support of non OpenGL platforms

[uwerat]

The implementation has one dependency on OpenGL - qt_gl_read_framebuffer ( = glReadPixels ) - for grabbing the current frame.
However with Qt6 the RHI ( Qt Rendering Hardware Interface ) abstraction layer has been introduced that might run on Direct3D or Metal backends.

As this project is for applications for the EGLFS platform we only have to take care of the OpenGL case, but the qt_gl_read_framebuffer call is all what prevents the server from being usable with other platforms. So it might be worth to find out out how to grab the frame buffer there.

[net147]

QQuickItem::grabToImage() can render offscreen asynchronously to QImage. Perhaps casting QWindow to QQuickWindow and accessing QQuickWindow::contentItem() to get the QQuickItem?

[uwerat]

The meat behind QQuickItem::grabToImage() can be found in QSGRhiLayer::toImage, what ends up at QRhiResourceUpdateBatch::readBackTexture. Seems to be a public but undocumented API - let's see how usable the QRhi abstraction is in this area.

In the meantime the vnc-eglfs code has been changed ( -> vajpeg branch ) so that the OpenGL back buffer is only copied to a texture on the scene graph thread and the application is not blocked by the VNC related postprocessing anymore. So there is more OpenGL related code now that has to be reimplemented using QRhiTexture and friends ( see https://github.com/uwerat/vnc-eglfs/blob/vajpeg/src/VncFrameGrabber.cpp )

However certain optional paths will probably remain being platform specific. F.e feeding the JPG encoder of the GPU without an additional roundtrip to the CPU requires using EGL/DMA-BUF calls. ( See https://github.com/uwerat/vnc-eglfs/blob/vajpeg/src/va/VncEgl.cpp ). It should be possible to find a corresponding implementation for Vulkan/DMA-BUF but I would be surprised to find QRhi code that hides these differences.