Reference for ultralytics/data/dataset.py

def build_transforms(self, hyp: Optional[Dict] = None) -> Compose:
    """
    Build and append transforms to the list.

    Args:
        hyp (dict, optional): Hyperparameters for transforms.

    Returns:
        (Compose): Composed transforms.
    """
    if self.augment:
        hyp.mosaic = hyp.mosaic if self.augment and not self.rect else 0.0
        hyp.mixup = hyp.mixup if self.augment and not self.rect else 0.0
        hyp.cutmix = hyp.cutmix if self.augment and not self.rect else 0.0
        transforms = v8_transforms(self, self.imgsz, hyp)
    else:
        transforms = Compose([LetterBox(new_shape=(self.imgsz, self.imgsz), scaleup=False)])
    transforms.append(
        Format(
            bbox_format="xywh",
            normalize=True,
            return_mask=self.use_segments,
            return_keypoint=self.use_keypoints,
            return_obb=self.use_obb,
            batch_idx=True,
            mask_ratio=hyp.mask_ratio,
            mask_overlap=hyp.overlap_mask,
            bgr=hyp.bgr if self.augment else 0.0,  # only affect training.
        )
    )
    return transforms

def cache_labels(self, path: Path = Path("./labels.cache")) -> Dict:
    """
    Cache dataset labels, check images and read shapes.

    Args:
        path (Path): Path where to save the cache file.

    Returns:
        (dict): Dictionary containing cached labels and related information.
    """
    x = {"labels": []}
    nm, nf, ne, nc, msgs = 0, 0, 0, 0, []  # number missing, found, empty, corrupt, messages
    desc = f"{self.prefix}Scanning {path.parent / path.stem}..."
    total = len(self.im_files)
    nkpt, ndim = self.data.get("kpt_shape", (0, 0))
    if self.use_keypoints and (nkpt <= 0 or ndim not in {2, 3}):
        raise ValueError(
            "'kpt_shape' in data.yaml missing or incorrect. Should be a list with [number of "
            "keypoints, number of dims (2 for x,y or 3 for x,y,visible)], i.e. 'kpt_shape: [17, 3]'"
        )
    with ThreadPool(NUM_THREADS) as pool:
        results = pool.imap(
            func=verify_image_label,
            iterable=zip(
                self.im_files,
                self.label_files,
                repeat(self.prefix),
                repeat(self.use_keypoints),
                repeat(len(self.data["names"])),
                repeat(nkpt),
                repeat(ndim),
                repeat(self.single_cls),
            ),
        )
        pbar = TQDM(results, desc=desc, total=total)
        for im_file, lb, shape, segments, keypoint, nm_f, nf_f, ne_f, nc_f, msg in pbar:
            nm += nm_f
            nf += nf_f
            ne += ne_f
            nc += nc_f
            if im_file:
                x["labels"].append(
                    {
                        "im_file": im_file,
                        "shape": shape,
                        "cls": lb[:, 0:1],  # n, 1
                        "bboxes": lb[:, 1:],  # n, 4
                        "segments": segments,
                        "keypoints": keypoint,
                        "normalized": True,
                        "bbox_format": "xywh",
                    }
                )
            if msg:
                msgs.append(msg)
            pbar.desc = f"{desc} {nf} images, {nm + ne} backgrounds, {nc} corrupt"
        pbar.close()

    if msgs:
        LOGGER.info("\n".join(msgs))
    if nf == 0:
        LOGGER.warning(f"{self.prefix}No labels found in {path}. {HELP_URL}")
    x["hash"] = get_hash(self.label_files + self.im_files)
    x["results"] = nf, nm, ne, nc, len(self.im_files)
    x["msgs"] = msgs  # warnings
    save_dataset_cache_file(self.prefix, path, x, DATASET_CACHE_VERSION)
    return x

def close_mosaic(self, hyp: Dict) -> None:
    """
    Disable mosaic, copy_paste, mixup and cutmix augmentations by setting their probabilities to 0.0.

    Args:
        hyp (dict): Hyperparameters for transforms.
    """
    hyp.mosaic = 0.0
    hyp.copy_paste = 0.0
    hyp.mixup = 0.0
    hyp.cutmix = 0.0
    self.transforms = self.build_transforms(hyp)

@staticmethod
def collate_fn(batch: List[Dict]) -> Dict:
    """
    Collate data samples into batches.

    Args:
        batch (List[dict]): List of dictionaries containing sample data.

    Returns:
        (dict): Collated batch with stacked tensors.
    """
    new_batch = {}
    batch = [dict(sorted(b.items())) for b in batch]  # make sure the keys are in the same order
    keys = batch[0].keys()
    values = list(zip(*[list(b.values()) for b in batch]))
    for i, k in enumerate(keys):
        value = values[i]
        if k in {"img", "text_feats"}:
            value = torch.stack(value, 0)
        elif k == "visuals":
            value = torch.nn.utils.rnn.pad_sequence(value, batch_first=True)
        if k in {"masks", "keypoints", "bboxes", "cls", "segments", "obb"}:
            value = torch.cat(value, 0)
        new_batch[k] = value
    new_batch["batch_idx"] = list(new_batch["batch_idx"])
    for i in range(len(new_batch["batch_idx"])):
        new_batch["batch_idx"][i] += i  # add target image index for build_targets()
    new_batch["batch_idx"] = torch.cat(new_batch["batch_idx"], 0)
    return new_batch

def get_labels(self) -> List[Dict]:
    """
    Return dictionary of labels for YOLO training.

    This method loads labels from disk or cache, verifies their integrity, and prepares them for training.

    Returns:
        (List[dict]): List of label dictionaries, each containing information about an image and its annotations.
    """
    self.label_files = img2label_paths(self.im_files)
    cache_path = Path(self.label_files[0]).parent.with_suffix(".cache")
    try:
        cache, exists = load_dataset_cache_file(cache_path), True  # attempt to load a *.cache file
        assert cache["version"] == DATASET_CACHE_VERSION  # matches current version
        assert cache["hash"] == get_hash(self.label_files + self.im_files)  # identical hash
    except (FileNotFoundError, AssertionError, AttributeError):
        cache, exists = self.cache_labels(cache_path), False  # run cache ops

    # Display cache
    nf, nm, ne, nc, n = cache.pop("results")  # found, missing, empty, corrupt, total
    if exists and LOCAL_RANK in {-1, 0}:
        d = f"Scanning {cache_path}... {nf} images, {nm + ne} backgrounds, {nc} corrupt"
        TQDM(None, desc=self.prefix + d, total=n, initial=n)  # display results
        if cache["msgs"]:
            LOGGER.info("\n".join(cache["msgs"]))  # display warnings

    # Read cache
    [cache.pop(k) for k in ("hash", "version", "msgs")]  # remove items
    labels = cache["labels"]
    if not labels:
        raise RuntimeError(
            f"No valid images found in {cache_path}. Images with incorrectly formatted labels are ignored. {HELP_URL}"
        )
    self.im_files = [lb["im_file"] for lb in labels]  # update im_files

    # Check if the dataset is all boxes or all segments
    lengths = ((len(lb["cls"]), len(lb["bboxes"]), len(lb["segments"])) for lb in labels)
    len_cls, len_boxes, len_segments = (sum(x) for x in zip(*lengths))
    if len_segments and len_boxes != len_segments:
        LOGGER.warning(
            f"Box and segment counts should be equal, but got len(segments) = {len_segments}, "
            f"len(boxes) = {len_boxes}. To resolve this only boxes will be used and all segments will be removed. "
            "To avoid this please supply either a detect or segment dataset, not a detect-segment mixed dataset."
        )
        for lb in labels:
            lb["segments"] = []
    if len_cls == 0:
        LOGGER.warning(f"Labels are missing or empty in {cache_path}, training may not work correctly. {HELP_URL}")
    return labels

def update_labels_info(self, label: Dict) -> Dict:
    """
    Update label format for different tasks.

    Args:
        label (dict): Label dictionary containing bboxes, segments, keypoints, etc.

    Returns:
        (dict): Updated label dictionary with instances.

    Note:
        cls is not with bboxes now, classification and semantic segmentation need an independent cls label
        Can also support classification and semantic segmentation by adding or removing dict keys there.
    """
    bboxes = label.pop("bboxes")
    segments = label.pop("segments", [])
    keypoints = label.pop("keypoints", None)
    bbox_format = label.pop("bbox_format")
    normalized = label.pop("normalized")

    # NOTE: do NOT resample oriented boxes
    segment_resamples = 100 if self.use_obb else 1000
    if len(segments) > 0:
        # make sure segments interpolate correctly if original length is greater than segment_resamples
        max_len = max(len(s) for s in segments)
        segment_resamples = (max_len + 1) if segment_resamples < max_len else segment_resamples
        # list[np.array(segment_resamples, 2)] * num_samples
        segments = np.stack(resample_segments(segments, n=segment_resamples), axis=0)
    else:
        segments = np.zeros((0, segment_resamples, 2), dtype=np.float32)
    label["instances"] = Instances(bboxes, segments, keypoints, bbox_format=bbox_format, normalized=normalized)
    return label

def build_transforms(self, hyp: Optional[Dict] = None) -> Compose:
    """
    Enhance data transformations with optional text augmentation for multi-modal training.

    Args:
        hyp (dict, optional): Hyperparameters for transforms.

    Returns:
        (Compose): Composed transforms including text augmentation if applicable.
    """
    transforms = super().build_transforms(hyp)
    if self.augment:
        # NOTE: hard-coded the args for now.
        # NOTE: this implementation is different from official yoloe,
        # the strategy of selecting negative is restricted in one dataset,
        # while official pre-saved neg embeddings from all datasets at once.
        transform = RandomLoadText(
            max_samples=min(self.data["nc"], 80),
            padding=True,
            padding_value=self._get_neg_texts(self.category_freq),
        )
        transforms.insert(-1, transform)
    return transforms

def update_labels_info(self, label: Dict) -> Dict:
    """
    Add text information for multi-modal model training.

    Args:
        label (dict): Label dictionary containing bboxes, segments, keypoints, etc.

    Returns:
        (dict): Updated label dictionary with instances and texts.
    """
    labels = super().update_labels_info(label)
    # NOTE: some categories are concatenated with its synonyms by `/`.
    # NOTE: and `RandomLoadText` would randomly select one of them if there are multiple words.
    labels["texts"] = [v.split("/") for _, v in self.data["names"].items()]

    return labels

def build_transforms(self, hyp: Optional[Dict] = None) -> Compose:
    """
    Configure augmentations for training with optional text loading.

    Args:
        hyp (dict, optional): Hyperparameters for transforms.

    Returns:
        (Compose): Composed transforms including text augmentation if applicable.
    """
    transforms = super().build_transforms(hyp)
    if self.augment:
        # NOTE: hard-coded the args for now.
        # NOTE: this implementation is different from official yoloe,
        # the strategy of selecting negative is restricted in one dataset,
        # while official pre-saved neg embeddings from all datasets at once.
        transform = RandomLoadText(
            max_samples=min(self.max_samples, 80),
            padding=True,
            padding_value=self._get_neg_texts(self.category_freq),
        )
        transforms.insert(-1, transform)
    return transforms

def cache_labels(self, path: Path = Path("./labels.cache")) -> Dict[str, Any]:
    """
    Load annotations from a JSON file, filter, and normalize bounding boxes for each image.

    Args:
        path (Path): Path where to save the cache file.

    Returns:
        (Dict[str, Any]): Dictionary containing cached labels and related information.
    """
    x = {"labels": []}
    LOGGER.info("Loading annotation file...")
    with open(self.json_file) as f:
        annotations = json.load(f)
    images = {f"{x['id']:d}": x for x in annotations["images"]}
    img_to_anns = defaultdict(list)
    for ann in annotations["annotations"]:
        img_to_anns[ann["image_id"]].append(ann)
    for img_id, anns in TQDM(img_to_anns.items(), desc=f"Reading annotations {self.json_file}"):
        img = images[f"{img_id:d}"]
        h, w, f = img["height"], img["width"], img["file_name"]
        im_file = Path(self.img_path) / f
        if not im_file.exists():
            continue
        self.im_files.append(str(im_file))
        bboxes = []
        segments = []
        cat2id = {}
        texts = []
        for ann in anns:
            if ann["iscrowd"]:
                continue
            box = np.array(ann["bbox"], dtype=np.float32)
            box[:2] += box[2:] / 2
            box[[0, 2]] /= float(w)
            box[[1, 3]] /= float(h)
            if box[2] <= 0 or box[3] <= 0:
                continue

            caption = img["caption"]
            cat_name = " ".join([caption[t[0] : t[1]] for t in ann["tokens_positive"]]).lower().strip()
            if not cat_name:
                continue

            if cat_name not in cat2id:
                cat2id[cat_name] = len(cat2id)
                texts.append([cat_name])
            cls = cat2id[cat_name]  # class
            box = [cls] + box.tolist()
            if box not in bboxes:
                bboxes.append(box)
                if ann.get("segmentation") is not None:
                    if len(ann["segmentation"]) == 0:
                        segments.append(box)
                        continue
                    elif len(ann["segmentation"]) > 1:
                        s = merge_multi_segment(ann["segmentation"])
                        s = (np.concatenate(s, axis=0) / np.array([w, h], dtype=np.float32)).reshape(-1).tolist()
                    else:
                        s = [j for i in ann["segmentation"] for j in i]  # all segments concatenated
                        s = (
                            (np.array(s, dtype=np.float32).reshape(-1, 2) / np.array([w, h], dtype=np.float32))
                            .reshape(-1)
                            .tolist()
                        )
                    s = [cls] + s
                    segments.append(s)
        lb = np.array(bboxes, dtype=np.float32) if len(bboxes) else np.zeros((0, 5), dtype=np.float32)

        if segments:
            classes = np.array([x[0] for x in segments], dtype=np.float32)
            segments = [np.array(x[1:], dtype=np.float32).reshape(-1, 2) for x in segments]  # (cls, xy1...)
            lb = np.concatenate((classes.reshape(-1, 1), segments2boxes(segments)), 1)  # (cls, xywh)
        lb = np.array(lb, dtype=np.float32)

        x["labels"].append(
            {
                "im_file": im_file,
                "shape": (h, w),
                "cls": lb[:, 0:1],  # n, 1
                "bboxes": lb[:, 1:],  # n, 4
                "segments": segments,
                "normalized": True,
                "bbox_format": "xywh",
                "texts": texts,
            }
        )
    x["hash"] = get_hash(self.json_file)
    save_dataset_cache_file(self.prefix, path, x, DATASET_CACHE_VERSION)
    return x

def get_img_files(self, img_path: str) -> List:
    """
    The image files would be read in `get_labels` function, return empty list here.

    Args:
        img_path (str): Path to the directory containing images.

    Returns:
        (list): Empty list as image files are read in get_labels.
    """
    return []

def get_labels(self) -> List[Dict]:
    """
    Load labels from cache or generate them from JSON file.

    Returns:
        (List[dict]): List of label dictionaries, each containing information about an image and its annotations.
    """
    cache_path = Path(self.json_file).with_suffix(".cache")
    try:
        cache, _ = load_dataset_cache_file(cache_path), True  # attempt to load a *.cache file
        assert cache["version"] == DATASET_CACHE_VERSION  # matches current version
        assert cache["hash"] == get_hash(self.json_file)  # identical hash
    except (FileNotFoundError, AssertionError, AttributeError, ModuleNotFoundError):
        cache, _ = self.cache_labels(cache_path), False  # run cache ops
    [cache.pop(k) for k in ("hash", "version")]  # remove items
    labels = cache["labels"]
    self.verify_labels(labels)
    self.im_files = [str(label["im_file"]) for label in labels]
    if LOCAL_RANK in {-1, 0}:
        LOGGER.info(f"Load {self.json_file} from cache file {cache_path}")
    return labels

def verify_labels(self, labels: List[Dict[str, Any]]) -> None:
    """
    Verify the number of instances in the dataset matches expected counts.

    This method checks if the total number of bounding box instances in the provided
    labels matches the expected count for known datasets. It performs validation
    against a predefined set of datasets with known instance counts.

    Args:
        labels (List[Dict[str, Any]]): List of label dictionaries, where each dictionary
            contains dataset annotations. Each label dict must have a 'bboxes' key with
            a numpy array or tensor containing bounding box coordinates.

    Raises:
        AssertionError: If the actual instance count doesn't match the expected count
            for a recognized dataset.

    Note:
        For unrecognized datasets (those not in the predefined expected_counts),
        a warning is logged and verification is skipped.
    """
    expected_counts = {
        "final_mixed_train_no_coco_segm": 3662412,
        "final_mixed_train_no_coco": 3681235,
        "final_flickr_separateGT_train_segm": 638214,
        "final_flickr_separateGT_train": 640704,
    }

    instance_count = sum(label["bboxes"].shape[0] for label in labels)
    for data_name, count in expected_counts.items():
        if data_name in self.json_file:
            assert instance_count == count, f"'{self.json_file}' has {instance_count} instances, expected {count}."
            return
    LOGGER.warning(f"Skipping instance count verification for unrecognized dataset '{self.json_file}'")

def close_mosaic(self, hyp: Dict) -> None:
    """
    Set mosaic, copy_paste and mixup options to 0.0 and build transformations.

    Args:
        hyp (dict): Hyperparameters for transforms.
    """
    for dataset in self.datasets:
        if not hasattr(dataset, "close_mosaic"):
            continue
        dataset.close_mosaic(hyp)

@staticmethod
def collate_fn(batch: List[Dict]) -> Dict:
    """
    Collate data samples into batches.

    Args:
        batch (List[dict]): List of dictionaries containing sample data.

    Returns:
        (dict): Collated batch with stacked tensors.
    """
    return YOLODataset.collate_fn(batch)

def __getitem__(self, i: int) -> Dict:
    """
    Return subset of data and targets corresponding to given indices.

    Args:
        i (int): Index of the sample to retrieve.

    Returns:
        (dict): Dictionary containing the image and its class index.
    """
    f, j, fn, im = self.samples[i]  # filename, index, filename.with_suffix('.npy'), image
    if self.cache_ram:
        if im is None:  # Warning: two separate if statements required here, do not combine this with previous line
            im = self.samples[i][3] = cv2.imread(f)
    elif self.cache_disk:
        if not fn.exists():  # load npy
            np.save(fn.as_posix(), cv2.imread(f), allow_pickle=False)
        im = np.load(fn)
    else:  # read image
        im = cv2.imread(f)  # BGR
    # Convert NumPy array to PIL image
    im = Image.fromarray(cv2.cvtColor(im, cv2.COLOR_BGR2RGB))
    sample = self.torch_transforms(im)
    return {"img": sample, "cls": j}

def __len__(self) -> int:
    """Return the total number of samples in the dataset."""
    return len(self.samples)

def verify_images(self) -> List[Tuple]:
    """
    Verify all images in dataset.

    Returns:
        (list): List of valid samples after verification.
    """
    desc = f"{self.prefix}Scanning {self.root}..."
    path = Path(self.root).with_suffix(".cache")  # *.cache file path

    try:
        check_file_speeds([file for (file, _) in self.samples[:5]], prefix=self.prefix)  # check image read speeds
        cache = load_dataset_cache_file(path)  # attempt to load a *.cache file
        assert cache["version"] == DATASET_CACHE_VERSION  # matches current version
        assert cache["hash"] == get_hash([x[0] for x in self.samples])  # identical hash
        nf, nc, n, samples = cache.pop("results")  # found, missing, empty, corrupt, total
        if LOCAL_RANK in {-1, 0}:
            d = f"{desc} {nf} images, {nc} corrupt"
            TQDM(None, desc=d, total=n, initial=n)
            if cache["msgs"]:
                LOGGER.info("\n".join(cache["msgs"]))  # display warnings
        return samples

    except (FileNotFoundError, AssertionError, AttributeError):
        # Run scan if *.cache retrieval failed
        nf, nc, msgs, samples, x = 0, 0, [], [], {}
        with ThreadPool(NUM_THREADS) as pool:
            results = pool.imap(func=verify_image, iterable=zip(self.samples, repeat(self.prefix)))
            pbar = TQDM(results, desc=desc, total=len(self.samples))
            for sample, nf_f, nc_f, msg in pbar:
                if nf_f:
                    samples.append(sample)
                if msg:
                    msgs.append(msg)
                nf += nf_f
                nc += nc_f
                pbar.desc = f"{desc} {nf} images, {nc} corrupt"
            pbar.close()
        if msgs:
            LOGGER.info("\n".join(msgs))
        x["hash"] = get_hash([x[0] for x in self.samples])
        x["results"] = nf, nc, len(samples), samples
        x["msgs"] = msgs  # warnings
        save_dataset_cache_file(self.prefix, path, x, DATASET_CACHE_VERSION)
        return samples