QPainter changes the rendering of OpenGLWidget

[BlondXXX]

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

[BlondXXX]

The solution of the problem was, that after using QPainter the OPenGL state was lost, so all the glEnable() were ignored and therefore the depth test as well, so I put again all the glEnable calls after QPainter call and it works well.

[SGaist]

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