Optimizing Qt GUI Applications with OpenGL: Using QSurfaceFormat::version() Effectively

Purpose

• In Qt applications that utilize OpenGL for graphics rendering, QSurfaceFormat::version() plays a crucial role in specifying the desired OpenGL version your application wants to use.

Functionality

• By calling version(), you can retrieve the currently set major and minor OpenGL versions stored within the QSurfaceFormat object.
• It's a member function of the QSurfaceFormat class, which is responsible for configuring the format (or capabilities) of the underlying graphics surface used for rendering.

Default Version

• If you haven't explicitly set the version using setVersion(), the default behavior is to use OpenGL version 2.0.

Setting the Version

• This ensures that Qt attempts to negotiate with the underlying graphics system (e.g., your graphics card driver) to provide a surface that supports the requested version.
• To request a specific OpenGL version, use the setVersion(majorVersion, minorVersion) function on a QSurfaceFormat object before creating your Qt widgets that require OpenGL.

Example

#include <QApplication>
#include <QOpenGLWidget>

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

    // Request OpenGL version 3.3 (modify as needed)
    QSurfaceFormat format;
    format.setVersion(3, 3);
    QSurfaceFormat::setDefaultFormat(format);

    QOpenGLWidget widget;
    widget.show();

    return app.exec();
}

• For broader compatibility, consider checking for the available versions using QOpenGLContext::versionFunctions() before setting a specific version.
• Not all systems may support all OpenGL versions. Qt will try its best to provide a compatible surface, but it might fall back to a lower version if the requested one isn't available.

---

Checking Available OpenGL Versions

#include <QApplication>
#include <QOpenGLContext>

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

    QOpenGLContext context;
    QOpenGLFunctions* functions = context.versionFunctions();

    if (functions) {
        int major = functions->majorVersion();
        int minor = functions->minorVersion();
        qDebug() << "Highest supported OpenGL version: " << major << "." << minor;
    } else {
        qDebug() << "Failed to obtain OpenGL version functions";
    }

    // ... (rest of your application code)

    return app.exec();
}

This code snippet first creates a dummy QOpenGLContext object. Then, it retrieves the QOpenGLFunctions pointer using versionFunctions(). If successful, it extracts the major and minor versions using majorVersion() and minorVersion(), and prints them for your reference.

Handling Compatibility Issues

#include <QApplication>
#include <QOpenGLWidget>

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

    // Request OpenGL version 3.3 (modify as needed)
    QSurfaceFormat format;
    format.setVersion(3, 3);

    // Check for available versions before setting default
    QOpenGLContext context;
    QOpenGLFunctions* functions = context.versionFunctions();
    if (functions && functions->majorVersion() >= 3 && functions->minorVersion() >= 3) {
        QSurfaceFormat::setDefaultFormat(format);
    } else {
        qDebug() << "Requested OpenGL version (3.3) not supported. Using default.";
    }

    QOpenGLWidget widget;
    widget.show();

    return app.exec();
}

This example first attempts to set the desired version (3.3 in this case). However, it then checks for available versions using versionFunctions(). If the requested version isn't supported, it prints a message and falls back to the default format provided by Qt.

---

Using a Higher-Level API (if applicable)

• If you're using a higher-level graphics library on top of Qt, like Qt 3D or Qt Quick 3D, these libraries might offer their own mechanisms for setting the OpenGL context or rendering engine. Consult their documentation to see if they provide built-in functionality for version selection.

Creating a Custom QWindow Subclass (Advanced)

• As a more advanced approach, you could potentially create a subclass of QWindow. Within this subclass, you could override the createPlatformWindow() function to have more control over the underlying window creation process. Here, you might interact with platform-specific APIs (like WGL on Windows) to try influencing the chosen OpenGL version. However, this approach is significantly more complex and less portable across different platforms.

Checking Available Versions Beforehand

• Even if you don't have a direct way to force a specific version, you can still use QOpenGLContext::versionFunctions() (as shown in the previous example) to check which versions are supported before running your application. This allows you to adjust your code or display informative messages based on the available capabilities.

Important Considerations

• Directly interacting with platform-specific APIs (like in approach 2) can lead to portability issues and introduce maintenance challenges as platforms evolve.
• Forcing a specific OpenGL version might not always be practical or desirable. Modern systems often support various profiles (core vs. compatibility) within a major version. You might have more success requesting a minimum version and allowing Qt to negotiate the best profile.

• In most cases, it's recommended to use QSurfaceFormat::version() in conjunction with checking available versions using QOpenGLContext::versionFunctions(). This approach provides a balance between flexibility and platform compatibility. If you encounter version-related issues, adjust your code or consider alternative rendering libraries that might offer more granular version control.