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Derive timer interval in the momentum scroller from previous scroll events

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Kovid Goyal
2026-01-09 10:40:24 +05:30
parent 3637e31ca3
commit 48de08ded1
1 changed files with 30 additions and 8 deletions
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38
glfw/momentum-scroll.c vendored
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@@ -24,22 +24,29 @@ typedef struct MomentumScroller {
min_velocity, // Minimum velocity before stopping
max_velocity, // Maximum velocity to prevent runaway scrolling
velocity_scale; // Scale factor for initial velocity
unsigned timer_interval_ms;
GLFWid timer_id, window_id;
ScrollSamples samples;
ScrollerState state;
struct { double x, y; } velocity;
int keyboard_modifiers;
monotonic_t timer_interval;
} MomentumScroller;
static MomentumScroller s = {
.friction = 0.04,
static const MomentumScroller defaults = {
.friction = 0.035,
.min_velocity = 0.5,
.max_velocity = 100,
.velocity_scale = 0.9,
.timer_interval_ms = 10,
};
static MomentumScroller s = defaults;
GLFWAPI void
glfwConfigureMomentumScroller(double friction, double min_velocity, double max_velocity, double velocity_scale) {
#define S(w) s.w = w >= 0 ? w : defaults.w
S(friction); S(min_velocity); S(max_velocity); S(velocity_scale);
#undef S
}
static void
cancel_existing_scroll(void) {
@@ -89,6 +96,7 @@ add_velocity(double x, double y) {
static void
set_velocity_from_samples(void) {
s.timer_interval = ms_to_monotonic_t(8);
trim_old_samples(monotonic());
ScrollSample ss;
switch (deque_size(&s.samples)) {
@@ -105,15 +113,29 @@ set_velocity_from_samples(void) {
monotonic_t first_time = deque_peek_front(&s.samples)->timestamp;
monotonic_t last_time = deque_peek_back(&s.samples)->timestamp;
double time_span = MAX(1, last_time - first_time);
double total_time_between_events = 0;
monotonic_t prev_timestamp = 0;
double ts_weight = 0;
for (size_t i = 0; i < deque_size(&s.samples); i++) {
const ScrollSample *ss = deque_at(&s.samples, i);
double weight = 1.0 + (ss->timestamp - first_time) / time_span;
total_dx += ss->dx * weight; total_dy += ss->dy * weight;
total_weight += weight;
if (prev_timestamp) {
monotonic_t t = ss->timestamp - prev_timestamp;
total_time_between_events += t * weight;
ts_weight += weight;
}
prev_timestamp = ss->timestamp;
}
deque_clear(&s.samples);
if (total_weight <= 0) return;
add_velocity((total_dx / total_weight) * s.velocity_scale, (total_dy / total_weight) * s.velocity_scale);
if (ts_weight <= 0) return;
s.timer_interval = (monotonic_t)(total_time_between_events / ts_weight);
double dy = total_dy / total_weight, dx = total_dx / total_weight;
add_velocity(dx * s.velocity_scale, dy * s.velocity_scale);
if (false) timed_debug_print(
"momentum scroll: event velocity: %.1f final velocity: %.1f interval: %d ms\n",
dy, s.velocity.y, monotonic_t_to_ms(s.timer_interval));
}
static void
@@ -149,7 +171,7 @@ static void
start_momentum_scroll(void) {
set_velocity_from_samples();
send_momentum_event(true);
s.timer_id = glfwAddTimer(ms_to_monotonic_t(s.timer_interval_ms), true, momentum_timer_fired, NULL, NULL);
s.timer_id = glfwAddTimer(s.timer_interval, true, momentum_timer_fired, NULL, NULL);
}
void
@@ -166,7 +188,7 @@ glfw_handle_scroll_event_for_momentum(
}
s.window_id = w->id;
s.keyboard_modifiers = ev->keyboard_modifiers;
if (is_finger_based) {
if (is_finger_based && s.friction > 0) {
add_sample(ev->x_offset, ev->y_offset);
s.state = stopped ? MOMENTUM_IN_PROGRESS : PHYSICAL_EVENT_IN_PROGRESS;
} else {