QPainter::setCompositingMode without effect

[Lykurg]

Quick guess: is anti aliasing turned off? And what happens if you code your paint function like this of the image composition example?

(And have you notified that "The RasterOp denoted blend modes are not supported for pens and brushes with alpha components."?)

[olidem]

Hmm, ok, I do not think that there is a connection to antialiasing.

The only idea I have is about the the format of the QImages. In the referred example, all images are kept in a premultiplied format (see the loadImage function), but I think this is mainly for rescaling reasons.

The problem I have is that the first step of there rendering process always at first fills the whole image with transparent color

painter.setCompositionMode(QPainter::CompositionMode_Source);
painter.fillRect(resultImage.rect(), Qt::transparent);

painter.setCompositionMode(QPainter::CompositionMode_Source);
painter.fillRect(resultImage.rect(), Qt::transparent);

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which I can't do because I would delete other things.

What about the attached file? Do you see errors there?

[olidem]

News:

I have now converted everything into the premultiplied format, with no changes, still the destinationOut does not what I expect.

Thanks for help in advance!!!
Olli

[olidem]

Hmm,

I ahve found a workaround:

I just use another temporary QImage where I render the image together with the cutmask. This image is then rendered into the actual painter given by the paint(...) -method:

void GraphicsHairstyleItem::paint(QPainter *painter,
		const QStyleOptionGraphicsItem *option, QWidget *widget)
{
	Q_UNUSED(option);
	Q_UNUSED(widget);
 
 
 
	QImage toPaint (m_img_warped.width(), m_img_warped.height(), m_img_warped.format());
	QPainter p(&toPaint);
	p.setCompositionMode(QPainter::CompositionMode_Source);
	p.fillRect(m_imgrect, Qt::transparent);
	p.setCompositionMode(QPainter::CompositionMode_SourceOver);
	p.drawImage(0, 0, m_img_warped);
	p.setCompositionMode(QPainter::CompositionMode_DestinationOut);
	p.drawImage(0, 0, m_img_cutmask);
	p.end();
 
 
	painter->setCompositionMode(QPainter::CompositionMode_SourceOver);
	painter->drawImage(m_imgrect, toPaint);
}

void GraphicsHairstyleItem::paint(QPainter *painter,
		const QStyleOptionGraphicsItem *option, QWidget *widget)
{
	Q_UNUSED(option);
	Q_UNUSED(widget);



	QImage toPaint (m_img_warped.width(), m_img_warped.height(), m_img_warped.format());
	QPainter p(&toPaint);
	p.setCompositionMode(QPainter::CompositionMode_Source);
	p.fillRect(m_imgrect, Qt::transparent);
	p.setCompositionMode(QPainter::CompositionMode_SourceOver);
	p.drawImage(0, 0, m_img_warped);
	p.setCompositionMode(QPainter::CompositionMode_DestinationOut);
	p.drawImage(0, 0, m_img_cutmask);
	p.end();


	painter->setCompositionMode(QPainter::CompositionMode_SourceOver);
	painter->drawImage(m_imgrect, toPaint);
}

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This does what it should.

But this is not elegant. I cannot understand why it should not be possible to do this DestinationOut rendering dierctly in the event-painter.

Can anyone clarify my problem?

Thnaks a lot in advance,
Olli