How to capture viewport render at original HMD resolution

[Marcos_Cabana]

Hi there, we are developing an eye test over a vive pro, and for comparison to an existing test we need to capture exactly what viewport is rendering at the original resolution (1400x1600 per eye).

Does anybody  think about a solution or aproach to get this. Put another camera in the rig is not the solution, we need to know which pixels are the user seeing and which not. @Corvus @MariosBikos_HTC