[Qt-interest] If Opacity Less than 1, QImage Not Painted

[John McClurkin]

Josiah Bryan wrote:
> I've got a really weird problem here:
> 
> QImage image((const 
> uchar*)byteArray.constData(),size.width(),size.height(),QImage::Format_RGB32); 
> 
> painter->setOpacity(0.5);
> painter->drawImage(rect, image);
> 
> This draws NOTHING. Not a white square, nothing - background visible at 
> that location. However, if I take out the 'setOpacity' line or set it to 
> 1.0, then the image draws fine. What in the world could be causing this?
> 
> If I call image.convertToFormat(QImage::Format_ARGB32) prior to 
> painting, it paints correctly with partial opacity - BUT this adds 
> another 5ms latency to the painting - quite unacceptable, really. I need 
> to paint it without having to call converToFormat just to get partial 
> opacity.
> 
> For the record, this is in a QGraphicsItem with a QGLWidget as the 
> QGraphicsView viewport - BUT, I've also tested it without the QGLWidget 
> viewport, and same problem.
> 
> The image data is coming from a V4L2 capture interface using the 
> V4L2_PIX_FMT_BGR32 pixel format - this works just fine, the image 
> displays correctly. I just can't get it to paint with a partial opacity.
> 
> Any ideas, anyone?
> 

Opacity is controlled by the alpha channel. From your email, it seems 
that the original data is in RGB format. The convertToFormat adds the 
alpha channel that partial opacity needs.