QOpenGLWidget getting wrong pixel at mouse position

[Alojz Holubek]

Good day,

I am learning the QT framework through a project that uses a QTOpenGLWidget to visualize a 3D object, i would now like to allow the user to be able to pick any triangle face of the mesh, for now i would just like to highlight that face in a different color.

I decided to go with 'color picking' as my method of selection, I render a special version of the model where each triangle side has a different color, then i get the position of the mouse within the widget and read the pixel color from it, thus getting a unique ID for the selected face.

My program basically works as described, but the mouse position seems to have a weird offset and I am not sure where it is coming from, here are some examples (apologies for the color scheme, doing simple RGB for now):

In this example, i clicked on the bottom right side of the cube, but the top left side is highlighted.

Here i am clicking on the top left part, but no face is highlighted.

Finally here i clicked completely outside the boundaries of the cube, but i still get a highlighted face.

Here is the code i think is the most relevant to this issue, first the left click event handler:

void Visualizer::mousePressEvent(QMouseEvent* event)
{
    QPoint currentPos = event->pos();
    // left button: rotate the model
    if (event->button() == Qt::LeftButton) {
        isRotating = true;
        lastMousePos = event->pos();


        mouseX = currentPos.x();  //mouseX and mouseY are of type int
        mouseY = currentPos.y();
        update();

    }
    // right button: drag and move the object around
    if (event->button() == Qt::RightButton) {
        isDragging = true;
        lastMousePos = event->pos();
    }
}

Here is the paintGL function that first paints the color pick mesh before clearing the screen and painting the actual mesh, I understand that this is a 'hacky' way of implementing color picking, but since i do not plan on working with large number of faces it should suffice for my needs:

void Visualizer::paintGL()
{
    projectionMatrix.setToIdentity();
    viewMatrix.setToIdentity();
    modelMatrix.setToIdentity();

    float aspectRatio = static_cast<float>(width()) / static_cast<float>(height());
    projectionMatrix.perspective(45, aspectRatio, 0.1f, 10000.0f);

    viewMatrix.translate(0, 0, translateZ);
    viewMatrix.rotate(m_rotationAngleX, 1.0f, 0.0f, 0.0f);
    viewMatrix.rotate(m_rotationAngleY, 0.0f, 1.0f, 0.0f);
    
    modelMatrix.translate(translateX, translateY, 0.0f);
    modelMatrix.scale(1.0f / zoomLevel);

    //combine the matrices to form mvp matrix
    mvpMatrix = projectionMatrix * viewMatrix * modelMatrix;


    QMatrix4x4 invViewMatrix = viewMatrix.inverted();
    QVector3D cameraPosition = invViewMatrix.column(3).toVector3D();
    //qDebug() << cameraPosition;


    glClearColor(0.0f, 0.0f, 1.0f, 1.0f);
    glClear(GL_COLOR_BUFFER_BIT | GL_DEPTH_BUFFER_BIT);
    glEnable(GL_DEPTH_TEST);

    if (objectLoaded) {
        drawPickFrame();
        glFinish();
        glClearColor(0.0f, 0.0f, 1.0f, 1.0f);
        glClear(GL_COLOR_BUFFER_BIT | GL_DEPTH_BUFFER_BIT);
        glEnable(GL_DEPTH_TEST);
        

        shaderProgram.bind();
        shaderProgram.setUniformValue("mvpMatrix", mvpMatrix);

        
        //QVector3D cameraPosition = viewMatrix.inverted().column(3).toVector3D();
        QVector3D lightDirection = cameraPosition.normalized();
        shaderProgram.setUniformValue("lightDirection", lightDirection);

        glBindVertexArray(vao);
        glBindBuffer(GL_ELEMENT_ARRAY_BUFFER, indexBuffer);
        glFinish();
        for (int i = 0; i < mesh.indices.size(); i += 3) {
            
            QVector3D v0 = mesh.vertices[mesh.indices[i]].normalized();
            QVector3D v1 = mesh.vertices[mesh.indices[i + 1]].normalized();
            QVector3D v2 = mesh.vertices[mesh.indices[i + 2]].normalized();

            QVector3D edge1 = (v1 - v0).normalized();
            QVector3D edge2 = (v2 - v0).normalized();
            QVector3D faceNormal = QVector3D::crossProduct(edge1, edge2).normalized();
            float shading = qMax(0.0f, QVector3D::dotProduct(faceNormal, lightDirection ));

            int isSelected = 0;
            if (i / 3 == selectedFace) {
                isSelected = 1;


			}

            shaderProgram.setUniformValue("selectedFace", isSelected);

            //qDebug() << shading;
            shaderProgram.setUniformValue("shadingValue", shading);
            glDrawElements(GL_TRIANGLES, 3, GL_UNSIGNED_INT, reinterpret_cast<void*>(sizeof(GLuint) * i));
        }
    }
    shaderProgram.release();
    glBindVertexArray(0);
    //update();
}

Finally, here is the color picking frame function, like i said it seems to be working correctly, it's just the actual pixel location seems to be offsetted by some values...

void Visualizer::drawPickFrame() {
    glClearColor(1.0f, 1.0f, 1.0f, 1.0f);
    glClear(GL_COLOR_BUFFER_BIT | GL_DEPTH_BUFFER_BIT);
    glEnable(GL_DEPTH_TEST);

    for (size_t i = 0; i < mesh.indices.size(); i += 3) {
        GLuint id = i / 3; // ID corresponding to the triangle
        int red = (id >> 16) & 0xFF;
        int green = (id >> 8) & 0xFF;
        int blue = id & 0xFF;
        //qDebug() << "ID: " << id;
        QColor color(red, green, blue);

        QVector3D vertex1 = mesh.vertices[mesh.indices[i]];
        QVector3D vertex2 = mesh.vertices[mesh.indices[i + 1]];
        QVector3D vertex3 = mesh.vertices[mesh.indices[i + 2]];

        vertex1 = mvpMatrix.map(vertex1);
        vertex2 = mvpMatrix.map(vertex2);
        vertex3 = mvpMatrix.map(vertex3);


        float r = color.redF();
        float g = color.greenF();
        float b = color.blueF();


        glBegin(GL_TRIANGLES);
        glColor3f(r, g, b);
        glVertex3f(vertex1.x(), vertex1.y(), vertex1.z());
        glVertex3f(vertex2.x(), vertex2.y(), vertex2.z());
        glVertex3f(vertex3.x(), vertex3.y(), vertex3.z());
        glEnd();
    }
    glFinish();
    unsigned char pixel[3];
    glReadPixels(mouseX, height() - mouseY, 1, 1, GL_RGB, GL_UNSIGNED_BYTE, &pixel);


    //qDebug() << mouseX << mouseY;

    GLuint id = (pixel[0] << 16) | (pixel[1] << 8) | pixel[2];
    qDebug() << "within ID: " << id;
    selectedFace = id;
    glFinish();
}

Finally i add my initializeGL and resizeGL functions, although i don't think the problem will lie here:

void Visualizer::initializeGL()
{
    initializeOpenGLFunctions();

    glEnable(GL_DEPTH_TEST);
    glClearColor(0.0f, 0.0f, 1.0f, 1.0f); // Set the clear color to blue
    
   
    // Create and bind the VAO, VBO 
    glGenVertexArrays(1, &vao);
    glBindVertexArray(vao);

    glGenBuffers(1, &vbo);
    glBindBuffer(GL_ARRAY_BUFFER, vbo);

    // Set up the vertex buffer data with a null buffer
    glBufferData(GL_ARRAY_BUFFER, 0, nullptr, GL_STATIC_DRAW);

    glGenBuffers(1, &indexBuffer);
    glBindBuffer(GL_ELEMENT_ARRAY_BUFFER, indexBuffer);
    // Set up the index buffer data with a null buffer
    glBufferData(GL_ELEMENT_ARRAY_BUFFER, 0, nullptr, GL_STATIC_DRAW);
    // Specify the format of the vertex data (position in this case)
    glVertexAttribPointer(0, 3, GL_FLOAT, GL_FALSE, sizeof(QVector3D), nullptr);
    glEnableVertexAttribArray(0);



    // Create and bind the  buffer
    glGenBuffers(1, &shadingBuffer);
    glBindBuffer(GL_ARRAY_BUFFER, shadingBuffer);
    glBufferData(GL_ARRAY_BUFFER, 0, nullptr, GL_STATIC_DRAW);
    glVertexAttribPointer(1, 3, GL_FLOAT, GL_FALSE, sizeof(QVector3D), nullptr);
    glEnableVertexAttribArray(1);

    glBindVertexArray(0);

    // Load the shader program
    shaderProgram.addShaderFromSourceFile(QOpenGLShader::Vertex, "vertex_shader.glsl");
    shaderProgram.addShaderFromSourceFile(QOpenGLShader::Fragment, "fragment_shader.glsl");
    //shaderProgram.addShaderFromSourceFile(QOpenGLShader::Geometry, "geometry_shader.glsl");
    shaderProgram.link();

    // Set the default matrices
    viewMatrix.setToIdentity();
    projectionMatrix.setToIdentity();
    modelMatrix.setToIdentity();
    float aspectRatio = static_cast<float>(width()) / static_cast<float>(height());
    projectionMatrix.perspective(45.0f, aspectRatio, 0.1f, 10000.0f); // fov?
}

/*
*   Function: inherited function that is used when the user resizes the application
*/
void Visualizer::resizeGL(int w, int h)
{
    glViewport(0, 0, w, h);

    projectionMatrix.setToIdentity();
    float aspectRatio = static_cast<float>(width()) / static_cast<float>(height());
    projectionMatrix.perspective(45.0f, aspectRatio, 0.1f, 10000.0f);
}

Any help would be much appreciated, thank you.

[Alojz Holubek]

Dug around other similar topics and found a solution, posting it here in case anyone has a similar problem to mine. The solution is in this thread

It seems this is a 'problem' with high DPI displays in particular. I changed the way the program gets the X and Y coordinates from the mouse by taking into account the devicePixelRatio() as outlined in the linked thread, which solved the offsetting.