convert cv::Mat into QPixmap

JacobNovitsky

I need simply convert cv::Mat into QPixmap
My first attempt for two last days led me to forum :)
Any pointers are highly appreciated!

pro.file

QT       += core gui

greaterThan(QT_MAJOR_VERSION, 4): QT += widgets

CONFIG += c++17

SOURCES += main.cpp
    
INCLUDEPATH += /usr/local/Qt-6.4.2/include
LIBS += -L/usr/local/Qt-6.4.2/lib -lQt6Core -lQt6Gui

# OpenCV
INCLUDEPATH += /usr/include/opencv4
LIBS += `pkg-config --cflags --libs opencv4`

# Remove redundant OpenCV libraries and make sure only pkg-config handles them

HEADERS +=

code:

#include <opencv2/opencv.hpp>
#include <QPixmap>
#include <QImage>
#include <QLabel>
#include <QGuiApplication>  // For QGuiApplication
#include <QApplication>  // For QGuiApplication

QPixmap MatToQPixmap(const cv::Mat &image) {
    // Check if image is empty
    if (image.empty()) {
        std::cerr << "Empty image" << std::endl;
        return QPixmap();
    }

    // Convert the cv::Mat to QImage
    QImage qimage;

    // For color images (CV_8UC3, RGB format)
    if (image.type() == CV_8UC3) {
        cv::Mat rgb;
        cv::cvtColor(image, rgb, cv::COLOR_BGR2RGB); // Convert BGR to RGB
        qimage = QImage(rgb.data, rgb.cols, rgb.rows, static_cast<int>(rgb.step), QImage::Format_RGB888);
    }
    // For grayscale images (CV_8UC1)
    else if (image.type() == CV_8UC1) {
        qimage = QImage(image.data, image.cols, image.rows, static_cast<int>(image.step), QImage::Format_Grayscale8);
    }
    // Handle other types if needed (e.g., CV_8UC4, CV_16UC3, etc.)
    else {
        std::cerr << "Unsupported image format" << std::endl;
        return QPixmap();
    }

    // Convert QImage to QPixmap
    QPixmap pixmap = QPixmap::fromImage(qimage);

    // Check if pixmap conversion is successful
    if (pixmap.isNull()) {
        std::cerr << "Failed to convert cv::Mat to QPixmap." << std::endl;
    }

    return pixmap;
}

int main(int argc, char *argv[]) {
    // Initialize QGuiApplication or QApplication (needed for QPixmap)
    QGuiApplication app(argc, argv);

    // // Load the image (example)
    cv::Mat image = cv::imread("/home/j/images/1.png");  // Replace with your image path

    // // Convert to QPixmap
    QPixmap pixmap = MatToQPixmap(image);

    // // Display the image using QLabel
    QLabel label;
    label.setPixmap(pixmap);
    label.show();

    // Enter the application's event loop
    return app.exec();
}

qt.qpa.plugin: Could not find the Qt platform plugin "wayland" in "" (Jacob: I consider its just warning, nothing fatal here, I see it usually, am I correct?)
qtc.process_stub: Inferior error: QProcess::Crashed "Process crashed"

Pl45m4

@JacobNovitsky said in convert cv::Mat into QPixmap:

I need simply convert cv::Mat into QPixmap

The code to create a QPixmap from cv:Mat looks correct.

Where do you see the wayland error? This is caused by something else and not how you convert Mat to QPixmap

Edit:

Why do you create QGuiApplication and not QApplication?

QGuiApplication might not work for every QWidget...
And why include both anyway?!

#include <QGuiApplication>  // For QGuiApplication
#include <QApplication>  // For QGuiApplication

Change to

int main(int argc, char *argv[]) {
    // Initialize QGuiApplication or QApplication (needed for QPixmap)
    QApplication app(argc, argv);
    // . . . 
}

IgKh

@JacobNovitsky said in convert cv::Mat into QPixmap:

qt.qpa.plugin: Could not find the Qt platform plugin "wayland" in "" (Jacob: I consider its just warning, nothing fatal here, I see it usually, am I correct?)

This is of no concern. It means that you are running under a Wayland compositor but don't have the Qt wayland plugin installed. That's fine, it will just fallback to working with Xwayland through the xcb plugin.

@JacobNovitsky said in convert cv::Mat into QPixmap:

qtc.process_stub: Inferior error: QProcess::Crashed "Process crashed"

This is of great concern. Run your project under the debugger and look at the resulting full stack trace to see where exactly it crashed. It could likely indeed be due to trying to create a QLabel widget instance when the application instance is not QApplication as @Pl45m4 suggested.

Bonnie

I used to find this project very helpful: https://github.com/dbzhang800/QtOpenCV
It converts between cv::Mat and QImage for a lot of pixel formats, but it is quite old and I'm not sure whether the code would work for Qt6 and the latest opencv.

JacobNovitsky

@Pl45m4 this worked for me, thanks

Bondrusiek

@JacobNovitsky hi,
you can see also this article: https://asmaloney.com/2013/11/code/converting-between-cvmat-and-qimage-or-qpixmap/ maybe it helps you.