Open the mainwindow.ui in Qt Creator and add the camera control buttons.

Qt UI button

In the MainWindow constructor, I check the camera status and initialize the camera object and surface object:

QList<QCameraInfo> cameras = QCameraInfo::availableCameras();
if (cameras.size() > 0) 
{
    for( int i = 0; i < cameras.count(); ++i )
    { 
        QCameraInfo cameraInfo = cameras.at(i);
        qDebug() << cameraInfo.deviceName();
        qDebug() << cameraInfo.description();
        camera = new QCamera(cameraInfo);
        surface = new MyVideoSurface(this, ui, reader);
        camera->setViewfinder(surface);
        break;
    }
}
else {
    ui->pushButton_open->setEnabled(false);
    ui->pushButton_stop->setEnabled(false);
}
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After that, I bind the button click events to the corresponding functions:

connect(ui->pushButton_open, SIGNAL(clicked()), this, SLOT(startCamera()));
connect(ui->pushButton_stop, SIGNAL(clicked()), this, SLOT(stopCamera()));

void MainWindow::startCamera()
{
    surface->reset();
    camera->start();
}

void MainWindow::stopCamera()
{
    camera->stop();
}
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The barcode scanner is ready to use. Let's build and run the application:

mkdir build
cd build
cmake -G "MinGW Makefiles" ..
cmake --build .
BarcodeReader.exe
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Qt C++ GUI barcode reader

Source Code

https://github.com/yushulx/cmake-cpp-barcode-qrcode/tree/main/examples/9.x/qt

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