Which OCR engine

Which OCR engine

When a PDF page is a scan, --pdf-ocr has an OCR engine read it. --ocr-engine chooses which one. Four run on your own machine; they differ in what they read well, what they install and whether they download anything.

python make_book.py --book_name scan.pdf --to-epub --pdf-ocr \
  --ocr-engine ocrmac --ocr-lang iso:en --language zh-hans

Try two pages first (--pages 1-2) and read source.md in the bundle before you pay for a translation. The run prints the engine it used (OCR engine: …).

If you need X, use Y

your situation	use	why
A Mac, and you do not want to download anything	ocrmac (already there with the pdf extra)	Apple's own engine, part of macOS: no model download. The fastest in the measurement (6 s per two pages) and second-best on English, tied with the best on typewriter and clean pages. Weaker than rapidocr on simplified Chinese.
You have not chosen	auto (the default)	Takes the first installed of ocrmac, rapidocr, easyocr, and names it. With the pdf extra that is ocrmac on a Mac and rapidocr elsewhere.
Simplified Chinese	rapidocr	Best on simplified Chinese: mean Han-character error 0.054, against ocrmac 0.097 and tesseract 0.116.
Traditional Chinese (horizontal)	ocrmac or rapidocr	Close on the one page measured (0.043 and 0.060). tesseract (0.350) and easyocr (0.724) were far behind. Vertical text comes out in the wrong column order with any engine: check source.md.
Japanese or Korean	ocrmac or rapidocr, with --ocr-lang iso:ja or iso:ko	Not measured. Both engines have these languages; tesseract needs the jpn or kor language file installed.
English typewriter or other degraded scans	tesseract or ocrmac	Tied on typewriter pages (error 0.082 and 0.086; rapidocr 0.206). On old print and a magazine, tesseract was clearly ahead (0.222; ocrmac 0.361, rapidocr 0.383).
A machine without a GPU	anything but easyocr	Every number here was measured on the CPU. easyocr took 37 s per two pages (others 6 to 12 s) and up to 6.6 GB of memory.
Linux or Windows	tesseract for English, rapidocr for Chinese	ocrmac exists only on macOS.
A script none of the above reads	easyocr	It has models for 80+ languages. In the measurement it was last on English and Chinese and dropped text.

Installing each engine

engine	install	downloads on first use	size
rapidocr	Comes with the pdf extra, with onnxruntime.	Nothing: its models are inside the package.	rapidocr 31 MB, onnxruntime 62 MB
ocrmac	Comes with the pdf extra on macOS (macOS only).	Nothing.	28 MB
easyocr	pip install easyocr	A detector (83 MB) and one model per script: English 15 MB, Latin 15 MB, simplified Chinese 22 MB, traditional Chinese 226 MB, into ~/.EasyOCR/model.	about 150 MB of packages
tesseract	The tesseract program and its language files, on PATH: brew install tesseract on macOS, apt install tesseract-ocr tesseract-ocr-chi-sim on Debian and Ubuntu, the installer from the tesseract project on Windows.	Nothing: add a language by installing its file (eng 4 MB, chi_sim 2.5 MB, chi_tra 2.4 MB in the fast set).	25 MB program plus its libraries

All four work off-line once installed and, for easyocr, once its models are downloaded.

An engine you name that is not installed, or ocrmac on Linux or Windows, is refused before any page is read, with the line that installs it. auto is never refused.

If you keep tesseract's language files in a folder of your own (TESSDATA_PREFIX), put tesseract's configs folder in it too: without it tesseract cannot write the table docling reads, and the extraction fails.

Language codes

--ocr-lang takes each engine's own codes, or a portable iso: tag that works on every engine. When you switch engines, the iso: tag is the one to use.

language	any engine	rapidocr	ocrmac	easyocr	tesseract
English	iso:en	en	en-US	en	eng
Simplified Chinese	iso:zh-Hans (or iso:zh)	ch	zh-Hans	ch_sim	chi_sim
Traditional Chinese	iso:zh-Hant	chinese_cht	zh-Hant	ch_tra	chi_tra
Japanese	iso:ja	japan	ja-JP	ja	jpn
Korean	iso:ko	korean	ko-KR	ko	kor

rapidocr reads one language per run (the first). Without --ocr-lang each engine reads its own defaults: rapidocr Chinese and English, ocrmac, easyocr and tesseract English, Spanish, French and German. A scan in another script then comes out wrong.

What was measured

20 scanned pages (14 English, 6 Chinese), the same language asked of each engine, all on the CPU. Error is the character error rate against a typed transcript: 0 is perfect, and it counts a paragraph put in the wrong place as errors.

pages	rapidocr	ocrmac	easyocr	tesseract
English typewriter (3)	0.206	0.086	0.100	0.082
English magazine and old print (3)	0.383	0.361	0.609	0.222
English, clean synthetic scan (4)	0.069	0.019	0.070	0.019
English, degraded synthetic scan (4)	0.080	0.124	0.333	0.116
Simplified Chinese book scan (1), Han characters	0.009	0.056	0.235	0.036
Traditional Chinese scan (1), Han characters	0.060	0.043	0.724	0.350
Simplified Chinese synthetic scans (4), Han characters	0.066	0.107	0.709	0.136
Time per two pages (median)	12 s	6 s	37 s	10 s

Who won where:

• English: tesseract, with ocrmac close behind. On typewriter and clean pages the two are tied (differences under 0.005). tesseract's lead is real on old print. On the degraded synthetic pages rapidocr's lower number comes from reading order: ignoring order, all three read the words equally well.
• Simplified Chinese: rapidocr, on the mean and on 3 of 5 pages; ocrmac was ahead on one clean synthetic page, ocrmac and tesseract on one degraded one.
• Traditional Chinese: ocrmac and rapidocr, within noise of each other on one page.
• easyocr was last on every class but typewriter, the slowest, and the heaviest in memory.

Every number, the pages, the scorer and the commands: OCR engines on scanned pages.