qmk_firmware/quantum/rgb_matrix_animations/splash_anim.h

#ifdef RGB_MATRIX_KEYREACTIVE_ENABLED
#if !defined(DISABLE_RGB_MATRIX_SPLASH) || !defined(DISABLE_RGB_MATRIX_MULTISPLASH)

#ifndef DISABLE_RGB_MATRIX_SPLASH
RGB_MATRIX_EFFECT(SPLASH)
#endif

#ifndef DISABLE_RGB_MATRIX_MULTISPLASH
RGB_MATRIX_EFFECT(MULTISPLASH)
#endif

#ifdef RGB_MATRIX_CUSTOM_EFFECT_IMPLS

void SPLASH_math(HSV* hsv, int16_t dx, int16_t dy, uint8_t dist, uint16_t tick) {
    uint16_t effect = tick - dist;
      if (effect > 255)
        effect = 255;
      hsv->h += effect;
      hsv->v = qadd8(hsv->v, 255 - effect);
}

#ifndef DISABLE_RGB_MATRIX_SPLASH
bool SPLASH(effect_params_t* params) {
    return effect_runner_reactive_splash(qsub8(g_last_hit_tracker.count, 1), params, &SPLASH_math);
}
#endif

#ifndef DISABLE_RGB_MATRIX_MULTISPLASH
bool MULTISPLASH(effect_params_t* params) {
    return effect_runner_reactive_splash(0, params, &SPLASH_math);
}
#endif

#endif // RGB_MATRIX_CUSTOM_EFFECT_IMPLS
#endif // !defined(DISABLE_RGB_MATRIX_SPLASH) || !defined(DISABLE_RGB_MATRIX_MULTISPLASH)
#endif // RGB_MATRIX_KEYREACTIVE_ENABLED