Reference for ultralytics/data/dataset.py

class YOLODataset(BaseDataset):
    """Dataset class for loading object detection and/or segmentation labels in YOLO format.

    This class supports loading data for object detection, segmentation, pose estimation, and oriented bounding box
    (OBB) tasks using the YOLO format.

    Attributes:
        use_segments (bool): Indicates if segmentation masks should be used.
        use_keypoints (bool): Indicates if keypoints should be used for pose estimation.
        use_obb (bool): Indicates if oriented bounding boxes should be used.
        data (dict): Dataset configuration dictionary.

    Methods:
        cache_labels: Cache dataset labels, check images and read shapes.
        get_labels: Return list of label dictionaries for YOLO training.
        build_transforms: Build and append transforms to the list.
        close_mosaic: Disable mosaic, copy_paste, mixup and cutmix augmentations and build transformations.
        update_labels_info: Update label format for different tasks.
        collate_fn: Collate data samples into batches.

    Examples:
        >>> dataset = YOLODataset(img_path="path/to/images", data={"names": {0: "person"}}, task="detect")
        >>> dataset.get_labels()
    """

    def __init__(self, *args, data: dict | None = None, task: str = "detect", **kwargs):
        """Initialize the YOLODataset.

        Args:
            data (dict, optional): Dataset configuration dictionary.
            task (str): Task type, one of 'detect', 'segment', 'pose', or 'obb'.
            *args (Any): Additional positional arguments for the parent class.
            **kwargs (Any): Additional keyword arguments for the parent class.
        """
        self.use_segments = task == "segment"
        self.use_keypoints = task == "pose"
        self.use_obb = task == "obb"
        self.data = data
        assert not (self.use_segments and self.use_keypoints), "Can not use both segments and keypoints."
        super().__init__(*args, channels=self.data.get("channels", 3), **kwargs)

def build_transforms(self, hyp: dict | None = 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
    if x["labels"]:
        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", "semantic_mask", "sem_masks"}:
            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
    if "batch_idx" in new_batch:
        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 list of label dictionaries 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, ModuleNotFoundError):
        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
    labels = cache["labels"]
    if not labels:
        issues = "\n  ".join(sorted(set(cache["msgs"]))) or "no error details"
        raise RuntimeError(f"No valid images found in {cache_path}.\n  {issues}\n{HELP_URL}")
    [cache.pop(k) for k in ("hash", "version", "msgs")]  # remove items
    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.

    Notes:
        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

class YOLOMultiModalDataset(YOLODataset):
    """Dataset class for loading object detection and/or segmentation labels in YOLO format with multi-modal support.

    This class extends YOLODataset to add text information for multi-modal model training, enabling models to process
    both image and text data.

    Methods:
        update_labels_info: Add text information for multi-modal model training.
        build_transforms: Enhance data transformations with text augmentation.

    Examples:
        >>> dataset = YOLOMultiModalDataset(img_path="path/to/images", data={"names": {0: "person"}}, task="detect")
        >>> batch = next(iter(dataset))
        >>> print(batch.keys())  # Should include 'texts'
    """

    def __init__(self, *args, data: dict | None = None, task: str = "detect", **kwargs):
        """Initialize a YOLOMultiModalDataset.

        Args:
            data (dict, optional): Dataset configuration dictionary.
            task (str): Task type, one of 'detect', 'segment', 'pose', or 'obb'.
            *args (Any): Additional positional arguments for the parent class.
            **kwargs (Any): Additional keyword arguments for the parent class.
        """
        super().__init__(*args, data=data, task=task, **kwargs)

@property
def category_names(self):
    """Return category names for the dataset.

    Returns:
        (set[str]): Set of class names.
    """
    names = self.data["names"].values()
    return {n.strip() for name in names for n in name.split("/")}  # category names

@property
def category_freq(self):
    """Return frequency of each category in the dataset."""
    texts = [v.split("/") for v in self.data["names"].values()]
    category_freq = defaultdict(int)
    for label in self.labels:
        for c in label["cls"].squeeze(-1):  # to check
            text = texts[int(c)]
            for t in text:
                t = t.strip()
                category_freq[t] += 1
    return category_freq

@staticmethod
def _get_neg_texts(category_freq: dict, threshold: int = 100) -> list[str]:
    """Get negative text samples based on frequency threshold."""
    threshold = min(max(category_freq.values()), 100)
    return [k for k, v in category_freq.items() if v >= threshold]

def build_transforms(self, hyp: dict | None = 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

class GroundingDataset(YOLODataset):
    """Dataset class for object detection tasks using annotations from a JSON file in grounding format.

    This dataset is designed for grounding tasks where annotations are provided in a JSON file rather than the standard
    YOLO format text files.

    Attributes:
        json_file (str): Path to the JSON file containing annotations.

    Methods:
        get_img_files: Return empty list as image files are read in get_labels.
        get_labels: Load annotations from a JSON file and prepare them for training.
        build_transforms: Configure augmentations for training with optional text loading.

    Examples:
        >>> dataset = GroundingDataset(img_path="path/to/images", json_file="annotations.json", task="detect")
        >>> len(dataset)  # Number of valid images with annotations
    """

    def __init__(self, *args, task: str = "detect", json_file: str = "", max_samples: int = 80, **kwargs):
        """Initialize a GroundingDataset for object detection.

        Args:
            json_file (str): Path to the JSON file containing annotations.
            task (str): Must be 'detect' or 'segment' for GroundingDataset.
            max_samples (int): Maximum number of samples to load for text augmentation.
            *args (Any): Additional positional arguments for the parent class.
            **kwargs (Any): Additional keyword arguments for the parent class.
        """
        assert task in {"detect", "segment"}, "GroundingDataset currently only supports `detect` and `segment` tasks"
        self.json_file = json_file
        self.max_samples = max_samples
        super().__init__(*args, task=task, data={"channels": 3}, **kwargs)

@property
def category_names(self):
    """Return unique category names from the dataset."""
    return {t.strip() for label in self.labels for text in label["texts"] for t in text}

@property
def category_freq(self):
    """Return frequency of each category in the dataset."""
    category_freq = defaultdict(int)
    for label in self.labels:
        for text in label["texts"]:
            for t in text:
                t = t.strip()
                category_freq[t] += 1
    return category_freq

@staticmethod
def _get_neg_texts(category_freq: dict, threshold: int = 100) -> list[str]:
    """Get negative text samples based on frequency threshold."""
    threshold = min(max(category_freq.values()), 100)
    return [k for k, v in category_freq.items() if v >= threshold]

def build_transforms(self, hyp: dict | None = 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.

    Notes:
        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}'")

class YOLOConcatDataset(ConcatDataset):

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.
    """
    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)

class SemanticDataset(YOLODataset):
    """Dataset for semantic segmentation with PNG mask labels.

    Expects a directory structure where each image has a corresponding PNG mask file with the same stem. Pixel values in
    masks represent class IDs, with 255 as the ignore label.

    The mask directory is specified in the dataset YAML via 'masks_dir' key, and mirrors the images/ directory structure
    (e.g., images/train/ -> masks/train/).

    Attributes:
        data (dict): Dataset configuration from YAML.
        mask_files (list[str]): List of mask file paths corresponding to images.
    """

    def __init__(self, *args, data: dict | None = None, **kwargs):
        """Initialize SemanticDataset.

        Args:
            *args (Any): Additional positional arguments for the parent class.
            data (dict): Dataset configuration dictionary.
            **kwargs (Any): Additional keyword arguments for the parent class.
        """
        self.data = data or {}
        self.label_mapping = self._parse_label_mapping(self.data.get("label_mapping"))
        self.mask_files = []
        super().__init__(*args, data=data, **kwargs)

def _parse_label_mapping(self, mapping):
    """Normalize label_mapping entries from dataset YAML into integer-to-integer ids."""
    if mapping is None:
        return {}
    if not isinstance(mapping, dict):
        raise TypeError(f"Expected 'label_mapping' to be a dict in dataset YAML, but got {type(mapping).__name__}.")

    normalized = {}
    for src, dst in mapping.items():
        src = int(src)
        if isinstance(dst, str):
            dst = dst.strip()
            dst = 255 if dst == "ignore_label" else int(dst)
        elif dst is None:
            dst = 255
        else:
            dst = int(dst)
        normalized[src] = dst
    return normalized

def _semantic_cache_hash(self, mask_files: list[str]) -> str:
    """Return a hash for semantic cache validation that also includes label_mapping changes."""
    mapping = json.dumps(self.label_mapping, sort_keys=True, separators=(",", ":"))
    return get_hash(self.im_files + mask_files + [f"label_mapping:{mapping}"])

def build_transforms(self, hyp=None):
    """Build transforms for semantic segmentation.

    Args:
        hyp (dict): Hyperparameters.

    Returns:
        (Compose): Composed transforms.
    """
    transforms = super().build_transforms(hyp)
    transforms[-1] = SemanticFormat()  # replace the last transform with SemanticFormat
    return transforms

def cache_labels(self, path: Path = Path("./labels.cache")) -> dict[str, Any]:
    """Cache semantic labels and image-mask pairing metadata.

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

    Returns:
        (dict[str, Any]): Cached semantic metadata.
    """
    x = {"labels": []}
    nm, nf, nc, msgs = 0, 0, 0, []  # missing, found, corrupt, messages
    desc = f"{self.prefix}Scanning {path.parent / path.stem}..."
    total = len(self.im_files)

    with ThreadPool(NUM_THREADS) as pool:
        results = pool.imap(
            func=verify_image_mask,
            iterable=zip(self.im_files, self.mask_files, repeat(self.prefix)),
        )
        pbar = TQDM(results, desc=desc, total=total)
        for im_file, mask_file, shape, nm_f, nf_f, nc_f, msg in pbar:
            nm += nm_f
            nf += nf_f
            nc += nc_f
            if im_file:
                x["labels"].append(
                    {
                        "im_file": im_file,
                        "mask_file": mask_file,
                        "shape": shape,
                        "cls": np.array([], dtype=np.float32),
                        "bboxes": np.zeros((0, 4), dtype=np.float32),
                        "segments": [],
                        "normalized": True,
                        "bbox_format": "xywh",
                    }
                )
            if msg:
                msgs.append(msg)
            pbar.desc = f"{desc} {nf} images, {nm} missing masks, {nc} corrupt"
        pbar.close()
    x["hash"] = self._semantic_cache_hash(self.mask_files)
    x["results"] = nf, nm, nc, total
    x["msgs"] = msgs
    if x["labels"]:
        save_dataset_cache_file(self.prefix, path, x, DATASET_CACHE_VERSION)
    return x

def convert_label(self, label, inverse=False):
    """Convert label values using the dataset's label mapping.

    Args:
        label (np.ndarray): Segmentation label array to convert.
        inverse (bool): If True, apply inverse mapping (mapped -> original). Defaults to False.

    Returns:
        (np.ndarray): Label array with converted values.
    """
    temp = label.copy()
    if inverse:
        for v, k in self.label_mapping.items():
            label[temp == k] = v
    else:
        for k, v in self.label_mapping.items():
            label[temp == k] = v
    return label

def get_image_and_label(self, index):
    """Get image, label and semantic mask for the given index.

    Overrides parent to include semantic mask so that Mosaic/CopyPaste mix images
    also have their masks loaded.

    Args:
        index (int): Dataset index.

    Returns:
        (dict): Label dict with 'img', 'semantic_mask', and metadata.
    """
    label = super().get_image_and_label(index)
    h, w = label["img"].shape[:2]
    mask = self.load_mask(index, image_shape=(h, w))
    # Resize mask to match the resized image dimensions
    if mask.shape[:2] != (h, w):
        mask = cv2.resize(mask, (w, h), interpolation=cv2.INTER_NEAREST)
    label["semantic_mask"] = mask
    return label

def get_labels(self):
    """Load semantic labels from cache or scan image-mask paths.

    Returns:
        (list[dict]): List of label dictionaries with mask file paths and image shapes.
    """
    self.mask_files = img2label_paths(self.im_files, label_dir=self.data.get("masks_dir", "masks"), suffix=".png")
    cache_path = Path(self.mask_files[0]).parent.with_suffix(".cache")

    try:
        cache, exists = load_dataset_cache_file(cache_path), True
        assert cache["version"] == DATASET_CACHE_VERSION
        assert cache["hash"] == self._semantic_cache_hash(self.mask_files)
    except (FileNotFoundError, AssertionError, AttributeError, ModuleNotFoundError):
        cache, exists = self.cache_labels(cache_path), False

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

    [cache.pop(k) for k in ("hash", "version", "msgs")]
    labels = cache["labels"]
    if not labels:
        raise RuntimeError(f"No valid images found in {cache_path}. {HELP_URL}")
    self.im_files = [lb["im_file"] for lb in labels]
    self.mask_files = [lb["mask_file"] for lb in labels]
    return labels

def load_image(self, i, rect_mode=True):
    """Load an image for semantic segmentation, scaling the short side to imgsz when rect_mode=True."""
    return super().load_image(i, rect_mode=rect_mode, resize_short=self.augment)

def load_mask(self, index: int, image_shape: tuple[int, int] | None = None) -> np.ndarray:
    """Load a semantic mask and apply optional dataset label mapping."""
    mask_file = self.labels[index]["mask_file"]
    mask = cv2.imread(mask_file, cv2.IMREAD_GRAYSCALE)
    if mask is None:
        raise FileNotFoundError(f"Semantic mask not found or unreadable: {mask_file}")
    if int(self.data.get("nc", 0)) == 1:
        with Image.open(mask_file) as im:
            if im.mode == "1":  # cv2 expands 1-bit PNGs to {0, 255}; map only true 1-bit foreground to 1.
                mask[mask == 255] = 1
    if self.label_mapping:
        mask = self.convert_label(mask, inverse=False)
    return mask.astype(np.uint8, copy=False)

class PolygonSemanticDataset(SemanticDataset, YOLODataset):
    """Semantic segmentation dataset that rasterizes YOLO polygon labels into masks on the fly.

    Used when the dataset YAML lacks 'masks_dir'. Pixels not covered by any polygon become a dedicated background class.
    Requires `add_polygon_background(data)` to be called first: for nc > 1 it bumps `data['nc']` to user_nc + 1 with
    background at `nc - 1`; for nc == 1 it keeps nc=1 and rasterizes a {0=bg, 1=fg} binary mask for use with
    BCEWithLogitsLoss.
    """

    def __init__(self, *args, data: dict | None = None, **kwargs):
        """Initialize PolygonSemanticDataset.

        Args:
            *args (Any): Additional positional arguments for the parent class.
            data (dict): Dataset configuration dictionary.
            **kwargs (Any): Additional keyword arguments for the parent class.
        """
        nc = (data or {}).get("nc") or len((data or {}).get("names", {}))
        self.bg_class_idx = data.get("bg_class_idx", max(int(nc) - 1, 0))
        super().__init__(*args, data=data, **kwargs)

def cache_labels(self, path: Path = Path("./labels.cache")) -> dict[str, Any]:
    """Cache polygon labels via YOLODataset to keep the 5-tuple `results` format expected by get_labels."""
    return YOLODataset.cache_labels(self, path)

def get_labels(self):
    """Parse YOLO polygon .txt labels."""
    return YOLODataset.get_labels(self)

def load_mask(self, index: int, image_shape: tuple[int, int] | None = None) -> np.ndarray:
    """Rasterize this image's polygons into a (H, W) uint8 semantic mask, bg = self.bg_class_idx."""
    h, w = image_shape
    label = self.labels[index]
    cls = label.get("cls")
    segments = label.get("segments") or []
    if cls is None or len(cls) == 0 or len(segments) == 0:
        return np.full((h, w), self.bg_class_idx, dtype=np.uint8)

    # Denormalize polygons (stored as normalized xy) to pixel coordinates at (h, w).
    scale = np.array([w, h], dtype=np.float32)
    polys = [np.asarray(s, dtype=np.float32).reshape(-1, 2) * scale for s in segments]
    # Returns (H, W) instance index map: 0 = no polygon, 1..N = sorted instance index.
    inst, sorted_idx = polygons2masks_overlap((h, w), polys, downsample_ratio=1)
    out = np.full((h, w), self.bg_class_idx, dtype=np.uint8)
    fg = inst > 0
    if int(self.data.get("nc", 0)) == 1:  # binary: fg=1 regardless of label cls value
        out[fg] = 1
    else:
        cls_arr = np.asarray(cls).reshape(-1).astype(np.int32)[sorted_idx]
        out[fg] = cls_arr[inst[fg] - 1].astype(np.uint8)
    return out

class ClassificationDataset:
    """Dataset class for image classification tasks wrapping torchvision ImageFolder functionality.

    This class offers functionalities like image augmentation, caching, and verification. It's designed to efficiently
    handle large datasets for training deep learning models, with optional image transformations and caching mechanisms
    to speed up training.

    Attributes:
        cache_ram (bool): Indicates if caching in RAM is enabled.
        cache_disk (bool): Indicates if caching on disk is enabled.
        samples (list): A list of lists, each containing the path to an image, its class index, path to its .npy cache
            file (if caching on disk), and optionally the loaded image array (if caching in RAM).
        torch_transforms (callable): PyTorch transforms to be applied to the images.
        root (str): Root directory of the dataset.
        prefix (str): Prefix for logging and cache filenames.

    Methods:
        __getitem__: Return transformed image and class index for the given sample index.
        __len__: Return the total number of samples in the dataset.
        verify_images: Verify all images in dataset.
    """

    def __init__(self, root: str, args, augment: bool = False, prefix: str = ""):
        """Initialize YOLO classification dataset with root directory, arguments, augmentations, and cache settings.

        Args:
            root (str): Path to the dataset directory where images are stored in a class-specific folder structure.
            args (Namespace): Configuration containing dataset-related settings such as image size, augmentation
                parameters, and cache settings.
            augment (bool, optional): Whether to apply augmentations to the dataset.
            prefix (str, optional): Prefix for logging and cache filenames, aiding in dataset identification.
        """
        import torchvision  # scope for faster 'import ultralytics'

        # Base class assigned as attribute rather than used as base class to allow for scoping slow torchvision import
        if TORCHVISION_0_18:  # 'allow_empty' argument first introduced in torchvision 0.18
            self.base = torchvision.datasets.ImageFolder(root=root, allow_empty=True)
        else:
            self.base = torchvision.datasets.ImageFolder(root=root)
        self.samples = self.base.samples
        self.root = self.base.root

        # Initialize attributes
        if augment and args.fraction < 1.0:  # reduce training fraction
            self.samples = self.samples[: round(len(self.samples) * args.fraction)]
        self.prefix = colorstr(f"{prefix}: ") if prefix else ""
        self.cache_ram = args.cache is True or str(args.cache).lower() == "ram"  # cache images into RAM
        if self.cache_ram:
            LOGGER.warning(
                "Classification `cache_ram` training has known memory leak in "
                "https://github.com/ultralytics/ultralytics/issues/9824, setting `cache_ram=False`."
            )
            self.cache_ram = False
        self.cache_disk = str(args.cache).lower() == "disk"  # cache images on hard drive as uncompressed *.npy files
        self.samples = self.verify_images()  # filter out bad images
        self.samples = [[*list(x), Path(x[0]).with_suffix(".npy"), None] for x in self.samples]  # file, index, npy, im
        scale = (1.0 - args.scale, 1.0)  # (0.08, 1.0)
        self.torch_transforms = (
            classify_augmentations(
                size=args.imgsz,
                scale=scale,
                hflip=args.fliplr,
                vflip=args.flipud,
                erasing=args.erasing,
                auto_augment=args.auto_augment,
                hsv_h=args.hsv_h,
                hsv_s=args.hsv_s,
                hsv_v=args.hsv_v,
            )
            if augment
            else classify_transforms(size=args.imgsz)
        )

def __getitem__(self, i: int) -> dict:
    """Return transformed image and class index for the given sample index.

    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[tuple]): 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

    # NOTE: ModuleNotFoundError to prevent numpy version conflicts when loading cache files created with different numpy versions
    except (FileNotFoundError, AssertionError, AttributeError, ModuleNotFoundError):
        # 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