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//! The display's refresh period, as seen from the vsync callbacks a frame
//! loop asks for.
/// The refresh periods a display can plausibly have: 250 Hz down to 20 Hz.
const MIN_VSYNC_PERIOD_NS: i64 = 4_000_000;
const MAX_VSYNC_PERIOD_NS: i64 = 50_000_000;
/// Folds the gap between two vsync callbacks into the refresh-period
/// estimate `period_ns` (`0` while none is known) and returns the new one.
///
/// Callbacks come only for the vsyncs the loop asked for, so a gap can span
/// several vsyncs, and a loop paced on a period twice too long would run at
/// half rate. Callback timestamps are the vsyncs' own, so a gap that is a
/// whole number of periods is taken as that many vsyncs; only a gap that is
/// not, from a display that changed rate, replaces the estimate.
pub(crate) fn refine_vsync_period(period_ns: i64, gap_ns: i64) -> i64 {
if period_ns > 0 {
let vsyncs = ((gap_ns + period_ns / 2) / period_ns).max(1);
if (gap_ns - vsyncs * period_ns).abs() <= period_ns / 20 {
return gap_ns / vsyncs;
}
}
if (MIN_VSYNC_PERIOD_NS..=MAX_VSYNC_PERIOD_NS).contains(&gap_ns) {
gap_ns
} else {
period_ns
}
}
/// The first vsync after `at_ns` on the grid of the vsync at `vsync_ns`,
/// `period_ns` apart; `None` without a vsync or a period.
pub(crate) fn next_vsync_ns(at_ns: i64, vsync_ns: i64, period_ns: i64) -> Option<i64> {
if vsync_ns <= 0 || period_ns <= 0 {
return None;
}
Some(at_ns + period_ns - (at_ns - vsync_ns).rem_euclid(period_ns))
}
#[cfg(test)]
#[path = "tests/vsync_period_tests.rs"]
mod tests;