Customize Postprocessing Method

Guideline for Postprocessing Module

Common Protocols

1. Each postprocessing module is a class with a callable function.
2. The input to the postprocessing function is network prediction and additional data information if needed.
3. The output of the postprocessing function is a alwasy a dict, where the key is a field name, such as 'polys' for polygons in text detection, 'text' for text detection.

Detection Postprocessing API Protocols

1. class naming: Det{Method}Postprocess

2. class __init__() args:

   • box_type (string): options are ["quad', 'polys"] for quadriateral and polygon text representation.
   • rescale_fields (List[str]='polys'): indicates which fields in the output dict will be rescaled to the original image space. Field name: "polys" for polygons

3. __call__() method: If inherit from DetBasePostprocess, you don't need to implement this method in your Postproc. class. Execution entry for postprocessing, which postprocess network prediction on the transformed image space to get text boxes (by self._postprocess() function) and then rescale them back to the original image space (by self.rescale() function).

   • Input args:

     • pred (Union[Tensor, Tuple[Tensor]]): network prediction for input batch data, shape [batch_size, ...]
     • shape_list (Union[List, np.ndarray, ms.Tensor]): shape and scale info for each image in the batch, shape [batch_size, 4]. Each internal array of length 4 is [src_h, src_w, scale_h, scale_w], where src_h and src_w are height and width of the original image, and scale_h and scale_w are their scale ratio after image resizing respectively.
     • **kwargs: args for extension

   • Return: detection result as a dictionary with the following keys

     • polys (List[List[np.ndarray]): predicted polygons mapped on the original image space, shape [batch_size, num_polygons, num_points, 2]. If box_type is 'quad', num_points=4, and the internal np.ndarray is of shape [4, 2]
     • scores (List[float]): confidence scores for the predicted polygons, shape (batch_size, num_polygons)

4. _postprocess() method: Implement your postprocessing method here if inherit from DetBasePostprocess Postprocess network prediction to get text boxes on the transformed image space (which will be rescaled back to original image space in call function)

   • Input args:

     • pred (Union[Tensor, Tuple[Tensor]]): network prediction for input batch data, shape [batch_size, ...]
     • **kwargs: args for extension

   • Return: postprocessing result as a dict with keys:

     • polys (List[List[np.ndarray]): predicted polygons on the transformed (i.e. resized normally) image space, of shape (batch_size, num_polygons, num_points, 2). If box_type is 'quad', num_points=4.
     • scores (np.ndarray): confidence scores for the predicted polygons, shape (batch_size, num_polygons)

   • Notes:

     • Please cast pred to the type you need in your implementation. Some postprocesssing steps use ops from mindspore.nn and prefer Tensor type, while some steps prefer np.ndarray type required in other libraries.
     • _postprocess() should NOT round the text box polys to integer in return, because they will be recaled and then rounded in the end. Rounding early will cause larger error in polygon rescaling and results in evaluation performance degradation, especially on small datasets.

5. About rescaling the polygons back to the original image spcae

   • The rescaling step is necessary for a fair evaluation and is needed in cropping text regions from the orginal image in inference.
   • To enable rescaling for evaluation
     1. add "shape_list" to the eval.dataset.output_columns in the YAML config file of the model.
     2. make sure rescale_fields is not None (default is ["polys"])
   • To enable rescaling in inference:
     1. directly parse shape_list (which is got from data["shape_list"] after data loading) to the postprocessing function. It works with rescale_fields to decide whether to do rescaling and which fields are to be rescaled.
   • shape_list is originally recorded in image resize transformation, such as DetResize.

Example Code: DetBasePostprocess and DetDBPostprocess

Recognition Postprocessing API Protocols

1. class __init__() should support the follow args: - character_dict_path - use_space_char - blank_at_last - lower Please see the API docs in RecCTCLabelDecode for argument illustration.

2. __call__() method:

   • Input args:

     • pred (Union[Tensor, Tuple[Tensor]]): network prediction
     • **kwargs: args for extension

   • Return: det_res as a dictionary with the following keys

     • texts (List[str]): list of preditected text string
     • confs (List[float]): confidence of each prediction

Example code: RecCTCLabelDecode