Beyond glProgramUniform4ui(): Exploring Uniform Setting Options in Qt OpenGL

Understanding QOpenGLExtraFunctions

In Qt, QOpenGLFunctions is a class that provides access to OpenGL functions. However, for functions beyond OpenGL 3.0 (or the version supported by your graphics driver), you'll need QOpenGLExtraFunctions. It inherits from QOpenGLFunctions and offers additional functions.

glProgramUniform4ui() Function

• Parameters
  
  • programId: The ID of the shader program where the uniform variable is located (obtained using glCreateProgram).
  • location: The location of the uniform variable within the program (obtained using glGetUniformLocation).
  • v0, v1, v2, v3: The four integer values to set for the uniform variable.
• Purpose
  Sets the values of a uniform variable in a shader program.

Qt Context and Usage

#include <QtOpenGLWidgets>

// ... other code

// In a function where you have set up your OpenGL context
void setUniformValues(QOpenGLShaderProgram *program) {
    QOpenGLExtraFunctions functions;

    GLint uniformLocation = functions.glGetUniformLocation(program->programId(), "myUniform");
    if (uniformLocation != -1) {
        functions.glProgramUniform4ui(program->programId(), uniformLocation, 10, 20, 30, 40);
    } else {
        // Handle error: uniform variable not found
    }
}

In this example:

1. We include the QtOpenGLWidgets header for OpenGL functionality in Qt.
2. We obtain the programId from a QOpenGLShaderProgram object (which you would have created using Qt's shader programming features).
3. We use glGetUniformLocation to get the location of a uniform variable named "myUniform" within the shader program.
4. If the location is valid (uniformLocation != -1), we use glProgramUniform4ui to set the values for the four integer components of the uniform variable.

• Understand shader programming concepts to effectively use uniform variables.
• It's used within a Qt application's OpenGL rendering context.
• QOpenGLExtraFunctions is for advanced OpenGL functionality, not directly related to Qt GUI development.

---

#include <QApplication>
#include <QOpenGLWidget>
#include <QOpenGLShaderProgram>
#include <QMouseEvent>

const char *vertexShaderSource =
    "attribute vec4 vertexPos;\n"
    "uniform vec4 ourColor;\n"
    "void main() {\n"
    "  gl_Position = vertexPos;\n"
    "}\n";

const char *fragmentShaderSource =
    "uniform vec4 ourColor;\n"
    "void main() {\n"
    "  gl_FragColor = ourColor;\n"
    "}\n";

class OpenGLWidget : public QOpenGLWidget {
    Q_OBJECT

public:
    OpenGLWidget(QWidget *parent = nullptr) : QOpenGLWidget(parent) {}

protected:
    void initializeGL() override {
        // Initialize OpenGL context
        initializeOpenGLFunctions();

        // Create vertex shader
        QOpenGLShader *vertexShader = new QOpenGLShader(QOpenGLShader::Vertex, this);
        if (!vertexShader->compileSourceCode(vertexShaderSource)) {
            qWarning() << "Vertex shader compilation error:" << vertexShader->log();
            return;
        }

        // Create fragment shader
        QOpenGLShader *fragmentShader = new QOpenGLShader(QOpenGLShader::Fragment, this);
        if (!fragmentShader->compileSourceCode(fragmentShaderSource)) {
            qWarning() << "Fragment shader compilation error:" << fragmentShader->log();
            return;
        }

        // Create shader program
        shaderProgram = new QOpenGLShaderProgram(this);
        shaderProgram->addShader(vertexShader);
        shaderProgram->addShader(fragmentShader);
        if (!shaderProgram->link()) {
            qWarning() << "Shader program linking error:" << shaderProgram->log();
            return;
        }

        // Get uniform location
        uniformColorLocation = shaderProgram->uniformLocation("ourColor");
        if (uniformColorLocation == -1) {
            qWarning() << "Uniform 'ourColor' not found";
            return;
        }

        // Set initial color (replace with your desired values)
        currentColor[0] = 1.0f; // Red
        currentColor[1] = 0.5f; // Green
        currentColor[2] = 0.0f; // Blue
        currentColor[3] = 1.0f; // Alpha
    }

    void paintGL() override {
        // Clear screen
        glClearColor(0.0f, 0.0f, 0.0f, 1.0f);
        glClear(GL_COLOR_BUFFER_BIT);

        // Bind shader program
        shaderProgram->bind();

        // Set uniform color
        QOpenGLExtraFunctions functions(this);
        functions.glProgramUniform4f(shaderProgram->programId(), uniformColorLocation,
                                     currentColor[0], currentColor[1], currentColor[2], currentColor[3]);

        // Vertex data for a triangle
        static const float vertices[] = {
            0.5f,  0.5f, 0.0f, // Top right
            -0.5f, -0.5f, 0.0f, // Bottom left
            0.5f, -0.5f, 0.0f  // Bottom right
        };

        // Set vertex attribute pointer
        glVertexAttribPointer(0, 3, GL_FLOAT, GL_FALSE, 0, vertices);
        glEnableVertexAttribArray(0);

        // Draw triangle
        glDrawArrays(GL_TRIANGLES, 0, 3);

        // Release shader program
        shaderProgram->release();
    }

    void mousePressEvent(QMouseEvent *event) override {
        // Update color on mouse click (replace with your logic)
        currentColor[0] = (float)(event->x() % 256) / 255.0f;
        currentColor[1] = (float)(event->y() % 256) / 255.0f;
        update();
    }

private:
    QOpenGLShaderProgram *shaderProgram;
    GLint uniformColorLocation;
    float currentColor[4];
};

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

---

For setting different data types

• glProgramUniform1ui (uint)
  Sets a single unsigned integer value.
• glProgramUniform1i (int)
  Sets a single integer value for the uniform variable.
• glProgramUniform4f (vec4)
  Sets a 4D vector of floating-point values.
• glProgramUniform3f (vec3)
  Sets a 3D vector of floating-point values.
• glProgramUniform2f (vec2)
  Sets a 2D vector of floating-point values.
• glProgramUniform1f (float)
  Sets a single floating-point value for the uniform variable.

For setting arrays of data

• glProgramUniform1fv (float array)
  Sets an array of floating-point values for the uniform variable. There are similar functions for other data types with a v at the end (e.g., glProgramUniform3iv for an array of integers).

Using Qt-specific methods (might not be available for all data types)

• QOpenGLShaderProgram::setUniformValue
  This method offers a more convenient way to set uniform values of various types based on C++ data types. However, it might not support all data types as glProgramUniform functions do.

Choosing the right alternative

The best alternative depends on the specific data type you need to set for the uniform variable. Use the function that matches the data type and number of components you want to set.

Example

If you want to set a uniform variable named "myColor" to a vec3 value (red, green, blue), you could use:

float red = 1.0f, green = 0.5f, blue = 0.0f;
QOpenGLExtraFunctions functions(this);
functions.glProgramUniform3f(program->programId(), uniformColorLocation, red, green, blue);