[Interest] QDirectFbWindow: performance issues due to alphachannel enabled

[Boettger, Heiko]

Hi,

we have a very slow arm v5 device without hardware acceleration. When using QT5 and directfb we see a huge performance difference compared to qt4. After looking deeper into the code to find the reason for this, I saw that the QDirectFbWindow::createDirectFbWindow method always sets the capability doublebuffered and alpha channel. As a result whenever something is painted on the window the content of update rect is blit using alpha blending:

   description.options = DFBWindowOptions(DWOP_ALPHACHANNEL);
        description.caps = DFBWindowCapabilities(DWCAPS_DOUBLEBUFFER|DWCAPS_ALPHACHANNEL);

Since the window is opaque I don't expect any transparency to be kept and a simple memcpy which Is fast enough on the device would be okay. I already tested this by modifying the code, but I am not sure what's the best way to fix this.

Is there another way to disable the use of alpha blending for the back buffer using a DEFINE or setting attribute somewhere?

Best Regards
Heiko

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