QOpenGLWidget and transparency - qt

Question

I have small problem with QOpenGLWidget and its background color.
When I want to create semi-transparent rect on my custom QOpenGLWidget using QPainter there are 2 different results:
If MyCustomWidget have parent - on every update rect's color multiplies (and after few repaints it is opaque, like previous painting result not cleaned)
If MyCustomWidget doesn't have parent - color doesn't repaints each time
Here is code example for QPainter:
class Widget : public QOpenGLWidget
{
Q_OBJECT
public:
Widget(QWidget *parent = 0)
: QOpenGLWidget(parent)
{
resize(800, 600);
Test *test = new Test(this);
}
~Widget(){}
protected:
void paintEvent(QPaintEvent *) {}
protected:
void initializeGL() {
if(paintEngine()->type() != QPaintEngine::OpenGL &&
paintEngine()->type() != QPaintEngine::OpenGL2)
qDebug() << "ERROR. Type is: " << paintEngine()->type();
}
void resizeGL(int, int) {}
void paintGL() {
QPainter p;
p.begin(this);
{
p.fillRect(rect(), Qt::white);
}
p.end();
}
private:
class Test : public QOpenGLWidget
{
public:
Test(QWidget *parent = 0) : QOpenGLWidget(parent) {
resize(100, 100);
}
protected:
void paintEvent(QPaintEvent *) {
QPainter p(this);
p.fillRect(rect(), QColor(125, 125, 125, 255/10));
}
};
};
Also by default it has black background (I don't know how to fix it. setAttribute(Qt::WA_TranslucentBackground) doesn't helps).
Also, when I'm trying to clear color using glClear it ignores alpha (both on QOpenGLWidget with parent and not). Here is Test class from previous code, but now it is using opengl to clear color:
class Test : public QOpenGLWidget
{
public:
Test(QWidget *parent = 0) : QOpenGLWidget(parent) {
resize(100, 100);
}
void initializeGL() {
QOpenGLFunctions *f = context()->functions();
f->glClearColor(0.0f, 1.0f, 0.0f, 0.1f);
}
void paintGL() {
QOpenGLFunctions *f = context()->functions();
f->glClear(GL_COLOR_BUFFER_BIT | GL_DEPTH_BUFFER_BIT);
}
};
How can I fix this problems?
I'm using Qt 5.5.0, Windows 10, MinGW 4.9.2

Answer 1

Xeed is correct when saying the QOpenGLWidget is painted first.
I'm not an expert but I think I found the solution. You need to set a widget attribute to always make the widget stacked on top (think of the widgets as layers on the window). Here is a link to where I got the following information:
P.S. As mentioned in the QQuickWidget post, there is a limitation regarding semi-transparency when using QQuickWidget or QOpenGLWidget as child widgets. For applications that absolutely need this, Qt 5.4 offers a workaround: the newly introduced Qt::WA_AlwaysStackOnTop widget attribute. This, at the expense of breaking the stacking order for other types of layouts, makes it possible to have a semi-transparent QQuickWidget or QOpenGLWidget with other widgets visible underneath. Of course, if the intention is only to make other applications on the desktop visible underneath, then the Qt::WA_TranslucentBackground attribute is sufficient
Solution in Python:
set attribute of OpenGL widget
setAttribute(Qt.WA_AlwaysStackOnTop)
Now the OpenGL widget is considered 'on top' in the window. Use 'glClearColor' function and specify the alpha channel to be zero (0.0).
glClearColor(0.0, 0.0, 0.0, 0.0)
I'm not sure how to write that in other languages but this worked for me. The OpenGL widget no longer has the default black background. It is transparent! Hope this helps.

Answer 2

As far as I know the QOpenGLWidget is always drawn first. Therefore you cannot show any widgets layered below. I'm currently looking into the same issue. I'll report back, when I find any solution.

Answer 3

I've had similar issue with QOpenGLWidget not repainting correctly in transparent areas and decided to switch to QOpenGLWindow wrapped inside QWidget::createWindowContainer()