QRenderCapture Always Shows Previous Frame

The QRenderCapture functionality appears to always wait until frame N+1 is being rendered before sending the images from frame N. This leads to erroneous behavior e.g. when attempting to display the results of replies within a UI. It can also lead to bugs if slots are waiting for a QRenderCaptureReply's completed() signal. These issues are most pronounced when the QRenderSettings' RenderPolicy is set to OnDemand, as this prevents successive frames from being immediately generated.

Attached is a project demonstrating the issue. It creates a QWindow, which is used as a surface for a Qt3D framegraph. The rendering is done in OnDemand mode and simply fills the surface with blue. A QRenderCapture provides a reply for the first frame, which generates an image when it is completed. The image is never created because there is never a second frame rendered.

The issue appears to be the result of having SendRenderCaptureJob execute at the beginning of a frame instead of the end. The captured images are all successfully created by the end of frame N, but the frontend is not notified until the jobs for frame N+1 are generated and run.