Draw pixel based graphics to a QWidget

Question

I have an application which needs to draw on a pixel by pixel basis at a specified frame rate (simulating an old machine). One caveat is that the main machine engine runs in a background thread in order to ensure that the UI remains responsive and usable during simulation.

Currently, I am toying with using something like this:

class QVideo : public QWidget {
public:
    QVideo(QWidget *parent, Qt::WindowFlags f) : QWidget(parent, f), screen_image_(256, 240, QImage::Format_RGB32) {

    }

    void draw_frame(void *data) {
        // render data into screen_image_
    }

    void start_frame() {
        // do any pre-rendering prep work that needs to be done before
        // each frame
    }

    void end_frame() {
        update(); // force a paint event
    }

    void paintEvent(QPaintEvent *) {
        QPainter p(this);
        p.drawImage(rect(), screen_image_, screen_image_.rect());
    }

    QImage screen_image_;
};

This is mostly effective, and surprisingly not very slow. However, there is an issue. The update function schedules a paintEvent, it may not hapen right away. In fact, a bunch of paintEvent's may get "combined" according to the Qt documentation.

The negative effect that I am seeing is that after a few minutes of simulation, the screen stops updating (image appears frozen though simulation is still running) until I do something that forces a screen update for example switching the window in and out of maximized.

I have experimented with using QTimer's and other similar mechanism to have the effect of the rendering being in the GUI thread so that I can force immediate updates, but this resulted in unacceptable performance issues.

Is there a better way to draw pixels onto a widget constantly at a fixed interval. Pure Qt solutions are preferred.

EDIT: Since some people choose to have an attitude instead of reading the whole question, I will clarify the issue. I cannot use QWidget::repaint because it has a limitation in that it must be called from the same thread as the event loop. Otherwise, no update occurs and instead I get qDebug messages such as these:

QPixmap: It is not safe to use pixmaps outside the GUI thread
QPixmap: It is not safe to use pixmaps outside the GUI thread
QWidget::repaint: Recursive repaint detected
QPainter::begin: A paint device can only be painted by one painter at a time.
QWidget::repaint: It is dangerous to leave painters active on a widget outside of the PaintEvent
QWidget::repaint: It is dangerous to leave painters active on a widget outside of the PaintEvent

EDIT: to demonstrate the issue I have created this simple example code:

QVideo.h

#include <QWidget>
#include <QPainter>

class QVideo : public QWidget {
    Q_OBJECT;
public:
    QVideo(QWidget *parent = 0, Qt::WindowFlags f = 0) : QWidget(parent, f), screen_image_(256, 240, QImage::Format_RGB32) {

    }

    void draw_frame(void *data) {
        // render data into screen_image_
        // I am using fill here, but in the real thing I am rendering
        // on a pixel by pixel basis
        screen_image_.fill(rand());
    }

    void start_frame() {
        // do any pre-rendering prep work that needs to be done before
        // each frame
    }

    void end_frame() {
        //update(); // force a paint event
        repaint();
    }

    void paintEvent(QPaintEvent *) {
        QPainter p(this);
        p.drawImage(rect(), screen_image_, screen_image_.rect());
    }

    QImage screen_image_;
};

main.cc:

#include <QApplication>
#include <QThread>
#include <cstdio>
#include "QVideo.h"


struct Thread : public QThread {

    Thread(QVideo *v) : v_(v) {
    }

    void run() {
        while(1) {
            v_->start_frame();
            v_->draw_frame(0); // contents doesn't matter for this example
            v_->end_frame();
            QThread::sleep(1);
        }
    }

    QVideo *v_;
};

int main(int argc, char *argv[]) {
    QApplication app(argc, argv);
    QVideo w;
    w.show();

    Thread t(&w);
    t.start();

    return app.exec();
}

I am definitely willing to explore options which don't use a temporary QImage to render. It is just the only class in Qt which seems to have a direct pixel writing interface.

Answer 1

Try emitting a signal from the thread to a slot in the event loop widget that calls repaint(), which will then execute right away. I am doing something like this in my graphing program, which executes the main calculations in one thread, then tells the widget when it is time to repaint() the data.

Answer 2

In similar cases what I did was still using a QTimer, but doing several simulation steps instead of just one. You can even make the program auto-tuning the number of simulation steps to be able to get whatever frames per seconds you like for the screen update.