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Latencytool

This is/will be a set of programs that can be used to measure the round-trip latency between application, screen, and camera.

To use, attach USB camera, and run latency_cv_qt N where N is the corresponding /dev/videoN device. This should open a window that is either white or black; moving the camera directly in front of the window should start rapidly switching the window color to the opposite of whatever the camera sees. stdout will eventually print the average time for a color switch operation -- this is the round-trip latency. Tuning constants in backend_opencv.cpp may be helpful. The V4L backend, on the other hand, will not work correctly unless you adjust constants to match your camera.

It is recommended to test with as small a window as feasible, both to reduce computational overhead, and because staring at large blinking lights of 10-12 Hz for the several minutes needed to get a stable measurement will give you a headache.

Status

An OpenCV and a V4L backend have been written. Frontends are available for terminal output, xcb, Wayland, /dev/fb0, and Qt.

Uses

Given a camera with a reasonably high framerate and known latency, one can estimate the total delay between application rendering and the time the screen update completes.

For example, with a PS3 Eye at 187 Hz, and a 60 Hz laptop monitor from 2015, we can compare the amounts of lag introduced by various display server combinations. With latency_cv_qt:

  • X11, no compositing, small window: 25ms average round trip
  • X11, no compositing, large window: 30ms average round trip
  • kwin_wayland, small window: 45ms average round trip
  • sway, small window: 47ms average round trip
  • sway, small window, nested under X11: 43ms average round trip
  • weston, small window: 41ms average round trip

With latency_cv_xcb on X11 without compositing, the average round trip time is reduced to 24ms for small windows, and 25ms for large windows. It also reduces the average round trip time for sway to 44ms, and weston to 41ms.

With latency_cv_wayland on sway, as a small window, 44ms average round trip time is observed.

With latency_cv_fb, 25ms average round trip time is observed.

Installation

Current requirements are:

  • GNU make
  • pkgconfig
  • opencv (tested with 4.0.1)
  • Qt5 (tested with 5.12)
  • libxcb (tested with 1.13.1)
  • wayland (tested with 1.16.0)
  • wayland-protocols (tested with 1.17.1)
  • V4L (as preferred opencv backend)
  • Linux (for the framebuffer frontend, and the V4L backend)

To compile, run make.

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Measure round trip latency from application to screen to camera to application

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