QML application not rendered correctly in Qt Wayland Compositor examples

robgu

Hello,

I'm trying to get familiar with QML's Wayland compositor examples.
For that I have cross-compiled Qt 6.5.3 with a provided example (analogclock) for my Raspberry Pi running on Ubuntu 22.04 to make sure all is fine. I also compiled the "minimal-qml" compositor example and installed Weston for having a reference. Then I made a very simple QML app (code below).

First step was to check whether the compilation is fine: both analogclock and my QML applications run smoothly in gnome desktop on Raspberry Pi.
Then I switched to tty and started Weston compositor. Again both applications run smoothly when started with command

./application_name --platform wayland

Here's how my app looks like in Weston:

Finally, still in tty, I started the minimal-qml example compositor:

./minimal-qml --platform eglfs

When started as mentioned above, the analogclock application is fine, but my QML application does not render correctly:

The black dots are the text, and I suppose that the grey areas have something to do with the window decorations (title bar, frame, etc.), but even after removing them (Qt::FramelessWindowHint | Qt::CustomizeWindowHint) the result is far from acceptable. (Note: black area on the top and left is my physical screen frame.)

What am I missing? Is there some argument to be set or added to the launch of QML app? Or is there something missing in the definition of the compositor?

Code of my QML application:
main.cpp:

#include <QGuiApplication>
#include <QQmlApplicationEngine>

int main(int argc, char *argv[])
{
    QGuiApplication app(argc, argv);

    QQmlApplicationEngine engine;
    QObject::connect(&engine, &QQmlApplicationEngine::objectCreationFailed,
        &app, []() { QCoreApplication::exit(-1); },
        Qt::QueuedConnection);
    engine.loadFromModule("testqml", "Main");

    return app.exec();
}

Main.qml:

import QtQuick
import QtQuick.Window

Window {
    width: 400
    height: 400
    visible: true
    title: qsTr("Hello QML World")
    
    Rectangle {
        visible: true
        id: rectangle
        color: "red"
        width: 400
        height: 400

        Text {
            id: text
            text: "This is QML code"
            font.pointSize: 14
            anchors.centerIn: parent
        }
    }
}

CMakeLists.txt:

cmake_minimum_required(VERSION 3.16)

project(testqml VERSION 0.1 LANGUAGES CXX)

set(CMAKE_CXX_STANDARD_REQUIRED ON)

find_package(Qt6 6.5 REQUIRED COMPONENTS Quick)

set(CMAKE_AUTORCC ON)

qt_standard_project_setup(REQUIRES 6.5)

qt_add_executable(apptestqml
    main.cpp
)

qt_add_qml_module(apptestqml
    URI testqml
    VERSION 1.0
    QML_FILES Main.qml
)

# Qt for iOS sets MACOSX_BUNDLE_GUI_IDENTIFIER automatically since Qt 6.1.
# If you are developing for iOS or macOS you should consider setting an
# explicit, fixed bundle identifier manually though.
set_target_properties(apptestqml PROPERTIES
#    MACOSX_BUNDLE_GUI_IDENTIFIER com.example.apptestqml
    MACOSX_BUNDLE_BUNDLE_VERSION ${PROJECT_VERSION}
    MACOSX_BUNDLE_SHORT_VERSION_STRING ${PROJECT_VERSION_MAJOR}.${PROJECT_VERSION_MINOR}
    MACOSX_BUNDLE TRUE
    WIN32_EXECUTABLE TRUE
)

target_link_libraries(apptestqml
    PRIVATE Qt6::Quick
)

include(GNUInstallDirs)
install(TARGETS apptestqml
    BUNDLE DESTINATION .
    LIBRARY DESTINATION ${CMAKE_INSTALL_LIBDIR}
    RUNTIME DESTINATION ${CMAKE_INSTALL_BINDIR}
)

robgu

Hello,

I might have advanced a little bit with my problem.
I changed my application to represent 2 vertical rectangles, one red, and the second divided into two squares (yellow and blue). I also fixed the window size to 256 by 256 pixels (I noticed regular glitches with only 2 rectangles so I thought it might be a buffering problem). My current application, when run in Weston, looks like expected (sorry for rasterization):

However, in minimal-qml example compositor it is still not correct:

So it seems that the colors and the proportions are fine, but I have the impression that the buffer passed from my app to the compositor is read in an incorrect manner.

What am I missing? Surface origin? Surface orientation? Other specific setting?
Any help or suggestion will be appreciated...

robgu

I found the solution to my problem.

I set environmental variable WAYLAND_DEBUG=1 and observed the output of my compositor and my app. Then I compared the same output with the logs of weston (launched with --log=file.log and -lproto). I noticed that the latter was using different buffers attached to the created surfaces.
So I tested starting my application with QT_QUICK_BACKEND=software and it worked!
However, there was no support for 3D (Quick3D examples).
Checking further on the buffers I found that setting QT_WAYLAND_CLIENT_BUFFER_INTEGRATION=linux-dmabuf-unstable-v1 before starting the compositor is informing the clients to use that kind of buffers.
This has entirely solved my problem on my configuration, I have now both QML applications and Quick3D applications rendered correctly, no need for additional settings on the clients' side.

KH-219Design

This has been a gripping mystery thread to (silently) follow. Thank you for continuing to update this thread! You had a silent "cheering squad" in the audience, despite me not having any help to provide :)