regenerate_thumbnails.php

<?php


/**
 * Regenerate missing image sizes only.
 *
 * Note: Untested code
 */
function regenerate($image_size = '', $attachment_id = []) {
  global $wpdb;
  $sql = "SELECT ID FROM {$wpdb->posts} WHERE post_mime_type LIKE '%image%'";
  if (!empty($attachment_id)) {
    $sql .= "AND ID IN (" . esc_sql(implode(',', $attachment_id)) . ")";
  }
  $results = $wpdb->get_col($sql);
  foreach ($results as $key => $attachment_id) {
    echo "Attachment ID: {$attachment_id} \n";
    $metadata = [];
    $file = _load_image_to_edit_path($attachment_id);
    if (file_is_displayable_image($file)) {
      $imagesize = getimagesize($file);
      $metadata['width'] = $imagesize[0];
      $metadata['height'] = $imagesize[1];
      list($uwidth, $uheight) = wp_constrain_dimensions($metadata['width'], $metadata['height']);
      $metadata['hwstring_small'] = "height='$uheight' width='$uwidth'";
      $metadata['file'] = _wp_relative_upload_path($file);
      global $_wp_additional_image_sizes;
      foreach (get_intermediate_image_sizes() as $s) {
        $sizes[$s] = [
          'width' => '',
          'height' => '',
          'crop' => FALSE,
        ];
        if (isset($_wp_additional_image_sizes[$s]['width'])) {
          $sizes[$s]['width'] = intval($_wp_additional_image_sizes[$s]['width']);
        }
        else {
          $sizes[$s]['width'] = get_option("{$s}_size_w");
        }
        if (isset($_wp_additional_image_sizes[$s]['height'])) {
          $sizes[$s]['height'] = intval($_wp_additional_image_sizes[$s]['height']);
        }
        else {
          $sizes[$s]['height'] = get_option("{$s}_size_h");
        }
        if (isset($_wp_additional_image_sizes[$s]['crop'])) {
          $sizes[$s]['crop'] = intval($_wp_additional_image_sizes[$s]['crop']);
        }
        else {
          $sizes[$s]['crop'] = get_option("{$s}_crop");
        }
      }
      $sizes = apply_filters('intermediate_image_sizes_advanced', $sizes);
      $attachment_meta = wp_get_attachment_metadata($attachment_id);
      foreach ($sizes as $size => $size_data ) {
        if (isset($attachment_meta['sizes'][$size])) {
          $metadata['sizes'][$size] = $attachment_meta['sizes'][$size];
        }
        else {
          $resized = image_make_intermediate_size($file, $size_data['width'], $size_data['height'], $size_data['crop']);
          if ($resized) {
            $metadata['sizes'][$size] = $resized;
            echo "{$size} regenerated \n";
          }
          else {
            echo "{$size} failed \n";
          }
        }
      }
      if (!empty($image_size)) {
        $resized = image_make_intermediate_size($file, $sizes[$image_size]['width'], $sizes[$image_size]['height'], $sizes[$image_size]['crop']);
        if ($resized) {
          $metadata['sizes'][$image_size] = $resized;
          echo "{$image_size} regenerated \n";
        }
        else {
          echo "{$image_size} failed \n";
        }
      }
      if ($attachment_meta['image_meta']) {
        $metadata['image_meta'] = $attachment_meta['image_meta'];
      }
    }
    wp_update_attachment_metadata($attachment_id, $metadata);
    echo "----------\n";
  }
  return;
}

This code was written long time ago and may break your page. Please test before running on a production environment.