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Hello everyone! I have encountered a strange problem with a QOpenGLWidget when trying to paint some rectangle box with text over it with QPainter. I have the following code to draw the opengl geometry:

void GLWidgetGeometry::initializeGL()

initializeOpenGLFunctions();
glClearColor(0.0, 0.0, 0.0, 1.0);
// connect shader to the source files
if (!shaderProgram_.addShaderFromSourceFile(QOpenGLShader::Vertex, ":/vshader.glsl"))
    close();
if (!shaderProgram_.addShaderFromSourceFile(QOpenGLShader::Fragment, ":/fshader.glsl"))
    close();
if (!shaderProgram_.link())
    close();
glEnable(GL_DEPTH_TEST); // important to keep for realistic output of rays on the foreground of the texture
//glDisable(GL_DEPTH_TEST);
// to see transparent back faces
glDisable(GL_CULL_FACE);
//glShadeModel(GL_SMOOTH); // not available in windows qt version
//glHint(GL_PERSPECTIVE_CORRECTION_HINT, GL_NICEST);
// transparency
glEnable(GL_BLEND);
// blending of alpha-colors of surfaces in the system with the background
glBlendFunc(GL_SRC_ALPHA, GL_ONE_MINUS_SRC_ALPHA);
//glBlendFunc(GL_ONE, GL_ONE);
// initial position
rotation_ = QQuaternion::fromAxisAndAngle(QVector3D(1, 0, 0), -90) * rotation_;
rotation_ = QQuaternion::fromAxisAndAngle(QVector3D(0, 1, 0), 30) * rotation_;
void GLWidgetGeometry::paintGL()

glClear(GL_COLOR_BUFFER_BIT | GL_DEPTH_BUFFER_BIT);

// Calculate model view transformation (camera moves, but not object itself)
QMatrix4x4 matrix, viewMatrix;
viewMatrix.lookAt(QVector3D(0, 0, 1.0), QVector3D(0, 0, 0), QVector3D(0, 1, 0));
viewMatrix.translate(translation_);
viewMatrix.rotate(rotation_);
// in the future use scaling factor provided by ray tracer
viewMatrix(2, 3) -= 5.0f * exp(distanceExp_ / 1200.0f); // scaling the view
//double scaling = rayTracer_->scalingFactor();
viewMatrix.scale(rayTracer_->scalingFactor());
shaderProgram_.bind();
// Set modelview-projection matrix
shaderProgram_.setUniformValue("mvp_matrix", projection_*viewMatrix*matrix);
shaderProgram_.setUniformValue("mv_matrix", viewMatrix*matrix);
shaderProgram_.setUniformValue("normal_matrix", (viewMatrix*matrix).normalMatrix());
shaderProgram_.setUniformValue("lightPosition", QMatrix4x4((viewMatrix*matrix).normalMatrix())*QVector3D(1.0, 1.0, 1.0).normalized());
shaderProgram_.setUniformValue("lightIntensity", QVector3D(5.0, 5.0, 5.0));
rayTracer_->draw(shaderProgram_);
shaderProgram_.release();
drawLegend(title_) ; // without this everything works perfect!
So when I am just draw geometry with opengl and shader call everything looks great, once I call drawLegend function with QPainter everything becomes ugly.
void GLWidgetGeometry::drawLegend(const QString& text)

QPainter painter; // to draw a legend
painter.begin(this);
painter.setRenderHint(QPainter::TextAntialiasing);
QFont font = painter.font(); // enlarged the text
font.setPixelSize(14);
font.setBold(true);
painter.setFont(font);
QFontMetrics metrics = QFontMetrics(font);
int border = qMax(4, metrics.leading());
QRect rect = metrics.boundingRect(0, 0, width() - 2*border, height()/4,
                                  Qt::AlignLeft | Qt::TextWordWrap, text);
painter.setRenderHint(QPainter::TextAntialiasing);
QPen pen = painter.pen();
pen.setWidth(2);
pen.setColor(QColor(255, 255, 255, 200));
painter.setPen(pen);
QRect rectText(0, 0, rect.width() + 4*border, rect.height() + 4*border);
painter.setBrush(QColor(38, 181, 242, 120));
painter.drawRect(rectText);
painter.drawText(rectText.x() + 2*border, rectText.y(), rectText.width(), rectText.height(),
                 Qt::AlignLeft | Qt::AlignVCenter | Qt::TextWordWrap, text);
painter.end();
Picture before adding QPainter draw

Picture after adding QPainter draw

Hi and welcome to devnet,
IIRC when you want to mix both, you need to enclose your OpenGL primitives call between painter-> beginNativePainting() and painter->endNativePainting()
Hope it helps