qmk-firmware/quantum/rgb_matrix_animations/digital_rain_anim.h

#pragma once
#if defined(RGB_MATRIX_FRAMEBUFFER_EFFECTS) && !defined(DISABLE_RGB_MATRIX_DIGITAL_RAIN)

#ifndef RGB_DIGITAL_RAIN_DROPS
    // lower the number for denser effect/wider keyboard
    #define RGB_DIGITAL_RAIN_DROPS 24
#endif

extern uint8_t rgb_frame_buffer[MATRIX_ROWS][MATRIX_COLS];

bool rgb_matrix_digital_rain(effect_params_t* params) {
  // algorithm ported from https://github.com/tremby/Kaleidoscope-LEDEffect-DigitalRain
  const uint8_t drop_ticks           = 28;
  const uint8_t pure_green_intensity = 0xd0;
  const uint8_t max_brightness_boost = 0xc0;
  const uint8_t max_intensity        = 0xff;

  static uint8_t drop = 0;

  if (params->init) {
    rgb_matrix_set_color_all(0, 0, 0);
    memset(rgb_frame_buffer, 0, sizeof rgb_frame_buffer);
    drop = 0;
  }

  for (uint8_t col = 0; col < MATRIX_COLS; col++) {
    for (uint8_t row = 0; row < MATRIX_ROWS; row++) {
      if (row == 0 && drop == 0 && rand() < RAND_MAX / RGB_DIGITAL_RAIN_DROPS) {
        // top row, pixels have just fallen and we're
        // making a new rain drop in this column
        rgb_frame_buffer[col][row] = max_intensity;
      }
      else if (rgb_frame_buffer[col][row] > 0 && rgb_frame_buffer[col][row] < max_intensity) {
        // neither fully bright nor dark, decay it
        rgb_frame_buffer[col][row]--;
      }
      // set the pixel colour
      uint8_t led[LED_HITS_TO_REMEMBER];
      uint8_t led_count = rgb_matrix_map_row_column_to_led(row, col, led);

      // TODO: multiple leds are supported mapped to the same row/column
      if (led_count > 0) {
        if (rgb_frame_buffer[col][row] > pure_green_intensity) {
          const uint8_t boost = (uint8_t) ((uint16_t) max_brightness_boost * (rgb_frame_buffer[col][row] - pure_green_intensity) / (max_intensity - pure_green_intensity));
          rgb_matrix_set_color(led[0], boost, max_intensity, boost);
        }
        else {
          const uint8_t green = (uint8_t) ((uint16_t) max_intensity * rgb_frame_buffer[col][row] / pure_green_intensity);
          rgb_matrix_set_color(led[0], 0, green, 0);
        }
      }
    }
  }

  if (++drop > drop_ticks) {
    // reset drop timer
    drop = 0;
    for (uint8_t row = MATRIX_ROWS - 1; row > 0; row--) {
      for (uint8_t col = 0; col < MATRIX_COLS; col++) {
        // if ths is on the bottom row and bright allow decay
        if (row == MATRIX_ROWS - 1 && rgb_frame_buffer[col][row] == max_intensity) {
          rgb_frame_buffer[col][row]--;
        }
        // check if the pixel above is bright
        if (rgb_frame_buffer[col][row - 1] == max_intensity) {
          // allow old bright pixel to decay
          rgb_frame_buffer[col][row - 1]--;
          // make this pixel bright
          rgb_frame_buffer[col][row] = max_intensity;
        }
      }
    }
  }
  return false;
}

#endif // defined(RGB_MATRIX_FRAMEBUFFER_EFFECTS) && !defined(DISABLE_RGB_MATRIX_DIGITAL_RAIN)