scrcpy/app/src/clock.h

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#ifndef SC_CLOCK_H
#define SC_CLOCK_H
#include "common.h"
#include <assert.h>
#include "util/tick.h"
#define SC_CLOCK_RANGE 32
static_assert(!(SC_CLOCK_RANGE & 1), "SC_CLOCK_RANGE must be even");
struct sc_clock_point {
sc_tick system;
sc_tick stream;
};
/**
* The clock aims to estimate the affine relation between the stream (device)
* time and the system time:
*
* f(stream) = slope * stream + offset
*
* To that end, it stores the SC_CLOCK_RANGE last clock points (the timestamps
* of a frame expressed both in stream time and system time) in a circular
* array.
*
* To estimate the slope, it splits the last SC_CLOCK_RANGE points into two
* sets of SC_CLOCK_RANGE/2 points, and compute their centroid ("average
* point"). The slope of the estimated affine function is that of the line
* passing through these two points.
*
* To estimate the offset, it computes the centroid of all the SC_CLOCK_RANGE
* points. The resulting affine function passes by this centroid.
*
* With a circular array, the rolling sums (and average) are quick to compute.
* In practice, the estimation is stable and the evolution is smooth.
*/
struct sc_clock {
// Circular array
struct sc_clock_point points[SC_CLOCK_RANGE];
// Number of points in the array (count <= SC_CLOCK_RANGE)
unsigned count;
// Index of the next point to write
unsigned head;
// Sum of the first count/2 points
struct sc_clock_point left_sum;
// Sum of the last (count+1)/2 points
struct sc_clock_point right_sum;
// Estimated slope and offset
// (computed on sc_clock_update(), used by sc_clock_to_system_time())
double slope;
sc_tick offset;
};
void
sc_clock_init(struct sc_clock *clock);
void
sc_clock_update(struct sc_clock *clock, sc_tick system, sc_tick stream);
sc_tick
sc_clock_to_system_time(struct sc_clock *clock, sc_tick stream);
#endif