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clutter/frame-clock: Simplify next_presentation_time_us calculation

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When more than one refresh interval has passed since
last_presentation_time_us.

I honestly can't tell if the previous calculation was correct or not,
but I'm confident the new one is, and it's simpler.

v2:
* ASCII art diagram didn't make sense anymore, try to improve
  (Ivan Molodetskikh)

Part-of: <https://gitlab.gnome.org/GNOME/mutter/-/merge_requests/3330>
This commit is contained in:
Michel Dänzer 2023-08-07 18:48:42 +02:00 committed by Robert Mader
parent c58312e249
commit 9f8283a95a
1 changed files with 15 additions and 34 deletions
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49
clutter/clutter/clutter-frame-clock.c
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@ -511,48 +511,29 @@ calculate_next_update_time_us (ClutterFrameClock *frame_clock,
*/
if (next_presentation_time_us < now_us)
{
int64_t presentation_phase_us;
int64_t current_phase_us;
int64_t current_refresh_interval_start_us;
/*
* Let's say we're just past next_presentation_time_us.
*
* First, we compute presentation_phase_us. Real presentation times don't
* have to be exact multiples of refresh_interval_us and
* presentation_phase_us represents this difference. Next, we compute
* current phase and the refresh interval start corresponding to now_us.
* Finally, add presentation_phase_us and a refresh interval to get the
* next presentation after now_us.
*
* last_presentation_time_us
* / next_presentation_time_us
* / / now_us
* / / / new next_presentation_time_us
* |--|-------|---o---|-------|--> presentation times
* | __|
* | |presentation_phase_us
* | |
* | | now_us - presentation_phase_us
* | | /
* |-------|---o---|-------|-----> integer multiples of refresh_interval_us
* | \__/
* | |current_phase_us
* | \
* | current_refresh_interval_start_us
* 0
* First, we calculate current_phase_us, corresponding to the time since
* the last integer multiple of the refresh interval passed after the last
* presentation time. Subtracting this phase from now_us and adding a
* refresh interval gets us the next possible presentation time after
* now_us.
*
* last_presentation_time_us
* / next_presentation_time_us
* / / now_us
* / / / new next_presentation_time_us
* |-------|---o---|-------|--> possible presentation times
* \_/ \_____/
* / \
* current_phase_us refresh_interval_us
*/
presentation_phase_us = last_presentation_time_us % refresh_interval_us;
current_phase_us = (now_us - presentation_phase_us) % refresh_interval_us;
current_refresh_interval_start_us =
now_us - presentation_phase_us - current_phase_us;
next_presentation_time_us =
current_refresh_interval_start_us +
presentation_phase_us +
refresh_interval_us;
current_phase_us = (now_us - last_presentation_time_us) % refresh_interval_us;
next_presentation_time_us = now_us - current_phase_us + refresh_interval_us;
}
if (frame_clock->is_next_presentation_time_valid)