HiDPI and SVG icon resolution

[cle1109]

Thanks for your input, I really appreciate it. I'm starting to think that this might be a Qt bug (maybe specific to macOS).

[mrjj]

@cle1109
hi, you are most welcome. Wish i had a working answer.
i can only say on linux and windows, it did work for me on 1440p screens.
But if 4k is more special or its due to/only on MacOS, i dont know.

[SimonSchroeder]

The problem might be that you leave it to Qt to render the SVG to pixels. What you can try instead is rendering the SVG into a QPixmap first and set this for the icon. Something similar to this (untested code, C++):

MainWindow *mw = ...
QToolbar *toolbar = ...
...
QImage svgImage(""waves-24px.svg"");
QPixmap svgPixmap(toolbar->iconSize());
svgPixmap->setDevicePixelRatio(mw->pixelRatio());
QPainter painter;
painter.begin(&svgPixmap);
painter.drawImage(0,0,svgImage);
painter.end();
QIcon icon(svgPixmap);
QAction *action = new QAction(icon, "Test");
toolbar->addAction(action);
...

Maybe, you'll even need a larger pixmap:

QPixmap svgPixmap(toolbar->iconSize() * mw->pixelRatio());

As you can see, I am calling pixelRatio() on the MainWindow. The reason behind this is that with multiple monitors of different sizes and resolutions it is possible to have a different pixelRatio per screen (at least on Windows). This has extra problems if you move the widget from one screen to the other...

[cle1109]

Thanks @SimonSchroeder, I've translated the code to Python as follows:

import sys
from PyQt5.QtWidgets import QApplication, QMainWindow, QAction
from PyQt5.QtGui import QIcon, QImage, QPixmap, QPainter


app = QApplication(sys.argv)
main = QMainWindow()
toolbar = main.addToolBar("toolbar")
image = QImage("waves-24px.svg")
pixmap = QPixmap(toolbar.iconSize())
pixmap.setDevicePixelRatio(main.devicePixelRatio())
painter = QPainter()
painter.begin(pixmap)
painter.drawImage(0, 0, image)
painter.end()
icon = QIcon(pixmap)
action = QAction(icon, "Test")
toolbar.addAction(action)
toolbar.show()
main.show()
sys.exit(app.exec_())

Note that a QMainWindow doesn't have a pixelRatio method so I've replaced that with a call to devicePixelRatio.

However, there seems to be something wrong because the result looks like this:

The actual content of the icon changes with each program execution BTW.

On a related note, I found out that this is indeed a macOS-specific issue, because everything looks fine on Linux and Windows using the same 4K monitor in scaled mode (I'm using the original code snippet from my initial post):

I guess this is indeed a Qt bug then and I'll report it. Still, I'd be interested in getting the workaround using QPixmap to actually work - then I could use that on macOS until they've fixed the underlying issue.

[J.Hilk]

@cle1109 have you tried it with a QSvgWidget ? Thats the widget you're supposed to use when drawing/showing svgs.

I assume its more sophisticated for the task, than a simple QIcon

[cle1109]

@J-Hilk I'm not sure how I am supposed to set the icon of the action then, because this expects a QIcon.

[J.Hilk]

@cle1109 we would burn that bridge when we get there 😉.
I would rather first check if it makes a difference at all.

[cle1109]

It might, and I will test this if I have time, but at the same time I feel that it should just work with a plain and simple QIcon("waves-24px.svg") as on Linux and Windows.

[J.Hilk]

@cle1109 well yes, but sadly the SVG support by Qt is limited. Doesn't work at all on iOS for example, and some common sub formats don't work at all across all platforms :(

[SimonSchroeder]

@cle1109 That is unfortunate that the workaround does not work on MacOS. I have one more suggestion you could try. Instead of

pixmap = QPixmap(toolbar.iconSize())
pixmap.setDevicePixelRatio(main.devicePixelRatio())

just try

pixmap = QPixmap(toolbar.iconSize() * main.devicePixelRatio())

There is a slight chance that using pixmap.setDevicePixelRatio does not work properly with SVG on MacOS. In this case this would create a larger icon which is then scaled down when drawing it on the toolbar.

[cle1109]

@SimonSchroeder I think there is something wrong with the code because like in the first version, I get a more or less random icon with the second version as well. It looks like either the conversion icon = QIcon(pixmap) doesn't work or one of the preceding steps.

[SimonSchroeder]

@cle1109 You can try if it is one of the preceeding steps by calling pixmap.save(...) and have a look at the saved image.

[cle1109]

I did pixmap.save("pixmap.png") after the painter stuff and the resulting PNG doesn't look right. So either the painter doesn't paint correctly into the pixmap or the image is not read in correctly. Can I debug this further to find out what the problem is?

[SimonSchroeder]

To me, this means that the painter does not paint correctly. I'm out of ideas.

Maybe, one last thing to check: I am not sure how a QPython project would be set up. For C++ I am using qmake. One thing I noticed is that in the end SVG icons are working. However, it would be more correct to add Qt's SVG module, i.e. I would add the line

QT += svg

to my qmake project file. Is there something similar for Python?

[cle1109]

I don't think this is necessary in Python. It is sufficient to import the required packages. In the example, I don't explicitly require the PyQt5.QtSvg module, but I don't think this is a problem (the stuff I'm using should automatically use functions from that module if needed).

I will rewrite my example in C++ to see if this is a problem specific to the Python bindings. That way, it will be easier to decide where to file the bug report (since this is working on Windows and Linux). I'll keep you posted.

[cle1109]

I tried this example in C++ and the result is exactly the same. The SVG icon is rendered in a very low resolution.

#include <QApplication>
#include <QMainWindow>
#include <QIcon>
#include <QAction>
#include <QToolBar>


int main(int argc, char **argv)
{
    QApplication app(argc, argv);
    QMainWindow window;
    QIcon icon("waves-24px.svg");
    QAction action(icon, "Test");
    QToolBar *toolbar = new QToolBar(&window);
    toolbar->addAction(&action);
    window.addToolBar(toolbar);
    window.show();
    return app.exec();
}

[cle1109]

Also, here's my main.pro file I used to build the app. It doesn't make a difference whether or not I include svg.

TEMPLATE = app
TARGET = main

QT = core gui widgets

SOURCES += main.cpp

[SimonSchroeder]

You'll need to use the trick with the QPixmap and QPainter in C++ as well. In the background Python and C++ use the exact same functionality, so I wouldn't expect any other behavior without the workaround. (I have experienced this exact problem in C++ multiple times.)

For your main.pro you need to add the svg module also:

...
QT = core gui widgets svg
...

This is what I actually meant with my last post.

[cle1109]

You'll need to use the trick with the QPixmap and QPainter in C++ as well. In the background Python and C++ use the exact same functionality, so I wouldn't expect any other behavior without the workaround. (I have experienced this exact problem in C++ multiple times.)

I get exactly the same result as in Python (the icon is just a random pixel pattern).

Yes, I know what you meant, I tried adding svg but it didn't make any difference. Same for the workaround. So in conclusion, this seems to be a Qt bug on macOS which also affects PyQt5.

[cle1109]

Turns out there is a simple solution:

app.setAttribute(Qt.AA_UseHighDpiPixmaps)