[Interest] Qt 5.5 LinuxFb Plugin with Base Layer Transparency

Phillip Class phillip.d.class at gmail.com
Mon Apr 4 13:53:23 CEST 2016


Our application requires that we blend 3 layers with 3 framebuffers on a TI
device. The base layer is opaque video, the middle layer is video blended
on top of the base layer with a global alpha channel that we need to be
able to modify on the fly, and a top GUI layer with transparency. This
approach worked great with Qt 4.8 and we are trying to get it to work with
Qt 5.5.

The problem is that when we run the top level GUI application, we cannot
set the background of the to be fully transparent. It simply appears black.
This covers the 2 lower z-order applications and they are no longer visible.

We attempted to investigate this issue by looking at the Qt5.5 linuxfb
plugin source code. Beginning at line 251 of qfbscreen.cpp in the Qt5.5
source ( located in qt5base-5.5.1/src/platformsupport/fbconvenience ), we
see where the linuxfb plugin sets the base layer to black. If we change
that one line to "qt::transparent", and re-launch our app, then we see the
lower 2 layers "through" the GUI in the regions of the GUI where it is
fully transparent. Initially, the fully transparent GUI displays correctly.
It displays a menu, which initially appears correct. A problem occurs when
we then try to hide the menu. It remains displayed on the screen.
Apparently, the redraw of the base layer, which is transparent, does not
replace the pixels from the menu, but is instead blended with the menu

We have tried several composition modes in the base layer's paint event,
including Source and Clear, but neither fixes the problem. On previous
products ( using the DM3730 ), we used this same approach ( DSS blended
framebuffers ) with Qt 4.8 and it worked as expected. How can we make this
work with Qt 5.5?


        // we only expect one rectangle, but defensive coding...
        foreach (const QRect &rect, intersect.rects()) {
            bool firstLayer = true;
            if (layer == -1) {
                mCompositePainter->fillRect(rect, Qt::black);
                firstLayer = false;
                layer = mWindowStack.size() - 1;

            for (int layerIndex = layer; layerIndex != -1; layerIndex--) {
                if (!mWindowStack[layerIndex]->window()->isVisible())
                // if (mWindowStack[layerIndex]->isMinimized())
                //     continue;

                QRect windowRect =
                QRect windowIntersect = rect.translated(-windowRect.left(),

                QFbBackingStore *backingStore =

                if (backingStore) {
backingStore->image(), windowIntersect);
                if (firstLayer) {
                    firstLayer = false;

We greatly appreciate any help - thank you.
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