[Interest] allocate QImage data in QSharedMemory

[Joshua Grauman]

Brilliant! How did I miss that constructor?!

Josh

> Hi,Why not construct the image object from the shared memory buffer to begin with? Then lock the sm while drawing.
> Look for example this constructor: http://doc.qt.io/qt-5/qimage.html#QImage-3
> 
> On Mon, Dec 12, 2016 at 11:45 PM, Joshua Grauman <jnfo-c at grauman.com> wrote:
>       Hello all,
>
>       I have been able to generate a screen capture of my program by rendering a window into a QImage at 30fps (called from QTimer), and then copying the QImage to shared memory
>       (QSharedMemory) where another program grabs the frames and outputs them to ffmpeg. It all works great.
>
>       However, my cpu is barely keeping up if I do much on my app.
>
>       One of the ways I thought of to speed it up is to avoid the memcpy from the QImage (img) to QSharedMemory (sharedMemory) by deriving from QImage to a new class
>       (QSharedMemoryImage) which allocates its image data in shared memory to begin with so that the window->render() function renders the window right into shared memory so the copy
>       isn't necessary. QImage was a bit too complex for me to figure out how to do this.
>
>       For you experts, is this possible? How hard would it be?
>
>       Here's the relevant code in the QTimer:
>
>         window->render(&img, QPoint(), QRegion(), QWidget::DrawChildren);
>         sharedMemory.lock();
>         char *to = (char*)sharedMemory.data();
>         const unsigned char *from = img.constBits();
>         memcpy(to, from, sharedMemory.size());
>         sharedMemory.unlock();
>
>       Josh
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> 
> 
>