---
id: wiki-2026-0508-optical-character-recognition
title: Optical Character Recognition
category: 10_Wiki/Topics
status: verified
canonical_id: self
aliases: [OCR, Text Recognition, Document AI]
duplicate_of: none
source_trust_level: A
confidence_score: 0.9
verification_status: applied
tags: [ocr, document-ai, tesseract, paddleocr, donut, vision-language]
raw_sources: []
last_reinforced: 2026-05-10
github_commit: pending
tech_stack: { language: python, framework: paddleocr/transformers }
---

# Optical Character Recognition

## 한 줄
이미지·스캔·PDF에서 텍스트를 추출하는 기술 — 전통 CRNN 파이프라인부터 modern Donut/TrOCR/Surya 및 VLM 직접 추출까지.

## 핵심
- **Pipeline**: detection (DBNet/CRAFT) → recognition (CRNN/Transformer) → post-process(NMS, layout).
- **Engines**:
  - Tesseract (오래됨, 다국어, 무료, 정밀도 낮음)
  - **EasyOCR** (PyTorch, 80+ 언어)
  - **PaddleOCR** (강력, 중국어/한국어 우수)
  - **Surya** (modern multilingual, layout)
  - **TrOCR** (HF, 단어/라인 인식)
  - **Donut** (OCR-free document understanding)
- **VLM-OCR**: GPT-5 Vision, Claude Sonnet 4.7, Qwen2.5-VL — 복잡 양식·표·손글씨에서 강함.
- **Layout**: LayoutLMv3, DocFormer, Nougat(논문).
- 평가: CER(Character Error Rate), WER, F1 on key fields.

## 💻 패턴

```python
# 1. Tesseract — 빠른 baseline
# brew install tesseract tesseract-lang
import pytesseract
from PIL import Image

img = Image.open("invoice.png")
text = pytesseract.image_to_string(img, lang="eng+kor")
data = pytesseract.image_to_data(img, output_type=pytesseract.Output.DICT)
# data 안에 (text, conf, left, top, width, height) 포함
```

```python
# 2. EasyOCR — Python-only 다국어
# pip install easyocr
import easyocr

reader = easyocr.Reader(["en", "ko"])
results = reader.readtext("receipt.jpg", detail=1)
for box, text, conf in results:
    print(f"{conf:.2f}: {text}")
```

```python
# 3. PaddleOCR — 강력한 한·중·영
# pip install paddleocr paddlepaddle
from paddleocr import PaddleOCR

ocr = PaddleOCR(use_angle_cls=True, lang="korean", show_log=False)
res = ocr.ocr("doc.png", cls=True)
for line in res[0]:
    box, (txt, conf) = line
    print(txt, conf)
```

```python
# 4. Surya — modern, layout + reading order
# pip install surya-ocr
from surya.ocr import run_ocr
from surya.model.detection.model import load_model as det_model, load_processor as det_proc
from surya.model.recognition.model import load_model as rec_model
from surya.model.recognition.processor import load_processor as rec_proc
from PIL import Image

img = Image.open("page.png")
preds = run_ocr([img], [["ko", "en"]],
                det_model(), det_proc(),
                rec_model(), rec_proc())
for line in preds[0].text_lines:
    print(line.text, line.confidence)
```

```python
# 5. TrOCR (HuggingFace) — 라인 단위
from transformers import TrOCRProcessor, VisionEncoderDecoderModel
from PIL import Image

proc = TrOCRProcessor.from_pretrained("microsoft/trocr-large-printed")
model = VisionEncoderDecoderModel.from_pretrained("microsoft/trocr-large-printed")

img = Image.open("line.png").convert("RGB")
pix = proc(img, return_tensors="pt").pixel_values
ids = model.generate(pix)
print(proc.batch_decode(ids, skip_special_tokens=True)[0])
```

```python
# 6. Donut — OCR-free document QA / parsing
from transformers import DonutProcessor, VisionEncoderDecoderModel
import torch

proc = DonutProcessor.from_pretrained("naver-clova-ix/donut-base-finetuned-cord-v2")
model = VisionEncoderDecoderModel.from_pretrained("naver-clova-ix/donut-base-finetuned-cord-v2")

prompt = "<s_cord-v2>"
ids = proc.tokenizer(prompt, add_special_tokens=False, return_tensors="pt").input_ids
pix = proc(Image.open("receipt.png").convert("RGB"), return_tensors="pt").pixel_values
out = model.generate(pix, decoder_input_ids=ids, max_length=512)
print(proc.token2json(proc.batch_decode(out)[0]))
# {"menu":[...], "total":...} 직접 JSON 출력
```

```python
# 7. VLM 직접 OCR (Claude Vision)
import anthropic, base64

img_b64 = base64.b64encode(open("scan.png", "rb").read()).decode()
client = anthropic.Anthropic()
resp = client.messages.create(
    model="claude-sonnet-4-7",
    max_tokens=2048,
    messages=[{"role": "user", "content": [
        {"type": "image", "source": {"type": "base64",
            "media_type": "image/png", "data": img_b64}},
        {"type": "text", "text":
            "Extract all text. Output JSON with fields: title, date, line_items[{name, qty, price}], total."},
    ]}],
)
print(resp.content[0].text)
```

```python
# 8. PDF → 텍스트 (pdfplumber + OCR fallback)
import pdfplumber, pytesseract
from pdf2image import convert_from_path

def pdf_to_text(path):
    out = []
    with pdfplumber.open(path) as pdf:
        for i, page in enumerate(pdf.pages):
            t = page.extract_text() or ""
            if len(t.strip()) < 20:  # scanned page → OCR
                img = convert_from_path(path, first_page=i+1, last_page=i+1)[0]
                t = pytesseract.image_to_string(img)
            out.append(t)
    return "\n\n".join(out)
```

## 결정 기준

| 상황 | 추천 |
|---|---|
| 빠른 prototype, 인쇄 영문 | Tesseract |
| 다국어 일반 | PaddleOCR / EasyOCR |
| 한국어 정밀 | PaddleOCR(ko) / Surya |
| 현대 다국어 + layout | Surya |
| 라인 단위 정밀 | TrOCR |
| 영수증·송장·양식 → 구조 JSON | Donut / VLM |
| 복잡 표·손글씨 | Claude Sonnet / GPT-5 Vision |
| 학술 PDF (수식) | Nougat |
| 디지털 PDF | pdfplumber, OCR fallback |

## 🔗 Graph
- Related: `[[Document-AI]]`, `[[LayoutLM]]`, `[[Vision-Language-Models]]`, `[[Image-Preprocessing]]`, `[[PDF-Processing]]`, `[[Surya]]`, `[[PaddleOCR]]`

## 🤖 LLM 활용
- VLM은 OCR + reasoning 동시 수행 (영수증 → 항목별 카테고리 자동 분류).
- 전통 OCR + LLM 후처리: 오타·줄바꿈 normalize, 누락 필드 추론.

## ❌ 안티패턴
- 저화질 이미지를 그대로 입력 (전처리 필수: deskew, binarize, dpi≥300).
- Tesseract만 믿고 한국어 받침 깨진 채 배포.
- VLM에 큰 PDF를 한 번에 — chunk per page.
- confidence threshold 무시하고 raw text 사용.

## 🧪 검증
- Ground truth와 CER/WER 측정.
- Field-level F1 (date/total/total_tax 등 critical 필드).
- 다양한 폰트·해상도·회전 sample suite 유지.

## 🕓 Changelog
- 2026-05-08 Phase 1: 초안.
- 2026-05-10 Manual cleanup: 8 패턴, Surya/Donut/VLM 추가.
