[Interest] redirecting Qt pixel data to designated buffer

Selmeci, Tamás tselmeci at open-st.eu
Tue Jan 6 08:14:58 CET 2015

Hello all!

I’m a beginner when it comes to Qt and being stuck for a while with this

What I want to do is to force Qt-5.3.2 to render all widgets’ graphics
into the buffer I provide. Would be even nicer if I could specify the
pixel format too.

When I was adding QWebPage to a program I could make QWebPage render the
web page into a buffer by specifying my custom QPainter and passing it
somehow like this:


But now I need to redirect all pixel operations to my buffer. In other
words, I need offscreen rendering.

My program is executing a QML script (QQmlApplicationEngine) and
implements a GUI. I have to have the GUI graphics in the buffer, so that
I can put it into anywhere. Qt’s graphics is going to be copied into an
SDL surface. The final picture in the SDL window is composed of multiple
SDL surfaces (have to emulate /dev/fb0 and /dev/fb1 [overlay] by SDL).
At this stage of development neither 2D-acceleration, nor OpenGL must be
used. What I only want is Qt copy the pixel data into my buffer (and
notify me after the buffer was updated).

I’m aware that there are multiple platform plugins, including
‘offscreen’ as well. However I have no clue how to tell ‘offscreen’ my
buffer in which it must work.

I’ve also thought of implementing my own platform plugin, but I don’t
want to begin the implementation until someone says it’s the best
solution. I’m very unfamiliar with Qt, so I don’t know which aspects to
take care of when implementing a new platform plugin. ‘minimal’ seemed
as something useful, but there are articles on the net that ‘minimal’ is
not supported for production. Perhaps it could be changed to my needs
easily – by the way, my app fails to work with ‘minimal’ plugin (xcb
works well).

Is there any way I can make my QApplication or QQmlApplicationEngine
render graphics into my buffer instead of the screen? Of course without
OpenGL, EGL and other fancy stuff. Just the with the good old slow
software rendering/blitting.


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