ImageStyle::createDerivative

public ImageStyle::createDerivative($original_uri, $derivative_uri)

public function createDerivative($original_uri, $derivative_uri) {

  // If the source file doesn't exist, return FALSE without creating folders.
  $image = \Drupal::service('image.factory')->get($original_uri);
  if (!$image->isValid()) {
    return FALSE;
  }

  // Get the folder for the final location of this style.
  $directory = drupal_dirname($derivative_uri);

  // Build the destination folder tree if it doesn't already exist.
  if (!file_prepare_directory($directory, FILE_CREATE_DIRECTORY | FILE_MODIFY_PERMISSIONS)) {
    \Drupal::logger('image')->error('Failed to create style directory: %directory', array('%directory' => $directory));
    return FALSE;
  }

  foreach ($this->getEffects() as $effect) {
    $effect->applyEffect($image);
  }

  if (!$image->save($derivative_uri)) {
    if (file_exists($derivative_uri)) {
      \Drupal::logger('image')->error('Cached image file %destination already exists. There may be an issue with your rewrite configuration.', array('%destination' => $derivative_uri));
    }
    return FALSE;
  }

  return TRUE;
}