def _preprocess(self):
        """
        This function is a bit complex and ugly, what it does is
        1. Extract data from cuhk-03.mat and save as png images.
        2. Create 20 classic splits. (Li et al. CVPR'14)
        3. Create new split. (Zhong et al. CVPR'17)
        """
        print(
            "Note: if root path is changed, the previously generated json files need to be re-generated (delete them first)")
        if osp.exists(self.imgs_labeled_dir) and \
                osp.exists(self.imgs_detected_dir) and \
                osp.exists(self.split_classic_det_json_path) and \
                osp.exists(self.split_classic_lab_json_path) and \
                osp.exists(self.split_new_det_json_path) and \
                osp.exists(self.split_new_lab_json_path):
            return

        mkdir_if_missing(self.imgs_detected_dir)
        mkdir_if_missing(self.imgs_labeled_dir)

        print("Extract image data from {} and save as png".format(self.raw_mat_path))
        mat = h5py.File(self.raw_mat_path, 'r')

        def _deref(ref):
            return mat[ref][:].T

        def _process_images(img_refs, campid, pid, save_dir):
            img_paths = []  # Note: some persons only have images for one view
            for imgid, img_ref in enumerate(img_refs):
                img = _deref(img_ref)
                # skip empty cell
                if img.size == 0 or img.ndim < 3: continue
                # images are saved with the following format, index-1 (ensure uniqueness)
                # campid: index of camera pair (1-5)
                # pid: index of person in 'campid'-th camera pair
                # viewid: index of view, {1, 2}
                # imgid: index of image, (1-10)
                viewid = 1 if imgid < 5 else 2
                img_name = '{:01d}_{:03d}_{:01d}_{:02d}.png'.format(campid + 1, pid + 1, viewid, imgid + 1)
                img_path = osp.join(save_dir, img_name)
                if not osp.isfile(img_path):
                    imsave(img_path, img)
                img_paths.append(img_path)
            return img_paths

        def _extract_img(name):
            print("Processing {} images (extract and save) ...".format(name))
            meta_data = []
            imgs_dir = self.imgs_detected_dir if name == 'detected' else self.imgs_labeled_dir
            for campid, camp_ref in enumerate(mat[name][0]):
                camp = _deref(camp_ref)
                num_pids = camp.shape[0]
                for pid in range(num_pids):
                    img_paths = _process_images(camp[pid, :], campid, pid, imgs_dir)
                    assert len(img_paths) > 0, "campid{}-pid{} has no images".format(campid, pid)
                    meta_data.append((campid + 1, pid + 1, img_paths))
                print("- done camera pair {} with {} identities".format(campid + 1, num_pids))
            return meta_data

        meta_detected = _extract_img('detected')
        meta_labeled = _extract_img('labeled')

        def _extract_classic_split(meta_data, test_split):
            train, test = [], []
            num_train_pids, num_test_pids = 0, 0
            num_train_imgs, num_test_imgs = 0, 0
            for i, (campid, pid, img_paths) in enumerate(meta_data):

                if [campid, pid] in test_split:
                    for img_path in img_paths:
                        camid = int(osp.basename(img_path).split('_')[2]) - 1  # make it 0-based
                        test.append((img_path, num_test_pids, camid))
                    num_test_pids += 1
                    num_test_imgs += len(img_paths)
                else:
                    for img_path in img_paths:
                        camid = int(osp.basename(img_path).split('_')[2]) - 1  # make it 0-based
                        train.append((img_path, num_train_pids, camid))
                    num_train_pids += 1
                    num_train_imgs += len(img_paths)
            return train, num_train_pids, num_train_imgs, test, num_test_pids, num_test_imgs

        print("Creating classic splits (# = 20) ...")
        splits_classic_det, splits_classic_lab = [], []
        for split_ref in mat['testsets'][0]:
            test_split = _deref(split_ref).tolist()

            # create split for detected images
            train, num_train_pids, num_train_imgs, test, num_test_pids, num_test_imgs = \
                _extract_classic_split(meta_detected, test_split)
            splits_classic_det.append({
                'train': train, 'query': test, 'gallery': test,
                'num_train_pids': num_train_pids, 'num_train_imgs': num_train_imgs,
                'num_query_pids': num_test_pids, 'num_query_imgs': num_test_imgs,
                'num_gallery_pids': num_test_pids, 'num_gallery_imgs': num_test_imgs,
            })

            # create split for labeled images
            train, num_train_pids, num_train_imgs, test, num_test_pids, num_test_imgs = \
                _extract_classic_split(meta_labeled, test_split)
            splits_classic_lab.append({
                'train': train, 'query': test, 'gallery': test,
                'num_train_pids': num_train_pids, 'num_train_imgs': num_train_imgs,
                'num_query_pids': num_test_pids, 'num_query_imgs': num_test_imgs,
                'num_gallery_pids': num_test_pids, 'num_gallery_imgs': num_test_imgs,
            })

        write_json(splits_classic_det, self.split_classic_det_json_path)
        write_json(splits_classic_lab, self.split_classic_lab_json_path)

        def _extract_set(filelist, pids, pid2label, idxs, img_dir, relabel):
            tmp_set = []
            unique_pids = set()
            for idx in idxs:
                img_name = filelist[idx][0]
                camid = int(img_name.split('_')[2]) - 1  # make it 0-based
                pid = pids[idx]
                if relabel: pid = pid2label[pid]
                img_path = osp.join(img_dir, img_name)
                tmp_set.append((img_path, int(pid), camid))
                unique_pids.add(pid)
            return tmp_set, len(unique_pids), len(idxs)

        def _extract_new_split(split_dict, img_dir):
            train_idxs = split_dict['train_idx'].flatten() - 1  # index-0
            pids = split_dict['labels'].flatten()
            train_pids = set(pids[train_idxs])
            pid2label = {pid: label for label, pid in enumerate(train_pids)}
            query_idxs = split_dict['query_idx'].flatten() - 1
            gallery_idxs = split_dict['gallery_idx'].flatten() - 1
            filelist = split_dict['filelist'].flatten()
            train_info = _extract_set(filelist, pids, pid2label, train_idxs, img_dir, relabel=True)
            query_info = _extract_set(filelist, pids, pid2label, query_idxs, img_dir, relabel=False)
            gallery_info = _extract_set(filelist, pids, pid2label, gallery_idxs, img_dir, relabel=False)
            return train_info, query_info, gallery_info

        print("Creating new splits for detected images (767/700) ...")
        train_info, query_info, gallery_info = _extract_new_split(
            loadmat(self.split_new_det_mat_path),
            self.imgs_detected_dir,
        )
        splits = [{
            'train': train_info[0], 'query': query_info[0], 'gallery': gallery_info[0],
            'num_train_pids': train_info[1], 'num_train_imgs': train_info[2],
            'num_query_pids': query_info[1], 'num_query_imgs': query_info[2],
            'num_gallery_pids': gallery_info[1], 'num_gallery_imgs': gallery_info[2],
        }]
        write_json(splits, self.split_new_det_json_path)

        print("Creating new splits for labeled images (767/700) ...")
        train_info, query_info, gallery_info = _extract_new_split(
            loadmat(self.split_new_lab_mat_path),
            self.imgs_labeled_dir,
        )
        splits = [{
            'train': train_info[0], 'query': query_info[0], 'gallery': gallery_info[0],
            'num_train_pids': train_info[1], 'num_train_imgs': train_info[2],
            'num_query_pids': query_info[1], 'num_query_imgs': query_info[2],
            'num_gallery_pids': gallery_info[1], 'num_gallery_imgs': gallery_info[2],
        }]
        write_json(splits, self.split_new_lab_json_path)

        This function is a bit complex and ugly, what it does is
        1. Extract data from cuhk-03.mat and save as png images.
        2. Create 20 classic splits. (Li et al. CVPR'14)
        3. Create new split. (Zhong et al. CVPR'17)
        
data/datasets/cuhk03.py