I'm trying to use the method described here to use a QPainter and GDI calls on the same widget.
Unfortunately this tutorial seem to have been written on an earlier version of Qt and now it does not work.
I set the WA_PaintOnScreen flag and reimplement paintEngine() to return NULL.
Then on the paintEvent() I create a QPainter, use it and then use some GDI calls to paint a bitmap. The GDI calls work fine but the QPainter does nothing. I get the following error on the console:
QPainter::begin: Paint device returned engine == 0, type: 1
Is this simply not supported anymore? how can I do it?
I've also tried creating an additional widget on top of the GDI-painting widget but that didn't go well as well since the top widget appears black and blocks the GDI widget.
I got this working in QT 4.7-beta 2 as follows
- In the constructor call setAttribute(Qt::WA_PaintOnScreen,true);
- Do NOT reimplement paintEngine() to return NULL;
Use the following code in the paintEvent();
QPainter painter(this); HDC hdc = painter.paintEngine()->getDC(); // THIS IS THE CRITICAL STEP! HWND hwnd = winID(); // From this point on it is all regular GDI QString text("Test GDI Paint"); RECT rect; GetClientRect(hwnd, &rect); HBRUSH hbrRed = CreateSolidBrush(RGB(255,0,0)); FillRect(hdc, &rect, hbrRed); HBRUSH hbrBlue = CreateSolidBrush(RGB(40,40,255)); HPEN bpenGreen = CreatePen(PS_SOLID, 4, RGB(0,255,0)); SelectObject(hdc,bpenGreen); SelectObject(hdc,hbrBlue); Ellipse(hdc,10,10,rect.right-20,rect.bottom-20); SetTextAlign(hdc, TA_CENTER | TA_BASELINE); TextOutW(hdc, width() / 2, height() / 2, text.utf16(), text.size()); ReleaseDC(hwnd, hdc);
This worked with Qt 4.0 and 4.1, but stopped working in either 4.2 or 4.3, when Trolltech reimplemented the Windows paint engine. In the second edition of the Qt 4 book, we added the sentence:
"For this to work, we must also reimplement QPaintDevice::paintEngine() to return a null pointer and set the Qt::WA_PaintOnScreen attribute in the widget's constructor."
I haven't tested it using later versions of Qt (I'm no longer at Trolltech/Nokia and have no Windows machine) but I hope it will still work.
