Another idea of how to add a latency tester into the Rift

The application sends a command via USB to the HMD to start the test (let’s call this the START_TEST command). The HMD will now store its internal timestamp and send a command back (DRAW_MARKER command). The application will change the rendering when the DRAW_MARKER command will get parsed together with the other USB signals (head rotation etc.) basically, it takes the same “route” thru the application as all the user inputs. What changes in the rending is the top-left pixel colour: Normally the renderings for the Rift are surrounded by a black border due to the distortion shader but even on HMDs that use the whole screen, changing one pixel (which should be black normally not to trigger the parsing by accident) won’t get seen by the user. So let’s say it changes from black to white caused by the final shader, the distortion shader. The HMD will parse the incoming pixel stream – at least the first pixel – for this magic color value and if it finally gets detected, it will calculate the time the HMD has waited for this to happen. Remember, it stored the internal time when it started the DRAW_MARKER command! Now it can send back the time difference in a LATENCY_RESULT command.