Reference for ultralytics/data/dataset.py

def build_transforms(self, hyp=None):
    """Builds and appends transforms to the list."""
    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
        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("./labels.cache")):
    """
    Cache dataset labels, check images and read shapes.

    Args:
        path (Path): Path where to save the cache file. Default is Path('./labels.cache').

    Returns:
        (dict): labels.
    """
    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),
            ),
        )
        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}WARNING ⚠️ 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):
    """Sets mosaic, copy_paste and mixup options to 0.0 and builds transformations."""
    hyp.mosaic = 0.0  # set mosaic ratio=0.0
    hyp.copy_paste = 0.0  # keep the same behavior as previous v8 close-mosaic
    hyp.mixup = 0.0  # keep the same behavior as previous v8 close-mosaic
    self.transforms = self.build_transforms(hyp)

@staticmethod
def collate_fn(batch):
    """Collates data samples into batches."""
    new_batch = {}
    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 == "img":
            value = torch.stack(value, 0)
        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):
    """Returns dictionary of labels for YOLO training."""
    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:
        LOGGER.warning(f"WARNING ⚠️ No images found in {cache_path}, training may not work correctly. {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"WARNING ⚠️ 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"WARNING ⚠️ No labels found in {cache_path}, training may not work correctly. {HELP_URL}")
    return labels

def update_labels_info(self, label):
    """
    Custom your label format here.

    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:
        # list[np.array(1000, 2)] * num_samples
        # (N, 1000, 2)
        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=None):
    """Enhances data transformations with optional text augmentation for multi-modal training."""
    transforms = super().build_transforms(hyp)
    if self.augment:
        # NOTE: hard-coded the args for now.
        transforms.insert(-1, RandomLoadText(max_samples=min(self.data["nc"], 80), padding=True))
    return transforms

def update_labels_info(self, label):
    """Add texts information for multi-modal model training."""
    labels = super().update_labels_info(label)
    # NOTE: some categories are concatenated with its synonyms by `/`.
    labels["texts"] = [v.split("/") for _, v in self.data["names"].items()]
    return labels

def build_transforms(self, hyp=None):
    """Configures augmentations for training with optional text loading; `hyp` adjusts augmentation intensity."""
    transforms = super().build_transforms(hyp)
    if self.augment:
        # NOTE: hard-coded the args for now.
        transforms.insert(-1, RandomLoadText(max_samples=80, padding=True))
    return transforms

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

def get_labels(self):
    """Loads annotations from a JSON file, filters, and normalizes bounding boxes for each image."""
    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 = []
        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

            cat_name = " ".join([img["caption"][t[0] : t[1]] for t in ann["tokens_positive"]])
            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)
        lb = np.array(bboxes, dtype=np.float32) if len(bboxes) else np.zeros((0, 5), dtype=np.float32)
        labels.append(
            {
                "im_file": im_file,
                "shape": (h, w),
                "cls": lb[:, 0:1],  # n, 1
                "bboxes": lb[:, 1:],  # n, 4
                "normalized": True,
                "bbox_format": "xywh",
                "texts": texts,
            }
        )
    return labels

@staticmethod
def collate_fn(batch):
    """Collates data samples into batches."""
    return YOLODataset.collate_fn(batch)

def __getitem__(self, i):
    """Returns subset of data and targets corresponding to given indices."""
    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):
    """Verify all images in dataset."""
    desc = f"{self.prefix}Scanning {self.root}..."
    path = Path(self.root).with_suffix(".cache")  # *.cache file path

    try:
        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