[Interest] QPainter questions

[maitai]

Hello all,

I am using QPainter (Qt 5.8) to render quite complex QImages, meaning 
many calls to drawPath, drawPolygons and drawPolyLines;

I have noticed that a very costly operation is drawPolylines. It gets 
much better if I use drawLines, but the result is ugly due to non-joined 
lines and (as it seems) no antialising. It is also much faster if I just 
set antialiasing off, but again with poor image quality in the end.

Several questions here:
1) Is there something I can do to improve things with drawPolylines, 
even at a small cost in quality?
2) I tried to render on an openGL surface (QOffscreenSurface) instead of 
QImage, not changing anything else in my program, i.e. still calling 
QPainter::drawPolyLines and such, and converting back to a QImage in the 
end. The result is that it works, but it is actually slower than regular 
paintEngine... I was hoping for a big gain in speed especially 
concerning antialising, but it is actually the opposite (1000ms with 
openGL versus 800ms using QImage directly).
3) I also have to draw some other QImages in the final image with 
QPainter::drawImage(). It works but the alpha channel of these images 
appear in black when using the openGL version. I have tried a lot to 
play with openGL options and format but no way so far.

What I am doing basically is:
#ifndef USE_S57_OPENGL
     snapPix = QImage(myProj.getW(), myProj.getH(), 
QImage::Format_ARGB32_Premultiplied);
     snapPix.fill(Qt::transparent);
     QPainter pnt(&snapPix);
#else
     QSurfaceFormat format;
     format.setMajorVersion(4);
     format.setMinorVersion(3);
     QOpenGLContext context;
     //context.setFormat(format); //for some reason that does not work
     context.create();
     QOffscreenSurface surface;
     surface.setFormat(format);
     surface.create();
     context.makeCurrent(&surface);
     QOpenGLFramebufferObjectFormat fboFormat;
     fboFormat.setSamples(16); //or 4, does not make any difference
     
fboFormat.setAttachment(QOpenGLFramebufferObject::CombinedDepthStencil);
     QOpenGLFramebufferObject fbo(myProj.getW(), myProj.getH(), 
fboFormat);
     fbo.bind();
     QOpenGLPaintDevice device(QSize(myProj.getW(), myProj.getH()));
     QPainter pnt(&device);
#endif
     pnt.setRenderHint(QPainter::Antialiasing);
     pnt.setRenderHint(QPainter::SmoothPixmapTransform);
     (..... all the painting you can imagine ...)
     pnt.end()
#ifdef USE_S57_OPENGL
     fbo.release();
     snapPix = fbo.toImage();
#endif

Thanks in advance for any advice. I am not an expert in openGL, needless 
to say..
Philippe Lelong