How to read my application screen pixel data with Qt - c++

Question

I am trying to read my application screen pixel data in my Qt Qml application.
The first approach was grabToImage(), which is fast in my PC. But the same is taking seconds to grab image in my embedded device.
So I tried to read /dev/fb0, using QFile::readAll,
QFile file("/dev/fb0");
QFile dataFile("/home/icu/WorkSpace/Samples/FBRead/Exe/data.bin");
if(!file.open(QIODevice::ReadOnly))
{
qDebug() << Q_FUNC_INFO << file.errorString();
}
if(!dataFile.open(QIODevice::WriteOnly))
{
qDebug() << Q_FUNC_INFO << dataFile.errorString();
}
QTextStream in(&file);
QString data;
qDebug() << Q_FUNC_INFO << "start read";
// while(!in.atEnd())
// {
qDebug() << Q_FUNC_INFO << "read";
data = in.readAll();
// qDebug() << Q_FUNC_INFO << data;
// }
QByteArray dataBytes;
dataBytes.append(data);
dataFile.write(dataBytes);
file.close();
dataFile.close();
I tried to open the file with some tool which shows image reading from raw file. But it did not show any data. When I open the file data is like 01 01 01 00 01 01 01 00 ...
Is there any alternate way to read my screen pixel data fast? Thanks

Answer 1

I'm not sure if using a QTextStream and converting data to QString and then back to QByteArray is the best way to do it. As explained in Qt's documentation constructing a QString from QByteArray forces a conversion to UTF-8 and also stops copying at the first null character, so you are modifying your pixel data and not getting it completely. Try copying data directly between files instead:
bool grabScreenToFile(const QString& outPath) {
QFile inFile("/dev/fb0");
QFile outFile(outPath);
// ...
outFile.write(inFile.readAll());
return true;
}
To check if this is copying the actual content of the file and you have no problem with it (truncation, conversions, ...), compare with the file obtained using cat /dev/fb0 > grab.raw (note that it is not a JPEG or PNG image but a raw buffer).
/dev/fb0
This link contains a long explanation about grabbing the framebuffer from the /dev/fb0 device. Basically, it points out that you must pay attention to the depth of the buffer, since pixels may be packed in different ways.
On the other hand, have you tried any other of the grabbing methods provided by Qt, such as QScreen::grabWindow? Maybe its performance is better that the QML's QQuickItem::grabToImage (which is the one I guess you are using). Indeed, the documentation states:
This function will render the item to an offscreen surface and copy that surface from the GPU's memory into the CPU's memory, which can be quite costly.
Following code is a modification of the screenshot example.
auto screen = QGuiApplication::primaryScreen();
if (const QWindow *window = windowHandle()) {
screen = window->screen();
}
if (screen) {
const auto pixmap = screen->grabWindow(yourWidget->winId());
// ...
}