Qt: Setting Uniform Variables with QOpenGLExtraFunctions::glProgramUniform2f()

Functionality

Its purpose is to set a uniform variable in a shader program with a single 2-dimensional floating-point value (like a vec2 in GLSL).
It's a convenience wrapper around the core OpenGL function glProgramUniform2fv().
QOpenGLExtraFunctions::glProgramUniform2f() is a function provided by the Qt framework's QOpenGLExtraFunctions class.

Usage in Qt GUI

- To use glProgramUniform2f(), you'll need to include the <QOpenGLFunctions> header in your Qt GUI code:
```
#include <QOpenGLFunctions>
```

Obtain QOpenGLExtraFunctions Object

Create an instance of QOpenGLExtraFunctions either:

By passing the current OpenGL context:

QOpenGLContext* context = ...; // Obtain your OpenGL context
QOpenGLExtraFunctions functions(context);

Or by letting it resolve the context automatically:

QOpenGLExtraFunctions functions;
functions.initializeOpenGLFunctions(); // Must be called before use

Set the Uniform

Once you have the functions object, call glProgramUniform2f():

GLuint programId = ...; // Your shader program ID
GLint location = ...;   // Location of the uniform variable in the program
GLfloat v1, v2;         // The two floating-point values to set

functions.glProgramUniform2f(programId, location, v1, v2);

Key Points

v1 and v2 are the two floating-point values you want to set the uniform to.
location is the location (index) of the uniform variable within the program. You typically obtain this location using shader compilation functions like glGetUniformLocation().
programId is the ID of the shader program where you want to set the uniform.

Additional Considerations

Error handling is essential in real-world applications. Consider checking the return value of initializeOpenGLFunctions() to ensure the context is valid.
Make sure the uniform variable in your shader program is declared with a vec2 data type to match the format of glProgramUniform2f().
glProgramUniform2f() is limited to setting a single 2D float value. For arrays of uniforms or different data types, use other functions like glProgramUniform1f(), glProgramUniform3fv(), or glProgramUniformMatrix4fv().

#include <QtWidgets>
#include <QOpenGLWidget>
#include <QOpenGLShaderProgram>
#include <QOpenGLFunctions>

class MyWidget : public QOpenGLWidget {
    Q_OBJECT

public:
    MyWidget(QWidget *parent = nullptr);

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

private:
    GLuint m_programId;
    GLint m_uniformLocation;
    GLfloat m_uniformValue1;
    GLfloat m_uniformValue2;
    QOpenGLFunctions m_functions;
};

MyWidget::MyWidget(QWidget *parent)
    : QOpenGLWidget(parent),
      m_uniformValue1(0.5f),
      m_uniformValue2(0.7f)
{
}

void MyWidget::initializeGL() {
    // Initialize OpenGL context here (e.g., set up vertex buffer objects, etc.)

    // Create shader program
    m_programId = createShaderProgram();

    // Obtain uniform location (assuming the uniform is named "myUniform" in the shader)
    m_uniformLocation = m_functions.glGetUniformLocation(m_programId, "myUniform");

    // Enable using the shader program
    m_functions.glUseProgram(m_programId);
}

void MyWidget::paintGL() {
    // Clear the screen
    m_functions.glClearColor(0.0f, 0.0f, 0.0f, 1.0f);
    m_functions.glClear(GL_COLOR_BUFFER_BIT);

    // Set the uniform value before drawing (assuming you want to update it dynamically)
    m_functions.glProgramUniform2f(m_programId, m_uniformLocation, m_uniformValue1, m_uniformValue2);

    // Your drawing code here (using the shader program that references the uniform)

    // Swap contents to the screen
    swapBuffers();
}

GLuint MyWidget::createShaderProgram() {
    // Implement shader creation logic here
    // (loading shaders from files, compiling them, linking them, etc.)

    // ...

    return programId; // Return the created shader program ID
}

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

- QtWidgets for general Qt widgets.
- QOpenGLWidget for the OpenGL widget class.
- QOpenGLShaderProgram for shader program management.
- QOpenGLFunctions for the convenience wrapper functions.
MyWidget Class
- This class inherits from QOpenGLWidget to create a custom widget that supports OpenGL rendering.
- It has member variables to store the shader program ID, uniform location, and uniform values.
- An m_functions object of type QOpenGLFunctions is created.
initializeGL() Function
- This function gets called when the OpenGL context is initialized.
- You'd typically set up your OpenGL state here (e.g., vertex buffer objects, textures, etc.).
- The createShaderProgram() function is called to create the shader program.
- The uniform location is obtained using glGetUniformLocation() and stored in m_uniformLocation.
- The shader program is enabled using glUseProgram().
paintGL() Function
- This function gets called whenever the widget needs to be repainted.
- It clears the screen with a black background.
- Before drawing, the uniform value is set using glProgramUniform2f(). The values can be updated here for dynamic behavior.
- Your drawing code using the shader program that references the uniform variable would go here.
- The widget's contents are swapped to the screen using swapBuffers().
createShaderProgram() Function
- This function is a placeholder for your actual shader creation logic.
- You'd typically load vertex and fragment shaders from separate files, compile them, and link them into a shader program.
- The function should return the ID of the created shader program.
main() Function
- This function creates a Qt application, an instance of MyWidget, and shows the widget.

Direct OpenGL Function Calls
- You can directly use the core OpenGL function glProgramUniform2fv() instead of the Qt wrapper. This requires including the <GL/glew.h> header (assuming you're using GLEW for managing OpenGL extensions).
```
#include <GL/glew.h>

// ...

GLuint programId = ...;
GLint location = ...;
GLfloat v1, v2;

glProgramUniform2fv(programId, location, 1, &v1); // Pass the value as an array of size 1
```
Note
Using direct OpenGL functions might require additional setup for function pointers (e.g., using GLEW).

Custom Uniform Setting Function

You can create your own function to encapsulate uniform setting logic, potentially using function pointers or other techniques:

void setUniform2f(GLuint programId, GLint location, GLfloat v1, GLfloat v2) {
    // Use either glProgramUniform2fv() or program->setUniformValue() based on your Qt version
    // ...
}

// ...

setUniform2f(programId, location, v1, v2);

This approach offers more flexibility but requires managing the specific uniform setting logic.

Choosing the Right Alternative

If you're already using GLEW for managing extensions, direct OpenGL function calls might be a natural fit.
For more control or compatibility with older Qt versions, direct OpenGL function calls or a custom function might be suitable.
If you prefer a concise Qt-specific approach and are using Qt 5.15 or later, consider the QOpenGLShaderProgram methods.

Qt: Setting Uniform Variables with QOpenGLExtraFunctions::glProgramUniform2f()

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