2nd/10_Wiki/Topics/AI_and_ML/Data-Augmentation Strategies.md

---
id: wiki-2026-0508-data-augmentation
title: Data Augmentation Strategies
category: 10_Wiki/Topics
status: verified
canonical_id: self
aliases: [data augmentation, AutoAugment, RandAugment, MixUp, CutMix, back translation, Mosaic]
duplicate_of: none
source_trust_level: A
confidence_score: 0.93
verification_status: applied
tags: [data-augmentation, vision, nlp, audio, regularization, autoaugment, mixup, cutmix, generative-augment]
raw_sources: []
last_reinforced: 2026-05-10
github_commit: pending
tech_stack:
  language: Python
  framework: torchvision / Albumentations / Augly / nlpaug / Diffusers
---

# Data Augmentation

## 매 한 줄
> **"매 data 의 양 의 X — 매 모습 의 다양화"**. 매 invariance 의 학습 + 매 overfit 의 방지. 매 vision: rotation, flip, crop, MixUp, CutMix, AutoAugment. 매 NLP: back-translation, 매 LLM-aided. 매 modern: 매 generative augmentation (Stable Diffusion).

## 매 핵심 strategy

### Computer Vision
- **Geometric**: rotation, flip, crop, scale.
- **Color**: brightness, contrast, hue, saturation.
- **Noise**: Gaussian, salt-pepper.
- **MixUp**: 매 two image 의 linear combine.
- **CutMix**: 매 patch swap.
- **Cutout**: 매 random masking.
- **AutoAugment / RandAugment**: 매 learned policy.
- **TrivialAugment**: 매 random + 매 simple.
- **Mosaic** (YOLOv5+): 매 4 image 의 grid.

### NLP
- **Synonym replacement** (SR).
- **Random insertion / deletion / swap** (EDA).
- **Back-translation**: en → fr → en.
- **Paraphrase**: 매 LLM 의 generate.
- **Token noise**: 매 BERT-MLM-style.
- **Mixup-NLP**: 매 hidden representation mix.

### Audio
- **Speed / pitch shift**.
- **SpecAugment**: 매 time + 매 frequency mask.
- **Noise injection**.
- **Reverb / EQ**.
- **Mixup**.

### Tabular
- **SMOTE**: 매 minority class 의 synthetic.
- **Feature noise** (Gaussian).
- **Mixup-tabular**.

### Modern (Generative)
- **Diffusion-based augmentation**: Stable Diffusion 의 generate.
- **GAN-based**.
- **LLM-aided text**: 매 paraphrase / extend.
- **Domain randomization** (sim → real).

### 매 task-specific
- **Detection**: bbox-aware augment (Albumentations).
- **Segmentation**: mask-aware.
- **Pose**: keypoint-aware.
- **OCR**: distortion + perspective.

### 매 modern best practice
1. **Strong but realistic**: 매 over-augmented X.
2. **Test-time augmentation** (TTA): 매 inference 의 multiple view.
3. **AutoML for augmentation**: 매 task-specific policy.
4. **Curriculum**: 매 weak → strong.
5. **Domain awareness**: 매 vertical / horizontal flip 의 task 에 따른.

## 💻 패턴

### torchvision (vision)
```python
import torch
from torchvision import transforms

train_transform = transforms.Compose([
    transforms.RandomResizedCrop(224),
    transforms.RandomHorizontalFlip(),
    transforms.ColorJitter(0.4, 0.4, 0.4),
    transforms.RandomRotation(15),
    transforms.RandAugment(num_ops=2, magnitude=9),  # 매 modern default
    transforms.ToTensor(),
    transforms.Normalize([0.485, 0.456, 0.406], [0.229, 0.224, 0.225]),
])
```

### Albumentations (detection / segmentation)
```python
import albumentations as A
from albumentations.pytorch import ToTensorV2

transform = A.Compose([
    A.RandomResizedCrop(224, 224, scale=(0.8, 1.0)),
    A.HorizontalFlip(p=0.5),
    A.OneOf([
        A.GaussianBlur(),
        A.MotionBlur(),
    ], p=0.3),
    A.RandomBrightnessContrast(p=0.5),
    A.Normalize(mean=[0.485, 0.456, 0.406], std=[0.229, 0.224, 0.225]),
    ToTensorV2(),
], bbox_params=A.BboxParams(format='pascal_voc', label_fields=['class_labels']))

# 매 bbox + image 의 동시 transform.
```

### MixUp (loss-level)
```python
def mixup_data(x, y, alpha=0.2):
    lam = np.random.beta(alpha, alpha)
    idx = torch.randperm(x.size(0))
    mixed_x = lam * x + (1 - lam) * x[idx]
    return mixed_x, y, y[idx], lam

def mixup_loss(criterion, pred, y_a, y_b, lam):
    return lam * criterion(pred, y_a) + (1 - lam) * criterion(pred, y_b)

# 매 train loop
for x, y in loader:
    mixed_x, y_a, y_b, lam = mixup_data(x, y)
    pred = model(mixed_x)
    loss = mixup_loss(F.cross_entropy, pred, y_a, y_b, lam)
    loss.backward()
```

### CutMix
```python
def cutmix(x, y, alpha=1.0):
    lam = np.random.beta(alpha, alpha)
    idx = torch.randperm(x.size(0))
    
    H, W = x.size(2), x.size(3)
    cut_w = int(W * (1 - lam) ** 0.5)
    cut_h = int(H * (1 - lam) ** 0.5)
    cx, cy = np.random.randint(W), np.random.randint(H)
    
    x1, y1 = max(cx - cut_w // 2, 0), max(cy - cut_h // 2, 0)
    x2, y2 = min(cx + cut_w // 2, W), min(cy + cut_h // 2, H)
    
    x[:, :, y1:y2, x1:x2] = x[idx, :, y1:y2, x1:x2]
    lam = 1 - ((x2 - x1) * (y2 - y1) / (W * H))
    return x, y, y[idx], lam
```

### NLP — back-translation
```python
from transformers import pipeline

en_to_fr = pipeline('translation', model='Helsinki-NLP/opus-mt-en-fr')
fr_to_en = pipeline('translation', model='Helsinki-NLP/opus-mt-fr-en')

def back_translate(text):
    fr = en_to_fr(text, max_length=512)[0]['translation_text']
    return fr_to_en(fr, max_length=512)[0]['translation_text']

# 매 paraphrase 효과
augmented = back_translate(original)
```

### nlpaug (NLP utility)
```python
import nlpaug.augmenter.word as naw

# 매 contextual (BERT-based) synonym
aug = naw.ContextualWordEmbsAug(model_path='bert-base-uncased', action='substitute')
augmented = aug.augment('The quick brown fox jumps')
```

### LLM-aided augmentation
```python
def llm_paraphrase(text, n=5):
    prompt = f"""Paraphrase the following sentence in {n} different ways while preserving meaning:

Original: {text}

Output {n} paraphrases, each on a new line."""
    return llm.generate(prompt).split('\n')
```

### Audio (SpecAugment)
```python
import torchaudio.transforms as T

aug = torch.nn.Sequential(
    T.FrequencyMasking(freq_mask_param=30),
    T.TimeMasking(time_mask_param=80),
)

mel_spec_augmented = aug(mel_spec)
```

### SMOTE (tabular)
```python
from imblearn.over_sampling import SMOTE

smote = SMOTE(random_state=42)
X_resampled, y_resampled = smote.fit_resample(X, y)
```

### Diffusion-based augmentation
```python
from diffusers import StableDiffusionImg2ImgPipeline

pipe = StableDiffusionImg2ImgPipeline.from_pretrained('runwayml/stable-diffusion-v1-5').to('cuda')

# 매 original image + 매 prompt 의 variation
augmented = pipe(
    prompt='a {class_name} in different lighting / angle',
    image=original_image,
    strength=0.3,  # 매 small change
    num_inference_steps=20,
).images[0]
```

### Test-time augmentation (TTA)
```python
def tta_predict(model, image, n=5):
    """매 매 prediction 의 augment + 매 average."""
    augments = [normal_transform, flip_transform, crop1_transform, ...]
    preds = [model(aug(image)) for aug in augments[:n]]
    return torch.stack(preds).mean(dim=0)
```

## 매 결정 기준
| 상황 | Strategy |
|---|---|
| Image classification | RandAugment + MixUp |
| Detection | Albumentations + Mosaic |
| Segmentation | Mask-aware augment |
| NLP | Back-translation + LLM paraphrase |
| Audio | SpecAugment |
| Imbalanced tabular | SMOTE |
| Long-tail vision | Class-balanced augment |
| Generative augment | Diffusion (img2img) |

**기본값**: RandAugment / TrivialAugment + MixUp/CutMix (vision). LLM paraphrase (NLP).

## 🔗 Graph
- 부모: [[Data-Engineering]] · [[L1-and-L2-Regularization|Regularization]]
- 변형: [[MixUp]] · [[CutMix]] · [[AutoAugment]] · [[Back-Translation]] · [[SMOTE]]
- Adjacent: [[Bias vs Variance Trade-off]] · [[Cross-Entropy Loss]] · [[CV_Synthesis]] · [[Antifragility]]

## 🤖 LLM 활용
**언제**: 매 ML training. 매 small dataset. 매 imbalanced. 매 robustness 필요.
**언제 X**: 매 already strong model + abundant data.

## ❌ 안티패턴
- **Test set 의 augment**: 매 leakage.
- **Over-augment** (training + test 의 distribute mismatch).
- **Wrong domain augmentation** (e.g., flipping a "B" → "ⳝ" wrong text).
- **No bbox-aware** (detection): 매 wrong label.
- **MixUp 의 label 의 hard target 의 keep**: 매 wrong loss.
- **Generative augment 의 OOD**: 매 noise.

## 🧪 검증 / 중복
- Verified (Cubuk AutoAugment, Zhang MixUp, DeVries CutOut, SpecAugment).
- 신뢰도 A.
- Related: [[Bias vs Variance Trade-off]] · [[Cross-Entropy Loss]] · [[CV_Synthesis]] · [[Computer_Vision]] · [[Antifragility]].

## 🕓 Changelog
| 날짜 | 변경 |
|---|---|
| 2026-05-08 | Phase 1 |
| 2026-05-10 | Manual cleanup — strategy + 매 torchvision / Albumentations / MixUp / back-translate / TTA code |