I have an application written in C using GTK (although the language is probably unimportant for this question).
This application has a fullscreengtk_window with a single gtk_drawing_area. For the drawing area, I have registered a tick callback via gtk_widget_add_tick_callback which just calls gtk_widget_queue_draw every tick. Inside the drawing area draw callback, I change the color of the entire window at regular intervals (e.g., from black to white at 1Hz).
Say that in this call to the draw callback I want to change the window from black to white. I would like to know the precise time (down to the nearest ms) that the change is actually drawn on the screen (ideally in the same units as CLOCK_MONOTONIC). I don't think this is the same thing as the GdkFrameClock available in the tick callback, which, as I understand it, is about the time of the frame, not the time when the frame is actually displayed on the screen.
If I just measure the CLOCK_MONOTONIC time in the drawing callback, and then use a photo-diode to measure when the actual change is via an attached A2D, the actual change is the display is understandably delayed by a number of refresh intervals (in my case, 3 screen refreshes).
Just as a summary: if I am in a GTK widget draw callback, is there any way to know the time when the display will actually be shown on the monitor in the units of CLOCK_MONOTONIC? Or alternatively, is there a way that I can block a separate thread until a specific redraw that I care about is actually displayed on the screen (a function I can write like wait_for_screen_flip())?
Update: Ideally, the same solution would work for any Linux compositor (X11 or Wayland), which is why I am hoping for a GTK/GDK solution, where the compositor is abstracted away.
I just came across https://developer.gnome.org/gdk3/stable/gdk3-GdkFrameTimings.html#gdk-frame-timings-get-presentation-time which seems to do just like what you want and is part of Gdk. I do not know how to use it nor have I seen some example of it, but https://developer.gnome.org/gdk3/stable/gdk3-GdkFrameTimings.html#gdk3-GdkFrameTimings.description says
The information in GdkFrameTimings is useful for precise synchronization of video with the event or audio streams, and for measuring quality metrics for the application’s display, such as latency and jitter.