How to draw pixmap with same anti alias quality as QPushButton::icon?

[patrickkidd]

@Christian-Ehrlicher I thought that you cannot set the devicePixelRatio on a QWidget paint device?

[raven-worx]

@patrickkidd
did you see the painters renderhints?
https://doc.qt.io/qt-5/qpainter.html#RenderHint-enum

[patrickkidd]

@raven-worx said in How to draw pixmap with same anti alias quality as QPushButton::icon?:

@patrickkidd
did you see the painters renderhints?
https://doc.qt.io/qt-5/qpainter.html#RenderHint-enum

Those don’t apply to drawing pixmaps

[raven-worx]

@patrickkidd
QPainter::SmoothPixmapTransform??

[patrickkidd]

@Christian-Ehrlicher said in How to draw pixmap with same anti alias quality as QPushButton::icon?:

When you provide a c++ example we could test it out.

#include <QtWidgets>

class Widget : public QWidget {


public:

    QPixmap *m_pixmap;

    Widget(QWidget *parent=nullptr) :
        QWidget(parent) {
        m_pixmap = new QPixmap("away.png");
    }

    ~Widget() {
        delete m_pixmap;
    }

    void paintEvent(QPaintEvent *) {
        QPainter p(this);
        p.drawPixmap(QRect(100, 100, 28, 28), *m_pixmap, m_pixmap->rect());
    }
    
};

    
int main(int argc, char **argv) {

    QApplication app(argc, argv);
    Widget w;
    w.resize(300, 300);
    w.show();

    QPushButton b(QIcon(QPixmap("away.png")), "", &w);
    b.setIconSize(QSize(28, 28));
    b.setFlat(true);
    b.resize(28, 28);
    b.move(150, 100);
    b.show();
        
    app.exec();
}

@raven-worx said in How to draw pixmap with same anti alias quality as QPushButton::icon?:

@patrickkidd
QPainter::SmoothPixmapTransform??

This has no effect.

[patrickkidd]

Here are two screenshots from my app on a display with devicePixelRatio == 1. These show the reverse behavior from my code example.
Using QPushButton::icon in QPushButton::paintEvent:

Using QPainter::drawPixmap

Again, notice how the QIcon painting via QPushButton is better that QPainter::drawPixmap in these examples, but the opposite is true in both my python/c++ code examples.

The end goal is to get behavior like the first screenshot here when calling QPainter::drawPixmap.

[Christian Ehrlicher]

@patrickkidd said in How to draw pixmap with same anti alias quality as QPushButton::icon?:

Again, notice how the QIcon painting via QPushButton is better that QPainter::drawPixmap in these examples, but the opposite is true in both my python/c++ code examples.

So I first would try to change the example to behave the same as your application.

[patrickkidd]

@Christian-Ehrlicher said in How to draw pixmap with same anti alias quality as QPushButton::icon?:

@patrickkidd said in How to draw pixmap with same anti alias quality as QPushButton::icon?:

Again, notice how the QIcon painting via QPushButton is better that QPainter::drawPixmap in these examples, but the opposite is true in both my python/c++ code examples.

So I first would try to change the example to behave the same as your application.

That is the original question in this post that I am stuck on.

[Christian Ehrlicher]

But what's the question - how to create a minimal reproducer? Simplify your program until it either does no longer have to problem or it's really minimal - but this is basic stuff which has to be done for every bug you're hunting.

[patrickkidd]

@Christian-Ehrlicher I have provided a basic example that reproduces the problem, and in two different languages. The example clearly shows unpredictable behavior in the way pixmaps are painted.

[mrjj]

Hi
ok i could reproduce it using your c++ code

And you are right - it seems that drawPixmap somehow not using any render hints.

    p.setRenderHint(QPainter::Antialiasing,true);
    p.setRenderHint(QPainter::SmoothPixmapTransform,true);
    p.setRenderHint(QPainter::LosslessImageRendering,true);
    p.drawPixmap(QRect(100, 100, iconsize, iconsize), *m_pixmap, m_pixmap->rect());

makes no difference. I thought it was the scaling to fit rect but even with no scaling, it still not as smooth.

You can make it paint nicely with

        QIcon ico;
        ico.addPixmap(*m_pixmap);
        ico.paint(&p,QRect(100, 100, iconsize, iconsize));

Runnable test project if others want to check it out.
https://we.tl/t-Q6PXROE2Le

[patrickkidd]

@mrjj said in How to draw pixmap with same anti alias quality as QPushButton::icon?:

        QIcon ico;
        ico.addPixmap(*m_pixmap);
        ico.paint(&p,QRect(100, 100, iconsize, iconsize));

That certainly solves it. Wow. I looked into the QIcon paint code and found it TLDR;

[manuelschneid3r]

@patrickkidd what exactly have you found?