VLC Fullscreen Control On Raspberry Pi CM4 A Comprehensive Guide

by StackCamp Team 65 views

In this comprehensive guide, we will address the challenge of managing VLC's fullscreen behavior on a Raspberry Pi Compute Module 4 (CM4) within an X11 environment. The core issue revolves around VLC's tendency to monopolize the display when in fullscreen mode, preventing other applications, such as a volume OSD indicator, from appearing on top. This behavior can be problematic when building media solutions that require overlaying UI elements on the video playback. This article provides a detailed exploration of the problem, potential solutions, and step-by-step instructions to achieve the desired behavior.

The problem arises when using VLC in fullscreen mode within a Qt application on a Raspberry Pi CM4. When VLC enters fullscreen, it effectively takes over the entire display, causing other applications to render in the background, making them invisible to the user. This is particularly problematic when you need to display overlay elements, such as a volume OSD, on top of the video playback.

Use Case Scenario

Imagine you are building a custom media player using a Raspberry Pi CM4. Your application includes:

  • A Qt-based application that uses VLC for video playback in fullscreen.
  • A secondary Qt application, such as a volume OSD indicator, that should appear on top of the video when the volume is adjusted.

The desired behavior is that when a user adjusts the volume, the OSD should temporarily appear on top of the fullscreen video, providing visual feedback. However, due to VLC's fullscreen behavior, the OSD is rendered in the background and is not visible, even though the volume change is successfully registered in the backend.

Expected vs. Actual Behavior

The expected behavior is that VLC, even in fullscreen mode, should allow other X11 windows to appear above it, similar to typical fullscreen applications on Linux desktops. This would enable overlaying UI components like dialogs and OSDs.

However, the actual behavior is that VLC fully covers the screen, making it impossible for other applications to overlay UI elements. This blocks the OSD from being displayed, hindering the user experience.

To overcome this issue, we can explore several strategies and configurations. These approaches aim to make VLC coexist harmoniously with other applications in the X11 environment, allowing for the desired overlay behavior. Key areas to investigate include:

  • VLC Configuration: Adjusting VLC's settings to control its fullscreen behavior. This involves exploring options related to window management and video output.
  • X11 Window Management: Understanding how X11 manages windows and how to ensure that specific windows are always on top.
  • Qt Application Structure: Modifying the Qt application structure to handle window layering and visibility correctly.
  • Alternative Video Players: Considering alternative video players or libraries that may offer better control over fullscreen behavior.

We will delve into each of these areas, providing detailed instructions and examples to help you implement the most suitable solution for your specific use case.

1. VLC Configuration Adjustments

One of the primary approaches is to configure VLC itself to behave in a more cooperative manner within the X11 environment. VLC provides various options that can influence its fullscreen behavior. We will explore these options and how to adjust them to allow other windows to appear on top.

1.1. Disable Fullscreen Mode Override

VLC has a setting that controls whether it overrides the window manager's fullscreen behavior. By disabling this override, we can allow the window manager to handle fullscreen, potentially enabling other windows to appear on top. To disable this setting:

  1. Open VLC.
  2. Go to Tools > Preferences.
  3. Click the Video tab.
  4. Uncheck the box labeled "Fullscreen mode override".
  5. Save the changes and restart VLC.

This setting can also be adjusted via the command line or configuration file. For example, in the VLC configuration file (~/.config/vlc/vlcrc), you can set fullscreen-on-top=0.

1.2. Adjust Video Output Settings

The video output module used by VLC can also affect its fullscreen behavior. Some output modules may be more cooperative with window managers than others. Try changing the video output module to see if it resolves the issue.

  1. Open VLC.
  2. Go to Tools > Preferences.
  3. Click the Video tab.
  4. In the Output dropdown, try different options such as "X11 video output (XCB)" or "OpenGL video output".
  5. Save the changes and restart VLC.

Each output module interacts differently with the X11 windowing system, and experimenting with these options may lead to a better outcome for your specific setup. The "X11 video output (XCB)" is often a good starting point for compatibility on Linux systems.

1.3. Command-Line Options

VLC provides several command-line options that can be used to control its fullscreen behavior. These options can be useful when launching VLC from within your Qt application. Some relevant options include:

  • --no-fullscreen: Prevents VLC from starting in fullscreen mode.
  • --no-qt-fs-controller: Disables the Qt fullscreen controller, which may interfere with other applications.
  • --embedded-xid <xid>: Specifies the X11 window ID to embed VLC into, which can provide more control over window management.

For example, to launch VLC without fullscreen mode override and embed it into a Qt widget, you might use the following command:

vlc --no-fullscreen --no-qt-fs-controller --embedded-xid <widget_xid> <video_file>

Where <widget_xid> is the X11 window ID of the Qt widget you want to embed VLC into, and <video_file> is the path to the video file.

2. X11 Window Management

Understanding how X11 manages windows is crucial for ensuring that your OSD or other UI elements appear on top of VLC. X11 uses a stacking order to determine which windows are visible. By manipulating the stacking order, we can force specific windows to be always on top.

2.1. Using xprop to Inspect Window Properties

The xprop utility is a powerful tool for inspecting the properties of X11 windows. You can use it to identify the window ID (XID) of VLC and your OSD application and to view their current properties.

To use xprop:

  1. Open a terminal.
  2. Type xprop and press Enter.
  3. The cursor will change to a crosshair. Click on the VLC window and then on your OSD window.
  4. xprop will display a list of properties for each window, including the window ID.

2.2. Using xdotool to Manage Windows

xdotool is a command-line tool for scripting X11 window management tasks. It can be used to move, resize, raise, and lower windows, among other things. To ensure that your OSD window is always on top, you can use the xdotool windowraise command.

  1. Install xdotool if it's not already installed:

    sudo apt-get update
sudo apt-get install xdotool

  2. Get the window ID of your OSD application using xprop as described above.

  3. Use xdotool to raise the OSD window:

    xdotool windowraise <osd_window_id>

This command will bring the OSD window to the top of the stacking order, making it visible even when VLC is in fullscreen mode.

2.3. Setting Window Attributes in Qt

Within your Qt application, you can set window attributes to control their behavior in the X11 environment. For example, you can use the Qt::WindowStaysOnTopHint flag to ensure that a window remains on top of others.

#include <QApplication>
#include <QWidget>

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

 QWidget window;
 window.setWindowTitle("OSD Window");
 window.setWindowFlags(window.windowFlags() | Qt::WindowStaysOnTopHint);
 window.show();

 return app.exec();
}

This code snippet creates a simple Qt widget and sets the Qt::WindowStaysOnTopHint flag, which tells the window manager to keep the window on top of other windows.

3. Qt Application Structure

The structure of your Qt application can also influence how windows are layered and managed. Proper window management within your Qt application is essential for achieving the desired overlay behavior. This involves understanding how to create and manage windows, and how to control their visibility and stacking order.

3.1. Creating Top-Level Windows

In Qt, top-level windows are independent windows that are managed by the window manager. To create a top-level window for your OSD, you can use the QWidget class or a subclass thereof.

#include <QWidget>
#include <QLabel>
#include <QVBoxLayout>

class OSDWindow : public QWidget {
public:
 OSDWindow(QWidget *parent = nullptr) : QWidget(parent) {
 setWindowTitle("Volume OSD");
 QLabel *label = new QLabel("Volume: 50%");
 QVBoxLayout *layout = new QVBoxLayout(this);
 layout->addWidget(label);
 setLayout(layout);
 }
};

This code defines a simple OSDWindow class that inherits from QWidget. It creates a label to display the volume level and uses a vertical layout to organize the widgets.

3.2. Managing Window Visibility

To show and hide the OSD window, you can use the show() and hide() methods. For example, you might show the OSD window when the volume is adjusted and hide it after a short delay.

#include <QTimer>

void showOSD() {
 OSDWindow *osdWindow = new OSDWindow();
 osdWindow->show();

 QTimer::singleShot(2000, [osdWindow]() {
 osdWindow->hide();
 osdWindow->deleteLater();
 });
}

This function creates an OSDWindow, shows it, and then uses a QTimer to hide and delete the window after 2 seconds. The deleteLater() method ensures that the window is deleted safely when the event loop is idle.

3.3. Embedding VLC in a Qt Widget

To embed VLC into a Qt widget, you can use the --embedded-xid command-line option as described earlier. You need to obtain the X11 window ID of the Qt widget and pass it to VLC. Here’s how you can do it:

#include <QWidget>
#include <QProcess>
#include <QWindow>
#include <WId>

void embedVLC(QWidget *widget, const QString &videoFile) {
 QProcess *vlcProcess = new QProcess();
 QString widgetId = QString::number(widget->windowHandle()->winId());
 QStringList arguments;
 arguments << "--no-fullscreen";
 arguments << "--no-qt-fs-controller";
 arguments << "--embedded-xid" << widgetId;
 arguments << videoFile;
 vlcProcess->start("vlc", arguments);
}

This function takes a Qt widget and a video file path as input. It obtains the X11 window ID of the widget using windowHandle()->winId() and then launches VLC with the appropriate command-line arguments to embed it into the widget.

4. Alternative Video Players and Libraries

If VLC continues to pose challenges, it may be worth considering alternative video players or libraries that offer more flexibility and control over fullscreen behavior. Several options are available, each with its own strengths and weaknesses.

4.1. MPV

MPV is a free and open-source media player that is known for its flexibility and powerful command-line options. It offers excellent control over window management and can be easily embedded into other applications.

To use MPV, you can launch it from your Qt application with specific command-line options to control its behavior. For example, to embed MPV into a Qt widget, you can use the --wid option, which is similar to VLC's --embedded-xid option.

#include <QProcess>
#include <QWindow>
#include <WId>

void embedMPV(QWidget *widget, const QString &videoFile) {
 QProcess *mpvProcess = new QProcess();
 QString widgetId = QString::number(widget->windowHandle()->winId());
 QStringList arguments;
 arguments << "--no-border";
 arguments << "--wid" << widgetId;
 arguments << videoFile;
 mpvProcess->start("mpv", arguments);
}

This function launches MPV and embeds it into a Qt widget. The --no-border option removes the window decorations, and the --wid option specifies the X11 window ID to embed MPV into.

4.2. GStreamer

GStreamer is a powerful multimedia framework that provides a flexible and extensible way to build media applications. It supports a wide range of codecs and formats and can be easily integrated into Qt applications.

Using GStreamer directly in your Qt application provides a high degree of control over video playback and window management. You can create a GStreamer pipeline and render the video output to a Qt widget.

4.3. LibVLC

LibVLC is the library version of VLC, which allows you to embed VLC's playback capabilities directly into your application. While this may seem like the most straightforward approach, it can still exhibit the same fullscreen behavior issues as the standalone VLC player. However, using LibVLC gives you more control over the playback process and allows you to handle window management programmatically.

To help you implement a solution, let's outline a step-by-step guide that incorporates the strategies discussed above.

Step 1: Set Up Your Development Environment

  1. Ensure you have a Raspberry Pi CM4 with a suitable operating system (e.g., Raspberry Pi OS).

  2. Install Qt and the necessary development tools.

  3. Install VLC and xdotool:

    sudo apt-get update
sudo apt-get install vlc xdotool

Step 2: Create a Basic Qt Application

  1. Create a new Qt project.
  2. Add a QWidget to your main window to act as the video display area.
  3. Create a separate QWidget for your OSD.

Step 3: Embed VLC into the Qt Widget

  1. Use the embedVLC function (or a similar function for MPV or GStreamer) to launch VLC and embed it into the video display widget.
  2. Ensure that you pass the correct X11 window ID to VLC.

Step 4: Implement the OSD Window

  1. Create an OSDWindow class as described above.
  2. Set the Qt::WindowStaysOnTopHint flag for the OSD window.
  3. Implement functions to show and hide the OSD window, using a timer to automatically hide it after a short delay.

Step 5: Handle Volume Changes

  1. Implement a mechanism to detect volume changes (e.g., using keyboard input or an RCU).
  2. When a volume change is detected, update the OSD display and show the OSD window.

Step 6: Test and Refine

  1. Run your application and test the fullscreen behavior.
  2. If the OSD does not appear on top, use xdotool to raise the OSD window manually and see if that resolves the issue.
  3. Experiment with different VLC settings and command-line options to find the optimal configuration.

Managing VLC's fullscreen behavior on a Raspberry Pi CM4 can be challenging, but with the right configuration and techniques, it is possible to achieve the desired overlay behavior. By adjusting VLC's settings, leveraging X11 window management tools, structuring your Qt application correctly, and considering alternative video players, you can build a robust media solution that meets your specific needs. This comprehensive guide has provided you with the knowledge and steps necessary to tackle this issue effectively. Remember to experiment with different approaches and settings to find the best solution for your unique application.

By following these steps and exploring the various options, you can create a seamless media experience on your Raspberry Pi CM4, ensuring that your OSD and other UI elements are always visible when needed. The key is to understand the interplay between VLC, X11, and Qt, and to tailor your approach to the specific requirements of your project.