code/framework/data/transforms.py

"""Albumentations-based transform builder (the conventional-augmentation tier).

Key correctness guarantees for segmentation:
  * masks always use NEAREST interpolation (integer class ids never blended);
    every geometric transform sets mask_interpolation=cv2.INTER_NEAREST.
  * image and mask receive the SAME random spatial parameters (Albumentations
    applies one transform jointly to image= and mask=).

`aug` presets:  none (resize+normalize only) | standard | strong.
Returns a callable: (image HWC uint8, mask HW int) -> (FloatTensor[C,H,W], LongTensor[H,W]).
"""
from __future__ import annotations

from typing import Tuple

import cv2
import numpy as np
import torch
import albumentations as A
from albumentations.pytorch import ToTensorV2


_IMAGENET_MEAN = (0.485, 0.456, 0.406)
_IMAGENET_STD = (0.229, 0.224, 0.225)


def _normalize(in_channels: int, normalize: str) -> A.Normalize:
    if normalize == "none":
        mean = (0.0,) * in_channels
        std = (1.0,) * in_channels
    elif normalize == "imagenet" and in_channels == 3:
        mean, std = _IMAGENET_MEAN, _IMAGENET_STD
    else:  # auto
        if in_channels == 3:
            mean, std = _IMAGENET_MEAN, _IMAGENET_STD
        else:
            mean, std = (0.5,) * in_channels, (0.5,) * in_channels
    return A.Normalize(mean=mean, std=std, max_pixel_value=255.0)


def build_transform(img_size: int, in_channels: int, train: bool,
                    aug: str = "standard", normalize: str = "auto"):
    N = cv2.INTER_NEAREST
    L = cv2.INTER_LINEAR
    ops = []

    if train and aug != "none":
        ops += [
            A.Resize(img_size, img_size, interpolation=L, mask_interpolation=N),
            A.HorizontalFlip(p=0.5),
            A.VerticalFlip(p=0.5),
            A.Affine(scale=(0.9, 1.1), translate_percent=(0.0, 0.05),
                     rotate=(-15, 15), interpolation=L, mask_interpolation=N, p=0.5),
        ]
        if aug == "strong":
            ops += [
                A.ElasticTransform(alpha=30, sigma=6, interpolation=L,
                                   mask_interpolation=N, p=0.3),
                A.GridDistortion(num_steps=5, distort_limit=0.2, interpolation=L,
                                 mask_interpolation=N, p=0.3),
                A.RandomBrightnessContrast(p=0.5),
                A.GaussNoise(p=0.2),
            ]
            if in_channels == 3:
                ops.append(A.CLAHE(p=0.2))
    else:
        ops.append(A.Resize(img_size, img_size, interpolation=L, mask_interpolation=N))

    ops += [_normalize(in_channels, normalize), ToTensorV2()]
    compose = A.Compose(ops)

    def _apply(image: np.ndarray, mask: np.ndarray) -> Tuple[torch.Tensor, torch.Tensor]:
        out = compose(image=image, mask=mask)
        img = out["image"].float()                 # C,H,W
        msk = out["mask"].long()                    # H,W
        return img, msk

    return _apply