Translucent Qt widget

I am developing an application for Embedded Linux using Qt 4.6.2 where we need to display video along with UI. UI may have a hole along with other UI objects on screen where user can see the video displayed in background.

I apply following attributes to every widget in my application:

Code:

---

widget->setAttribute(Qt::WA_NoMousePropagation);
    widget->setAutoFillBackground(true);
    //widget->setAttribute(Qt::WA_OpaquePaintEvent);
    widget->setAttribute(Qt::WA_PaintOnScreen);

---

I use the following method to create a hole in UI at required place:

Code:

---

void DVRGlobal::createHole(QWidget *widget)
{
  if(widget==NULL)
    return;
  QWidget *parentWidget = widget;
  //Find out parent most widget
  while(parentWidget->parentWidget()!=NULL)
    parentWidget = parentWidget->parentWidget();
 
    parentWidget->setAttribute(Qt::WA_TranslucentBackground);
    if(parentWidget!=widget)
      widget->setStyleSheet("background:transparent");
}

---

This works with following two problems:
(1) Translucent widget does not repaint itself and I only see the background until I hide/show the complete widget, and it is very slow too.
(2) Any child widget on translucent widget does not repaint itself. i.e. when tool tip disappears on any widget, the background area is not repainted and remains black.

I think for playing video with overlay at a proper framerare you have to do some more tricks like having a graphics chip with two framebuffers: Decode video in one framebuffer, have the Qt ui in the other and let the graphics chip do the compositing.

I finally got it working with help of attributes like WA_PaintOnScreen and setAutofillBackground(true), thereafter I define a transparent color key in my system and fill widget with the same. It works very nice and fast.

Can you share the final code ?
It might be useful for others out here..

sure, I have following code for this purpose:

Code:

---

void Global::applyAttributes(QWidget *widget)
{
    if(widget==NULL)
        return;
 
    widget->setAttribute(Qt::WA_NoMousePropagation);
    widget->setAutoFillBackground(true);
    //widget->setAttribute(Qt::WA_OpaquePaintEvent);
    widget->setAttribute(Qt::WA_PaintOnScreen);
 
    foreach(QObject *child,widget->children())
    {
        if(child->isWidgetType())
            applyAttributes((QWidget*)child);
    }
}

---

I call applyAttributes method on parentMost widget in my application and thus every widget is set with WA_PaintOnScreen and autoFillBackground attributes.

Code:

---

void lobal::createHole(QWidget *widget)
{
  if(widget==NULL)
    return;
 
    widget->setBackgroundRole(QPalette::Window);
    QPalette transparentPal;
    transparentPal.setColor(QPalette::Window, QColor(128,64,128));
    widget->setPalette(transparentPal);
}

---

I call createHole() on the widget which needs to be transparent to show video. Above mentioned color key (128,64,128) can be any color which is our internal fbset color to make a transparent layer.

A few questions comments about this

1 - I don't think the recursive applyAttributes is needed. If you set a solid background color for your widget and use that as a chroma key (transparent value) and set setAutoFillBackground(true); for just the one widget you want the hole in you should get the same results. You could even just fillRect of that chroma key into the widget as well.

2 - There are a few problems with this common approach
a - The chroma key you choose, no matter what it is, will be found someplace else on the UI if you have other images/gradients/etc. which will then show up funny once you set the key.
b - There is no way to alpha-composite on top of the widget, i.e. any rendering controls, etc. which are children of the widget are alpha-blended to the widget's background, but then that isn't alpha-blended to the video showing in the background video plane, so it shows up solid.

Question is: How do you solve those problems!

correct omnilala,

for the same reasons I am not using the approach you mentioned and using the one which I mentioned.

transparentPal.setColor(QPalette::Window, QColor(128,64,128)); <-- when you have this color code, how can you tell your program to make this color translucent?

That is a frame buffer set color key.