CODEWITHSUNDEEP
qtqpixmap
Sonicpath
Sonicpath

Reputation: 241

How to rotate a QPixmap without changing size

How to rotate a QPixmap without changing size I use this code but when I rotate the image it change the size of the image.

The header file 

#ifndef CUSTOMDIAL_H
#define CUSTOMDIAL_H
#include <QDial>
class CustomDial : public QDial
{
Q_OBJECT
public:
CustomDial(QWidget * parent = nullptr);
private:
virtual void paintEvent(QPaintEvent*) override;
};
#endif

the CPP file

#include "Customdial.h"
#include <QGraphicsPixmapItem>
#include <QPainter>
#include <QColor>
#include <QPixmap>
 CustomDial::CustomDial(QWidget* parent)
                    : QDial(parent)
 {
// Default range
QDial::setRange(0,100);
}   
void CustomDial::paintEvent(QPaintEvent*)
{
int a = QDial::sliderPosition();
QPainter painter(this);
//load pixmap
QPixmap pix(":/img/img/knob.png");
//rotate pixmap
QMatrix rm;
        rm.rotate(a);
        pix = pix.transformed(rm);
//draw pixmap
painter.drawPixmap(QPointF(0,0),
                    pix.scaled(QDial::height(),
                    QDial::height(),
                    Qt::KeepAspectRatio),
                    pix.rect().adjusted(1, 1, -1000, -1000) );
}

How to rotate a QPixmap without changing size I use this code but when I rotate the image it change the size of the image the header file

Upvotes: 0

Views: 8100

Answers (2)

absolute.madness
absolute.madness

Reputation: 896

Concerning your question, you can just use QPixmap::copy() to extract the central part of the needed size:

//load pixmap
QPixmap pix(":/images/pixmap.png");
//rotate pixmap
QMatrix rm;
rm.rotate(a);
int pxw = pix.width(), pxh = pix.height();
pix = pix.transformed(rm);
pix = pix.copy((pix.width() - pxw)/2, (pix.height() - pxh)/2, pxw, pxh);

But that would be inefficient (see also note for QPixmap::transformed()). I would prefer drawing directly using QPainter without creating intermediary pixmaps especially because you already have it:

qreal scale = qMin(width(), height()) / qMax(pix.width(), pix.height());
QPointF center(width() / 2., height() / 2.); // widget center
// uncomment to trade performance for quality
//  painter.setRenderHint(QPainter::SmoothPixmapTransform);
painter.translate(center);
painter.rotate(degrees);
painter.scale(scale, scale);
painter.drawPixmap(-pix.rect().center(), pix);

Also consider loading QPixmap on initialization of your widget and storing it rather that doing it in paintEvent (again for the sake of efficiency).

The reason that QPixmap::transformed() increases the size on rotation is to get the whole image including the edges because you can always extract a smaller image from the bigger one but not vice versa. You probably have a round picture but for rectangular it's immediately obvious.

Upvotes: 1

Sonicpath
Sonicpath

Reputation: 241

The method that Qdial is using to calculate the background circle is this

int width = option.rect.width();
int height = option.rect.height();
qreal r2 = qMin(width, height) / 2;
r2 -= r2/50;
const qreal d_ = r2 / 6;
const qreal dx = option.rect.x() + d_ + (width - 2 * r2) / 2 + 1;
const qreal dy = option.rect.y() + d_ + (height - 2 * r2) / 2 + 1;
QRectF br = QRectF(dx + 0.5, dy + 0.5,
               int(r2 * 2 - 2 * d_ - 2),
               int(r2 * 2 - 2 * d_ - 2));

So QRect br is the rect for the background circle. Then just add the Pixmap resource file using those cordinates 

 int w=option.rect.width()/2;
 int h=option.rect.height()/2;
 painter.translate(w,h);
 painter.rotate(a);

 //draw pixmap
 painter.drawPixmap(-br.width()/2,
                    -br.height()/2,
                         pix.scaled(br.width(),br.height(),Qt::KeepAspectRatio));

And you get a perfect resizable and rotatable image inside your dial

Upvotes: 0

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