"Render" into QImage

[Imons]

Hi all,

i have a little probelm. I wanted to write a renderer and use qt as a frontend. Now my question is, how do i display and update my image _efficently_. As the rendering process needs time there are only a few pixels which gets updated in a certain interval.

I've written a little program which shows how i would solve this problem. But i'm not very happy with this solution because i don't know how much copying of the image is done under the hood. Maybe theres a better way?

Here the code:

class QMainWin : public QMainWindow
{
    Q_OBJECT
 
public:
    QMainWin();
 
private:
    QScrollArea *area;
    QImage *image;
    QLabel *imgLabel;
    int x, y;
 
private slots:
    void drawPixels();
};

class QMainWin : public QMainWindow
{
    Q_OBJECT

public:
    QMainWin();

private:
    QScrollArea *area;
    QImage *image;
    QLabel *imgLabel;
    int x, y;

private slots:
    void drawPixels();
};

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QMainWin::QMainWin()
{
    image = new QImage(200, 200, QImage::Format_ARGB32);
    imgLabel = new QLabel;
    imgLabel->setPixmap(QPixmap::fromImage(*image));
 
    area = new QScrollArea;
    area->setBackgroundRole(QPalette::Dark);
    area->setWidget(imgLabel);
 
    setCentralWidget(area);
 
    QAction* action = new QAction(tr("Draw something"), this);
    connect(action, SIGNAL(triggered()), this, SLOT(drawPixels()));
 
    QMenu* menu = menuBar()->addMenu(tr("Drawing"));
    menu->addAction(startAction);
 
    x = y = 0;
}
 
void QMainWin::drawPixels()
{
    for(int i = y; i < y+20; i++)
        for(int j = x; j < x+20; j++)
            image->setPixel(j, i, qRgb(255, 0, 0));
 
    x += 20;
    y += 20;
 
    // This line is needed, else the picture doesn't get updated
    imgLabel->setPixmap(QPixmap::fromImage(*image));
    imgLabel->update();
}

QMainWin::QMainWin()
{
    image = new QImage(200, 200, QImage::Format_ARGB32);
    imgLabel = new QLabel;
    imgLabel->setPixmap(QPixmap::fromImage(*image));

    area = new QScrollArea;
    area->setBackgroundRole(QPalette::Dark);
    area->setWidget(imgLabel);

    setCentralWidget(area);

    QAction* action = new QAction(tr("Draw something"), this);
    connect(action, SIGNAL(triggered()), this, SLOT(drawPixels()));

    QMenu* menu = menuBar()->addMenu(tr("Drawing"));
    menu->addAction(startAction);

    x = y = 0;
}

void QMainWin::drawPixels()
{
    for(int i = y; i < y+20; i++)
        for(int j = x; j < x+20; j++)
            image->setPixel(j, i, qRgb(255, 0, 0));

    x += 20;
    y += 20;

    // This line is needed, else the picture doesn't get updated
    imgLabel->setPixmap(QPixmap::fromImage(*image));
    imgLabel->update();
}

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I've omitted forward declarations, includes and so on.
Sry for the bad english.

Regards
Imons

[wysota]

What exactly are you not satisfied with? QImage (and QPixmap as well) is an implicitly shared class. If you copy an object to a new one only a shallow copy is performed but when you change at least one bit of information of any of the incarnations, a deep copy of the whole image is made for the instance being changed.

Anyway, the loop where you do the painting can be replaced by a single call to QPainter::drawRect(). Of course you need to modify the way you operate on QPixmap but then you can get rid of QImage at all.