def mean_pixel(model_variant=None):
  """Gets mean pixel value.

  This function returns different mean pixel value, depending on the input
  model_variant which adopts different preprocessing functions. We currently
  handle the following preprocessing functions:
  (1) _preprocess_subtract_imagenet_mean. We simply return mean pixel value.
  (2) _preprocess_zero_mean_unit_range. We return [127.5, 127.5, 127.5].
  The return values are used in a way that the padded regions after
  pre-processing will contain value 0.

  Args:
    model_variant: Model variant (string) for feature extraction. For
      backwards compatibility, model_variant=None returns _MEAN_RGB.

  Returns:
    Mean pixel value.
  """
  if model_variant in ['resnet_v1_50',
                       'resnet_v1_101'] or model_variant is None:
    return _MEAN_RGB
  else:
    return [127.5, 127.5, 127.5]