[Qt-interest] native painting in a QWidget on Windows

[John Weeks]

I must ask hard questions :)

In thinking about my problem (in the absence of a better solution) it seemed like perhaps I could create a memory device context (a bitmap context) and do my drawing into that. When the drawing is done, convert it to a QPixMap (presumably using QPixmap::fromWinHBITMAP) and then draw the QPixMap into my widget using QPainter::drawPixMap().

Has anyone done anything like this?

Will this result in poor performance? It seems like there is a possibility that the conversion+extra draw might take some time.

I'd be really interested in any reaction you folks might have to this idea, or a real solution to the problem outlined below.

Thanks!
John Weeks

WaveMetrics, Inc.
Phone (503) 620-3001
Fax   (503) 620-6754
email   support at WaveMetrics.com



On Dec 27, 2010, at 2:54 PM, John Weeks wrote:

> I have a need to use native GDI+ painting inside a QWidget. I wish I didn't, but can't get around it yet. I would be very grateful for any help with this.
> 
> I am able to paint using Core Graphics on Macintosh, but using the same code with GDI+ doesn't work. It appears that the QPainter calls are not drawing to the screen- if I step through my code in the debugger, I see my GDI+ drawing appear as a step over it. But the QPainter calls that came before don't paint until after paintEvent() returns.
> 
> I found QPainter::beginNativePainting(); the description sounds good, but the examples I can find seem to indicate that it is only for use with OpenGL.
> 
> I also found the Qt::WA_PaintOnScreen attribute, but that doesn't seem to fix my problem.
> 
> Here is my code, called from paintEvent(). It has an accumulation of attempted fixes in it...
> 
> void IgorTestContent::doPaint()
> {
> 	QWidget * widget = windowPtr->getContentWidget();
> 	QRect r = widget->geometry();
> 	QPainter painter(widget);
> 	
> 	// do some QPainter painting just to make sure that I can...
> 
> 	QLinearGradient titleGradient(r.width()/2, 0, r.width()/2, r.height());
> 	titleGradient.setColorAt(0, QColor(255, 0, 0));
> 	titleGradient.setColorAt(1, QColor(255, 255, 0));
> 	QBrush gradientBrush(titleGradient);
> 	painter.setBrush(gradientBrush);
> 	
> 	painter.drawRect(0, 0, r.width()-1, r.height()-1);
> 		
> 	painter.drawLine(r.x()+10, r.y()+10, r.x()+r.width()-10, r.y()+r.height()-10);
> 	
> 	painter.drawText(r.x()+10, r.y()+r.height()/2, "This text was drawn by QPainter.");
> 	
> 	// Try native painting. A Core Graphics version of this code works on Macintosh.
> 	painter.beginNativePainting();
> 	void * hdc = (void *)widget->getDC();	// void * so I don't have to have Windows API's here. Will be replaced eventually with a cross-platform version.
> 	doNativePaint(hdc);					// this function encapsulates native painting.
> 	widget->releaseDC((HDC)hdc);		// Again, will be replaced eventually with a cross-platform version.
> 	painter.endNativePainting();
> }
> 
> 
> Regards,
> John Weeks
> 
> WaveMetrics, Inc.
> Phone (503) 620-3001
> Fax   (503) 620-6754
> email   support at WaveMetrics.com
> 
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> Qt-interest mailing list
> Qt-interest at qt.nokia.com
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