| call_vision_api | helper function to send POST request to the Google Vision API | 
| create_request_body | helper function to create json for response request | 
| create_single_image_request | helper function to create a list of details of one image annotation request | 
| encode_image | helper function to base64 encode the image file | 
| extractor | helper function code to provide an extractor function for different feature types | 
| extract_annotations | helper function code to extract the annotations | 
| extract_error | helper function code to extract error from API response into a data.table | 
| extract_response | helper function code to extract the response data.frame | 
| face_detection_extractor | helper function code to extract API response into a data.table for given feature type | 
| gcv_get_available_feature_types | helper function code to record available feature types | 
| gcv_get_image_annotations | Get parsed image annotations from the Google Cloud Vision API | 
| gcv_get_raw_response | Get raw API response from the Google Cloud Vision API | 
| gcv_get_response | helper function to call the API for one batch of images | 
| get_bounding_boxes | helper function code to extract Bounding Box x,y coordinates for an API response element | 
| get_invalid_image_paths | helper function to validate input image paths | 
| label_detection_extractor | helper function code to extract API response into a data.table for given feature type | 
| landmark_detection_extractor | helper function code to extract API response into a data.table for given feature type | 
| logo_detection_extractor | helper function code to extract API response into a data.table for given feature type | 
| ocr_extractor | helper function code to extract API response into a data.table for given feature type | 
| split_to_chunks | helper function to split a vector to approximately equally sized chunks |