Qt: Understanding QOpenGLExtraFunctions::glActiveShaderProgram()

Understanding QOpenGLExtraFunctions

• QOpenGLExtraFunctions is a subclass that extends QOpenGLFunctions and includes functions specifically for OpenGL ES 3.0, 3.1, and 3.2. This allows you to develop cross-platform applications that use these OpenGL ES versions.
• Qt provides the QOpenGLFunctions class to interact with OpenGL functions within your Qt application.

Purpose of glActiveShaderProgram()

• It activates a shader program within a graphics pipeline object (PBO) in OpenGL ES 3.x or compatible OpenGL contexts (versions 3.x or 4.x).
• This function is a convenience wrapper around the core OpenGL function glActiveShaderProgram().

How it Works

• The function then calls the underlying OpenGL function to bind the specified shader program to the active PBO.
• You pass two arguments to glActiveShaderProgram():
  • pipeline (type: GLuint): The identifier of the PBO where you want to activate the shader program.
  • program (type: GLuint): The identifier of the shader program you want to activate.

Benefits of Using QOpenGLExtraFunctions::glActiveShaderProgram()

• Simplified API
  It provides a convenient way to activate shader programs within PBOs, reducing the need to directly call the core OpenGL function.
• Cross-Platform Compatibility
  By using QOpenGLExtraFunctions, your code can work on both desktop platforms (with OpenGL 3.x or 4.x) and embedded devices that support OpenGL ES 3.x. This reduces the need for code changes when deploying to different environments.

Important Considerations

• If you're using plain OpenGL without these versions or extensions, you might need to use a different approach for shader program activation.
• This function is only available in the contexts mentioned above (OpenGL ES 3.x, OpenGL 3.x, or 4.x).

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#include <QtWidgets>
#include <QOpenGLWidget>
#include <QOpenGLExtraFunctions>

class MyWidget : public QOpenGLWidget {
    Q_OBJECT

public:
    MyWidget(QWidget *parent = nullptr);

protected:
    void initializeGL() override;
    void paintGL() override;

private:
    GLuint m_vertexShader, m_fragmentShader, m_shaderProgram;
    QOpenGLExtraFunctions *m_funcs; // Pointer to extra functions
};

MyWidget::MyWidget(QWidget *parent) : QOpenGLWidget(parent) {
    m_funcs = nullptr; // Initialize to null
}

void MyWidget::initializeGL() {
    // Create a new OpenGL context
    makeCurrent();

    // Get extra functions for OpenGL ES 3.x or compatible contexts
    m_funcs = QOpenGLContext::currentContext()->extraFunctions();

    // (Shader source code goes here)
    // Assuming you have separate vertex and fragment shader source code in string variables

    // Compile vertex shader
    m_vertexShader = m_funcs->glCreateShader(GL_VERTEX_SHADER);
    const char *vertexShaderSource = vertexShaderCode.c_str();
    m_funcs->glShaderSource(m_vertexShader, 1, &vertexShaderSource, nullptr);
    m_funcs->glCompileShader(m_vertexShader);

    // Compile fragment shader (similar process)

    // Create shader program
    m_shaderProgram = m_funcs->glCreateProgram();
    m_funcs->glAttachShader(m_shaderProgram, m_vertexShader);
    m_funcs->glAttachShader(m_shaderProgram, m_fragmentShader);
    m_funcs->glLinkProgram(m_shaderProgram);

    // (Error checking for shader compilation and linking can be added here)
}

void MyWidget::paintGL() {
    // Clear the screen (assuming you have a clear color set)
    glClear(GL_COLOR_BUFFER_BIT);

    // **Activate the shader program**
    m_funcs->glActiveShaderProgram(0, m_shaderProgram); // Assuming PBO 0

    // Bind vertex data (VBO), set vertex attributes, etc.

    // Draw using appropriate OpenGL functions (e.g., glDrawArrays)

    // Deactivate the shader program (optional)
    m_funcs->glActiveShaderProgram(0, 0); // Deactivate or switch to another program
}

int main(int argc, char *argv[]) {
    QApplication app(argc, argv);
    MyWidget widget;
    widget.show();
    return app.exec();
}

1. • QtWidgets for basic Qt widgets.
   • QOpenGLWidget for the OpenGL widget class.
   • QOpenGLExtraFunctions for accessing OpenGL ES 3.x functions.

2. MyWidget Class
   

   • Inherits from QOpenGLWidget.
   • Defines member variables for shader program IDs and the extra functions pointer.

3. MyWidget Constructor
   

   • Initializes m_funcs to nullptr.

4. initializeGL() Function
   

   • Creates a new OpenGL context.
   • Obtains a pointer to QOpenGLExtraFunctions.
   • Creates and compiles vertex and fragment shaders (replace with your actual shader code).
   • Links the shaders into a shader program.
   • (Error checking for shader compilation and linking can be added.)

5. paintGL() Function
   

   • Clears the screen.
   • Activates the shader program using glActiveShaderProgram() with PBO ID 0 (assuming you're using the default PBO).
   • Binds vertex data, sets attributes, and prepares for drawing.
   • Draws using appropriate OpenGL functions.
   • Optionally deactivates the shader program by setting both program IDs to 0 (you might want to switch to a different program here).

Compiling and Running

1. Replace the placeholder shader code comments with your actual vertex and fragment shader source code.
2. Compile the code using a Qt compiler.
3. Run the application.

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Using Core OpenGL Function Directly (if applicable)

• If you're certain your target platforms all support plain OpenGL 3.x or 4.x (without the need for OpenGL ES), you can use the core OpenGL function glUseProgram(). This function has the same effect as glActiveShaderProgram():

void paintGL() {
  // ...
  // Activate the shader program
  glUseProgram(m_shaderProgram);
  // ...
}

Custom Shader Program Management

• If you have a more complex scenario where you need finer control over shader program management, you could create your own system for tracking active programs. This would involve keeping track of the currently bound program ID and manually activating programs as needed using glUseProgram(). This approach requires more manual handling but offers greater flexibility.

Context-Specific Approach (if porting to non-compatible platforms)

• If you're targeting platforms that don't support OpenGL ES 3.x or compatible versions, you might need to adopt a different approach altogether. This could involve using a different graphics API entirely or creating a separate code path for those platforms.

• For more complex scenarios or platforms with incompatible OpenGL versions, consider a custom management system or a different graphics API approach.
• If you're targeting platforms with plain OpenGL 3.x or 4.x support exclusively, you can use glUseProgram() directly.
• For standard Qt development using OpenGL ES 3.x or compatible contexts, QOpenGLExtraFunctions::glActiveShaderProgram() is the recommended approach for activating shader programs.