qmk-firmware/quantum/rgb_matrix_animations/cycle_up_down_anim.h

#pragma once
#ifndef DISABLE_RGB_MATRIX_CYCLE_UP_DOWN

extern rgb_counters_t g_rgb_counters;
extern rgb_led g_rgb_leds[DRIVER_LED_TOTAL];
extern rgb_config_t rgb_matrix_config;

bool rgb_matrix_cycle_up_down(effect_params_t* params) {
  RGB_MATRIX_USE_LIMITS(led_min, led_max);

  HSV hsv = { 0, rgb_matrix_config.sat, rgb_matrix_config.val };
  uint8_t time = scale16by8(g_rgb_counters.tick, rgb_matrix_config.speed / 4);
  for (uint8_t i = led_min; i < led_max; i++) {
    RGB_MATRIX_TEST_LED_FLAGS();
    point_t point = g_rgb_leds[i].point;
    hsv.h = point.y - time;
    RGB rgb = hsv_to_rgb(hsv);
    rgb_matrix_set_color(i, rgb.r, rgb.g, rgb.b);
  }
  return led_max < DRIVER_LED_TOTAL;
}

#endif // DISABLE_RGB_MATRIX_CYCLE_UP_DOWN