[Interest] redirecting Qt pixel data to designated buffer

[Rutledge Shawn]

On 6 Jan 2015, at 08:14, Selmeci, Tamás <tselmeci at open-st.eu> wrote:

> Hello all!
> 
> I’m a beginner when it comes to Qt and being stuck for a while with this
> issue.
> 
> 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:
> 
> 
> mainFrame()->documentElement().render(&myPainter);
> 
> 
> 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. 

QtQuick 2 items have no built-in support for rendering via QPainter, so it normally doesn’t work without OpenGL.  One way to get back an image after the rendering is done is QQuickItem::​grabToImage().  There is a commercial add-on to use the raster engine for rendering though, in case you really don’t have an OpenGL-capable GPU: http://doc.qt.io/QtQuick2DRenderer/index.html

QtQuick 1 items are rendered with QPainter.  QtQuick 1 is likely to be dropped eventually though.