@java.lang.Override
  public void writeTo(com.google.protobuf.CodedOutputStream output) throws java.io.IOException {
    for (int i = 0; i < tracks_.size(); i++) {
      output.writeMessage(3, tracks_.get(i));
    }
    if (!thumbnail_.isEmpty()) {
      output.writeBytes(4, thumbnail_);
    }
    getUnknownFields().writeTo(output);
  }
<pre>
The thumbnail of a person's face.
</pre>

<code>bytes thumbnail = 4;</code>

@return The thumbnail.
java-video-intelligence/proto-google-cloud-video-intelligence-v1p3beta1/src/main/java/com/google/cloud/videointelligence/v1p3beta1/FaceDetectionAnnotation.java