Some Pixels are differed by 1 when taking screenshots in 32 and 64 architecture

Disanth

We are using Qt Minimal plugin for our testing environment.
qputenv("QT_QPA_PLATFORM", "minimal");
When taking screenshot of current screen and the some of the pixels are differed by 1 (One) in 32 and 64 architecture.
32 arch Image Pixel = [255, 255, 255],
64 arch Image2 Pixel = [254, 254, 254]

What could be the possible reason for this issue?

Thanks in Advanced.

mvuori

In case someone knows about this, you may need to provide more information:

Which one is correct...
How do you draw the screen
How do you get the grab
Device information, including screen driver

Disanth

Hi @mvuori , thanks for the reply. I suspecting it would be because of floating point conversions in image processing but not having an idea on how to triage it. Here is the info that you asked

The first one is correct i.e. 32 arch Image Pixel = [255, 255, 255]
Qt is taking care of the screen drawing from minimal plugin (QMinimalBackingStore and QMinimalintegration Plugin files are the responsible classes here)

QMinimalintegration.cpp implementation

  m_primaryScreen = new QMinimalScreen();
  m_primaryScreen->mGeometry = QRect(0, 0, width, height);
  m_primaryScreen->mDepth = 32;
 m_primaryScreen->mFormat = QImage::Format_ARGB32_Premultiplied

 QWindowSystemInterface::handleScreenAdded(m_primaryScreen);

QMinimalBackingStore flush() & paintDevice() method implementation

void QMinimalBackingStore::flush(QWindow *window, const QRegion &region, const QPoint &offset)
{
       Q_UNUSED(window);
      Q_UNUSED(region);
      Q_UNUSED(offset);

      if (mDebug) {
             static int c = 0;
            QString filename = QString("/tmp/output%1.png").arg(c++, 4, 10, QLatin1Char('0'));
            bool saved = mImage.save(filename);
      }
}

 QPaintDevice *QMinimalBackingStore::paintDevice()
   {
       return &mImage;
   }

Note: here mImage is class level instance with QImage::Format_RGB32. ( but m_primaryScreen's Format is QImage::Format_ARGB32_Premultiplied)**

We are grabbing the image like below

      qobject_cast<QQuickWindow *>(QGuiApplication::focusWindow())->grabWindow().save("/tmp/screenshot.png")

Embedded Linux Device, Screen Driver is framebuffer in real qemu environment, but in testing environment we are using minimal plugin

Axel Spoerl

That’s a bug in the minimal backing store implementation.
It’s got nothing to do with 32 bit, which just exposes the bug.
Please file a ticket at https://bugreports.qt.io.