QPainter DrawImage CenterAligned - qt

Question

Is there any way to draw an image on QPainter center aligned? I see QPainter::drawText gives us this provision but drawImage does not. I have one source rect, target rect and an image. when the source size is small the image gets drawn on the left side of the page. I want it to be printed center aligned.

Answer 1

The painter doesn't have a size, but the device() it paints on does. You can use QRect(painter.device()->width(), painter.device()->height()) as the rectangle where you want to center your image in.
Then you'd paint the image centered like so:
QImage source;
QPainter painter(...);
...
QRect rect(source.rect());
QRect devRect(0, 0, painter.device()->width(), painter.device()->height());
rect.moveCenter(devRect.center());
painter.drawImage(rect.topLeft(), source);

Answer 2

I would try to do the following (please follow the source code comments):
The sample image that should be drawn
// The image to draw - blue rectangle 100x100.
QImage img(100, 100, QImage::Format_ARGB32);
img.fill(Qt::blue);
In the paint event handler
[..]
QRect source(0, 0, 100, 100);
QRect target(0, 0, 400, 400);
// Calculate the point, where the image should be displayed.
// The center of source rect. should be in the center of target rect.
int deltaX = target.width() - source.width();
int deltaY = target.height() - source.height();
// Just apply coordinates transformation to draw where we need.
painter.translate(deltaX / 2, deltaY / 2);
painter.drawImage(source, img);
Of course you should check whether source rectangle is smaller than the target before applying this approach. I omitted that code for simplicity reasons just to demonstrate how you can center your image.

Answer 3

I wanted to show a more complete example with a variable image size that stays within the bounds of the area provided to add to the other great answers.
void ImageView::paintEvent(QPaintEvent*)
{
if (this->imageBuffer.empty()){ return; }
double widgetWidth = this->width();
double widgetHeight = this->height();
QRectF target(0, 0, widgetWidth, widgetHeight);
QImage tempQImage = *this->imageBuffer.at(this->imageBuffer.count()-1);
tempQImage = tempQImage.scaled(rect().size(), Qt::KeepAspectRatio, Qt::SmoothTransformation);
double imageSizeWidth = static_cast<double>(tempQImage.width());
double imageSizeHeight = static_cast<double>(tempQImage.height());
QRectF source(0.0, 0.0, imageSizeWidth, imageSizeHeight);
int deltaX = 0;
int deltaY = 0;
if(source.width() < target.width())
deltaX = target.width() - source.width();
else
deltaX = source.width() - target.width();
if(source.height() < target.height())
deltaY = target.height() - source.height();
else
deltaY = source.height() - target.height();
QPainter painter(this);
painter.translate(deltaX / 2, deltaY / 2);
painter.drawImage(source, tempQImage);
}