README.md

KDE Bug 466373 – Qt apps don't use wp-fractional-scale-v1 for hidpi fractional scaling on Wayland

SUMMARY Qt client apps using system Qt 5.15.8 still use the old downscaling mechanism to deal with fractional scaling, instead of wp-fractional-scale-v1, leading to visual artefacts.

STEPS TO REPRODUCE

1. Set "Scale" to fractional eg 135% in Display Configuration
2. Open a Qt app

OBSERVED RESULT App is upscaled to 2x then downscaled. This produces aliasing artifacts as detailed previously here 1

EXPECTED RESULT Native scaling... like what is done by XWayland in "Apply scaling themselves" mode. See attached picture. On the left is XWayland (Platfom=xcb) and on the right Wayland.

SOFTWARE/OS VERSIONS KDE Neon based on ubuntu 22.04.2 (available in About System) KDE Plasma Version: 5.27.1 KDE Frameworks Version: 5.104.0 Qt Version: 5.15.8 libqt5core5a Version: 5.15.8+p22.04+tunstable+git20230209.0323-0 python3-pyqt5 Version: 5.15.9+dfsg-1+22.04+jammy+unstable+build21

ADDITIONAL INFORMATION

I believe the previous scaling is offered by Qt since 5.6 and is described here 2 and 3. And the new one was announced by Nate here 4.

I think the KWin part landed:

> wayland-info | rg frac
interface: 'wp_fractional_scale_manager_v1',             version:  1, name: 10

but maybe the Qt patches were not added to Neon's patch collection? Is this 5 related? how can I verify if the Qt5 client libs have support?

I attach the small PyQt sample 6 used to show that the scale given to the app is 2x and the screen geometry and physicalDotsPerInch is fake (dpi 115.834 instead of 163), with system Qt 5.15.8.

qt6-dpi.py

from datetime import datetime
import os
import sys

if sys.argv[1:] or '' == '5':
    # `exec` to avoid messing with the IDE smarts
    exec('''
from PyQt5.QtWidgets import \\
    QMainWindow, QLabel, QGraphicsRectItem,\\
    QGridLayout, QWidget, \\
    QGraphicsScene, QGraphicsView, QApplication
from PyQt5.QtGui import QPixmap, QColor
from PyQt5.QtCore import Qt, QT_VERSION, QT_VERSION_STR, PYQT_VERSION_STR
TextSelectableByMouse = Qt.TextInteractionFlag.TextSelectableByMouse
    ''')
else:
    from PyQt6.QtWidgets import \
        QMainWindow, QLabel, QGraphicsRectItem,\
        QGridLayout, QWidget, \
        QGraphicsScene, QGraphicsView, QApplication
    from PyQt6.QtGui import QPixmap, QColor
    from PyQt6.QtCore import Qt, QT_VERSION, QT_VERSION_STR, PYQT_VERSION_STR
    TextSelectableByMouse = Qt.TextInteractionFlag.TextSelectableByMouse


class MyApp(QWidget):

    def __init__(self):
        super().__init__()
        self.buildUI()
        self.initUI()
        self.signals()

    def signals(self):
        self.windowHandle().screenChanged.connect(self.update)
        for scr in app.screens():
            scr.logicalDotsPerInchChanged.connect(self.update)

    def buildUI(self):
        mainLayout = QGridLayout()

        scene = QGraphicsScene()
        # https://stackoverflow.com/a/48782280/2017781
        pxm = QPixmap(os.path.join(sys.path[0], 'dpitest1px.png'))
        pxit = scene.addPixmap(pxm)
        pxit.setPos(1, 1)
        # the image is 126x23 px, add 2*pen.width
        rect = QGraphicsRectItem(0, 0, 128, 25)
        rect.setPen(QColor(0, 255, 0))
        scene.addItem(rect)

        self.view = QGraphicsView(scene)
        self.view.setFixedSize(150, 50)
        mainLayout.addWidget(self.view)

        self.labf = QLabel()
        self.updateLabel()
        self.labf.setTextInteractionFlags(TextSelectableByMouse)
        mainLayout.addWidget(self.labf)

        self.setLayout(mainLayout)

    def update(self):
        self.updateLabel()

    def updateLabel(self):
        ss = ''
        for scr in app.screens():
            g = scr.availableGeometry().getRect()
            v = scr.availableVirtualGeometry().getRect()
            ss += f'''
            {scr.name()}: scale = {scr.devicePixelRatio()}
                physicalDotsPerInch = {round(scr.physicalDotsPerInch(), 4)}
                logicalDotsPerInch = {scr.logicalDotsPerInch()} 
                Geometry = {g}
                VirtualGeometry = {v}'''

        h = self.windowHandle()
        screenName = h.screen().name() if h else None

        self.labf.setText(f'''
        On {datetime.now().isoformat()}
        Qt {QT_VERSION_STR}
        PyQt {PYQT_VERSION_STR}
        app.platformName = {app.platformName()}
        app.devicePixelRatio = {app.devicePixelRatio()}
        screenName = {screenName}

        View:
            devicePixelRatioF = {self.view.devicePixelRatioF()}
            devicePixelRatio = {self.view.devicePixelRatio()}
            devicePixelRatioFScale = {self.view.devicePixelRatioFScale()}
            logicalDpiX = {self.view.logicalDpiX()}
            size = {self.view.size()}

        Screens: {ss}
        ''')

    def initUI(self):
        self.setWindowTitle(f'Qt {QT_VERSION_STR} {app.platformName()} demo')
        # self.adjustSize()
        self.center()
        self.show()

    def center(self):
        qr = self.frameGeometry()
        cp = app.primaryScreen().availableGeometry().center()
        # cp = app.activeWindow().screen.g windowHandle().sc
        qr.moveCenter(cp)
        self.move(qr.topLeft())


app = QApplication([])
ex = MyApp()
app.exec()

20231016_120713, Neon_unstable:

With display at 120%, Pixel ratio is 2 and dpi is 96.