Is there any way to smoothly scale and draw a section of a QPixmap that's defined by a QRectF?

Guy Gizmo

I'm working on a Qt 5 mac application and I'm looking for a method to smoothly scale and render a fractional section of a QPixmap as defined by a QRectF.

What I want to do actually is exactly what void QPainter::drawPixmap(const QRectF &targetRect, const QPixmap &pixmap, const QRectF &sourceRect) is supposed to do:

QRectF sourceRect = ...;
QRectF targetRect = ...;
painter.setRenderHints(QPainter::SmoothPixmapTransform | QPainter::Antialiasing);
painter.drawPixmap(targetRect, pixmap, sourceRect);

However, as mentioned in this thread and this bug report, QPainter::SmoothPixmapTransform is ignored and this method does not work.

Short of writing my own efficient bilinear scaling method from scratch (which I am not inclined to do), is there any other method I can use to accomplish the same thing?

QPixmap::scaled() only allows scaling to integer dimensions, and provides no way to crop out a section defined by a QRectF, so that's a non-starter.

Just tossing out ideas here, but is it possible to somehow use OpenGL to render the portion of the image I want at the right scale and then render it back into a QImage or QPixmap? This kind of scaling is one of the most basic things OpenGL can do very easily and quickly, but I don't have any experience using OpenGL in a Qt desktop app so far so I'm not sure if this is possible.

mpergand

Hi,
Don't know if it meets your use case, but you can try to apply a blur effect, here the code I used to create a thumbnail from a web page:

QImage TopSitesManager::blurImage(const QImage& img)
{
	if(img.isNull()) return img;

	QGraphicsScene scene;
	QGraphicsPixmapItem item;
	QGraphicsBlurEffect effect;

	effect.setBlurRadius(3.0);
	effect.setBlurHints(QGraphicsBlurEffect::QualityHint);
	item.setPixmap(QPixmap::fromImage(img));
	item.setGraphicsEffect(&effect);
	scene.addItem(&item);
	QImage blurImg(img.size(),QImage::Format_ARGB32);
	blurImg.fill(Qt::transparent);

	QPainter painter(&blurImg);
	QRectF source(0,0,img.size().width(),img.size().height());
	scene.render(&painter,source,source);
    painter.end();

	return blurImg;
}

Guy Gizmo

I'm afraid that doesn't help for my use case, since I need the images to appear as clear and clean as possible when being scaled down.