Optimizing Vulkan Window Resource Handling in Qt: Alternatives to QVulkanWindow::flags()

What it is

• QVulkanWindow is a Qt class that simplifies working with Vulkan windows within your Qt application.
• Vulkan is a graphics API that provides low-level control over graphics processing on modern GPUs.
• QVulkanWindow::flags() is a method in the Qt framework that allows you to control the behavior of a Vulkan window.

What it does

• QVulkanWindow::flags() lets you override this behavior using flags.
• By default, when a Vulkan window is hidden (e.g., minimized or obscured by another window), Qt releases all the graphics resources associated with that window. This is done to conserve memory and avoid unnecessary processing.
• This method specifically deals with flags that modify how QVulkanWindow handles graphics resources when the window becomes hidden or unexposed.

The Flag

• This can be useful if you want the window to be able to quickly render again when it becomes visible again, without the overhead of re-initializing all the graphics resources.
• When you set this flag using setFlags(QVulkanWindow::DontReleaseResourcesOnHide), Qt will not release graphics resources when the window becomes hidden.
• The relevant flag here is QVulkanWindow::DontReleaseResourcesOnHide.

Things to Consider

• The setFlags() method must be called before the window is made visible or at the latest in the QVulkanWindowRenderer::preInitResources() function. It has no effect if called afterwards.
• You'll need to weigh the performance benefits of faster re-rendering against the memory usage implications.
• Keeping resources allocated while the window is hidden can consume more memory.

• Use this flag cautiously, considering the trade-off between performance and memory usage.
• The DontReleaseResourcesOnHide flag helps you control resource management when the window is hidden.
• QVulkanWindow::flags() allows you to configure the behavior of a Vulkan window in Qt.

---

#include <QtWidgets>
#include <QtVulkan>

class MyVulkanWindow : public QVulkanWindow {
    Q_OBJECT

public:
    MyVulkanWindow(QWidget *parent = nullptr) : QVulkanWindow(parent) {}

protected:
    void preInitResources() override {
        // Set the flag to prevent resource release on hide
        setFlags(flags() | QVulkanWindow::DontReleaseResourcesOnHide);

        // Call the base class implementation for further initialization
        QVulkanWindow::preInitResources();
    }
};

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

    // Create your Vulkan instance and other Vulkan setup (not shown here)
    QVulkanInstance vulkanInstance;
    // ...

    MyVulkanWindow window;
    window.resize(800, 600);
    window.setVulkanInstance(&vulkanInstance);
    window.show();

    return app.exec();
}

1. We define a custom class MyVulkanWindow that inherits from QVulkanWindow.
2. In the preInitResources() function (called before resource initialization), we:
   • Use setFlags(flags() | QVulkanWindow::DontReleaseResourcesOnHide) to set the flag.
   • Call QVulkanWindow::preInitResources() to proceed with the base class initialization.
3. In main(), we:
   • Create a QVulkanInstance (not shown here, see Vulkan setup documentation).
   • Create a MyVulkanWindow instance.
   • Set window size, Vulkan instance, and make it visible.

• This is a basic example. You'll need additional code for rendering and managing your Vulkan resources.
• Replace the Vulkan setup part (commented out) with your actual Vulkan instance creation and device initialization code.

---

1. Manual Resource Management
   

   • Instead of relying on the window's flags, you can explicitly manage the lifecycle of your Vulkan resources in your code.
   • When the window hides:
     • Call functions like vkDeviceWaitIdle to ensure all GPU operations finish.
     • Free host-side memory buffers and textures using Vulkan functions.
   • When the window shows again:
     • Recreate the host-side memory buffers and textures.
     • Re-upload data to device memory.
     • Recreate any Vulkan objects that depend on these resources (e.g., pipelines, descriptors).

   Pros
   

   • Provides fine-grained control over resource management.
   • Can potentially be more memory-efficient than keeping resources allocated all the time.
   • Requires more code to handle resource creation/destruction on hide/show events.
   • Can introduce overhead if frequent window hiding/showing occurs.

2. Custom QVulkanWindow Subclass
   

   • Create a subclass of QVulkanWindow and override relevant functions like hideEvent() and showEvent().
   • In hideEvent(), perform manual resource management as described above.
   • In showEvent(), recreate and re-upload resources as needed.

   Pros
   

   • Encapsulates resource management logic within the window class.
   • Can be cleaner than scattering resource management throughout your code.

   Cons
   

   • Requires creating a new class and maintaining the subclass.
   • Similar trade-offs to manual resource management in terms of complexity and overhead.

Choosing the Right Approach

• QVulkanWindow::flags() with DontReleaseResourcesOnHide offers a simpler solution but might consume more memory.
• If code maintainability is a priority and you need to encapsulate resource logic within the window, a subclass might be better.
• If performance is critical and hiding/showing happens frequently, consider manual resource management.