[Interest] [Qt3D] QBufferCapture get buffer data from gpu to cpu

[Juan Jose Casafranca]

Hi,

I'm coding the mechanism to get the data from gpu buffers back to cpu side (
https://bugreports.qt.io/browse/QTBUG-57727) and I have different ideas on
how to do this, so I want your opinion.

The first idea is follow the QRenderCapture idea: An object in the
framegraph which creates a reply object in which the buffer content will
be. The buffer which you wanna download will be setted in the
QBufferCapture class (or maybe a vector of QBuffers) The reply object sends
a complete signals to know when the backend has finished the job. This is
easy but I think it has some drawbacks. First of all, you'll need or
several differents QBufferCapture objects in the framegraph for different
buffer (or a vector in the QBufferCapture object). I think this is a
horrible way of specifying the buffer you want to get as you can lost
control of which QBufferCapture object is downloading which buffer.

My second idea is to specify a QBufferCapture class but this class only
sets a flag (download the contents or don't download the contents). I will
add a property to the QBuffer class to specify if that specific buffer must
be downloaded or not. When the renderview has a QBufferCapture object, it
will ask if it must download the contents for that specific frame, and if
it should, each QBuffer which must be downloaded will create a job to get
the GPU data and transferred it to the cpu side. The QBuffer will be in
charge of sending a signal to notify that the content is ready. The
downloaded data will probably be stored in the QBuffer object (I still have
to think about this).

The last idea is a simplification of the second idea. The QBufferCapture
class will disappear and the renderview will consider that the user always
wants the data. The QBuffer will still have the download/not download flag
to specify if that specific buffer must be downloaded.

To be able to code this completely, I will need some help to fully
understand how Qt3D works. I'm mainly interested in synchronization stuff.
My main worry about this is what happens when a renderview has submit all
the download jobs. Is it possible that while I'm still downloading the
buffer contents, another renderview begins rendering and writes on the gpu
buffers that I'm still downloading? Or does renderviews do things in a
completely serialized manner?

Thanks!
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