Multiple Viewports and resize in QOpenGLWidget

[ABloom]

Hi there,
I have a problem drawing multiple viewports on a QOpenGLWidget. Everything seems to work fine until i resize the window (after that the second viewport content disappears forever). Testbed: QT Examples --> Qt-5.4 --> Opengl --> cube
changes to code:

cpp:

void MainWidget::resizeGL(int w, int h)
{
    this->w = w;
    this->h = h;

    // Calculate aspect ratio
    qreal aspect = qreal(w) / qreal(h ? h : 1);

    // Set near plane to 3.0, far plane to 7.0, field of view 45 degrees
    const qreal zNear = 3.0, zFar = 7.0, fov = 45.0;

    // Reset projection
    projection.setToIdentity();

    // Set perspective projection
    projection.perspective(fov, aspect, zNear, zFar);
    cam_projection.perspective(fov, 1.0f, zNear, zFar);
}
//! [5]

void MainWidget::paintGL()
{
    // Clear color and depth buffer
    glClear(GL_COLOR_BUFFER_BIT | GL_DEPTH_BUFFER_BIT);

    texture->bind();

//! [6]
    // Calculate model view transformation
    QMatrix4x4 matrix;
    matrix.translate(0.0, 0.0, -5.0);
    matrix.rotate(rotation);

    // Set modelview-projection matrix
    program.setUniformValue("mvp_matrix", projection * matrix);
//! [6]

    // Use texture unit 0 which contains cube.png
    program.setUniformValue("texture", 0);

    // Draw cube geometry
    geometries->drawCubeGeometry(&program);

    glViewport(0,0,100,100);

    // Set modelview-projection matrix
    program.setUniformValue("mvp_matrix", cam_projection * matrix);
    geometries->drawCubeGeometry(&program);

    glViewport(0,0,w,h);
}

add in h:

    // new
    int w,h;
    QMatrix4x4 cam_projection;

Any Help appreciated,

Regards,
Andrea

[ABloom]

Update:

tried with qt 5.5, same behaviour.

[Chris Kawa]

QMatrix4x4::perspective multiplies the existing matrix by another. In the resetGL() method the original projection matrix is set to identity before perspective is applied, but you're not doing that for your cam_projection, so it gets cumulatively more and more distorted on every resize. Simply adding cam_projection.setToIdentity(); before applying perspective fixes the issue.

[ABloom]

Thanks Chris. That solves the problem.