faster-paddle

Fast, CPU-only OCR in Rust with Python bindings — a self-contained reimplementation of PaddleOCR's PP-OCRv6 detection + recognition pipeline powered by ONNX Runtime.

• ⚡ CPU latency optimizations: fused image transforms, compact DB components, dynamic recognition scheduling, and model-specific input widths.
• 📦 Self-contained — the tiny + small ONNX models are bundled inside the wheel. No paddlepaddle, no model downloads for tiny/small.
• 🎚️ Three model sizes: tiny (default, fastest), small, and medium (higher accuracy; downloaded once on first use and cached).
• 🦀 Pure-Rust pre/post-processing (detection DB decode, minAreaRect, perspective crop, CTC decode, reading-order text reconstruction). No OpenCV.
• 🖥️ Prebuilt wheels for Linux, Windows, macOS (x86-64 + arm64).

Defaults automatically respect available physical cores, Linux CPU affinity, and container CPU limits. Reuse an engine to amortize model/session loading. See performance results and tradeoffs and the benchmark harnesses for reproducible measurements.

Install

pip install faster-paddle

Usage

import faster_paddle

# One-shot, using a shared default engine (lazily initialized):
with open("document.jpg", "rb") as f:
    result = faster_paddle.ocr(f.read())

print(result["text"])              # reading-order reconstructed text
for idx, b in result["bounds"].items():
    print(idx, b["text"], b["confidence"], b["topLeftCoord"], b["bottomRightCoord"])

Reuse an explicit engine (recommended for servers — load the models once):

from faster_paddle import OcrEngine

# model_size: "tiny" (default), "small", or "medium"
engine = OcrEngine(model_size="tiny", threads=None, det_max_side=1600)

result = engine.ocr(image_bytes)                 # raw jpeg/png/webp/bmp/tiff/gif bytes
result = engine.ocr_base64(b64_string)           # base64-encoded image

Optional preprocessing

ocr / ocr_base64 take four optional flags (all default False), applied — when enabled — in the optimal order, all in fast parallel Rust:

result = engine.ocr(
    image_bytes,
    resize=True,     # 1. downscale source to ≤ 2100×3000 (aspect preserved) if larger
    denoise=True,    # 2. fast Non-Local-Means denoise (grayscale)
    deskew=True,     # 3. detect skew (Canny + Hough) and rotate to straighten
    binarize=True,   # 4. Sauvola adaptive thresholding (clean black/white)
)

Order rationale: resize first (everything downstream is then faster), denoise before angle detection and thresholding, deskew on the cleaned image, binarize last to produce the final B/W. resize can reduce source/crop work, but may add an extra resampling pass when detection already hits its size cap; benchmark it on your input. Any of denoise/deskew/binarize converts the image to grayscale.

Returned bounds are always in the original image's coordinate space — even when resize or deskew changes the working image, the boxes are mapped back so they line up with your input.

Preprocess only (no OCR)

prepare runs the same preprocessing in one pass and returns the prepared image as PNG bytes (grayscale once any of denoise/deskew/binarize is on, else color). If every option is False the original bytes are returned unchanged.

prepared = engine.prepare(image_bytes, resize=True, denoise=True, deskew=True, binarize=False)
# or module-level:  faster_paddle.prepare(image_bytes, resize=True, ...)

with open("prepared.png", "wb") as f:
    f.write(prepared)
# you can also feed it straight back in:
result = engine.ocr(prepared)

Model sizes

tiny and small are embedded in the wheel (offline). medium exceeds PyPI's file-size limit, so the first OcrEngine(model_size="medium") downloads it once (needs network that time only) and caches it for subsequent runs.

Result shape

{
  "text": "full reconstructed text...",
  "structured_text": "layout-preserving text (see below)",
  "bounds": {
     0: {
        "topLeftCoord":     (x1, y1),
        "bottomRightCoord": (x2, y2),
        "text":             "line text",
        "confidence":       0.97,
     },
     1: { ... },
  }
}

text and bounds match the JSON contract of the original paddle-ocr-api service, so it is a drop-in replacement.

structured_text

A spatial reconstruction that reads left-to-right, top-to-bottom while preserving the visual layout: vertical whitespace gaps split the page into columns/panes (each read fully before the next), and within each one the rows are laid out as a monospace grid, so indentation (tree nesting) and aligned sub-columns (key/value tables) are kept. Single-glyph UI icon noise is dropped.

Use structured_text for screenshots, forms, table/tree UIs, and code — anything where spatial structure carries meaning. Use text for dense multi-column prose: there the absolute pixel spacing of structured_text produces very wide lines, so the column-merging text reconstruction reads better. Both are always returned, so you can pick per use case.

Example structured_text for a two-pane file-tree + settings UI:

Project
 src (14)
   main.rs
   parser.rs
   utils.rs
 tests
 docs

Setting                                            Value
        max_connections                            128
        request_timeout_seconds                    30
        cache_size_mb                              512

API

• resize/denoise/deskew/binarize: optional preprocessing (see above).
• model_size: "tiny" (default), "small", or "medium".
• threads: total CPU budget; defaults to available physical cores, limited by affinity and OS/container quotas. An explicit positive value overrides discovery.
• rec_batch: actual maximum crops per recognition tensor (default 1). Independent crops share the worker pool. Larger caps are available for tuning.
• det_max_side: detector long-side limit (default 1600); recognition crops still come from the original source. Lower values can miss small text.
• det_min_side: minimum detector short side (default 736). Set 0 to disable minimum-side upscaling. Dimensions are still rounded to multiples of 32.
• rec_min_width: recognition padding floor, automatically 64 for tiny, 96 for small, and 320 for medium. Set 320 for the reference padding behavior. Shorter padding changes context and can change text/confidence; validate your languages and documents when migrating.
• engine.config: dictionary of resolved thread counts, pool size and input limits, for diagnostics and reproducible benchmarks.

Calls release the GIL. Inference on a shared engine is serialized; decode and layout can overlap. Independent engines have independent CPU budgets, so set threads explicitly when running several engines concurrently.

Multiple images

engine = OcrEngine(model_size="small")
results = engine.ocr_batch([image_a_bytes, image_b_bytes, image_c_bytes])
# results[i] corresponds to input i and has the same shape as engine.ocr(...).
# Module-level faster_paddle.ocr_batch([...]) uses the shared tiny engine.

ocr_batch(images, *, batch_size=4, resize=False, denoise=False, deskew=False, binarize=False) processes bounded windows of images. It decodes/preprocesses in parallel and schedules recognition crops across pages. Each page keeps its own detector resolution and coordinate transform. batch_size controls the number of decoded pages resident in a window; it is independent of rec_batch. Input bytes are borrowed when possible. An empty list returns []; invalid images raise an error with their zero-based input index.

Batching helps fill recognition workers on sparse pages and can improve throughput. It is not guaranteed to accelerate dense pages, and the caller waits for the complete result list. Use ocr for minimum time to the first page's result. Minor floating-point confidence differences can occur between the single-line and multi-page execution paths.

Parallelism

• The detector uses up to 8 threads within the CPU budget.
• Recognition uses up to one worker per available physical core, capped at 32 (8 for medium) to limit model memory. Jobs are scheduled dynamically, largest estimated jobs first. Sparse long-line jobs use a separate pool of up to four sessions with up to four threads each, within the same CPU budget. Session construction is parallel in bounded groups to reduce startup latency.
• An engine-local Rayon pool handles image and geometry work within the same CPU budget. ONNX workers spin during inference to reduce wakeup latency, and stop spinning immediately when their inference call finishes.
• Input tensor buffers are reused. Recognition budgets include actual padding.

Advanced overrides (read at engine construction): OCR_THREADS, OCR_DET_THREADS, REC_POOL, REC_BUDGET, RAYON_NUM_THREADS, OCR_REC_MIN_WIDTH, OCR_DET_MIN_SIDE, OCR_MEMPAT, OCR_PREPACK, OCR_DET_SPIN, OCR_REC_SPIN, OCR_APPROX_GELU. Approximate GELU stays off: its measured end-to-end benefit was inconsistent and it changed a detection. Constructor arguments take precedence over their corresponding environment values; detector/worker/Rayon counts are capped by the total CPU budget. The CPU-count defaults are measured heuristics, not online autotuning; benchmark other CPU architectures with benchmarks/cpu_latency.py before overriding them.

How it works

The pipeline faithfully mirrors PaddleOCR's lightweight path:

1. Detection — resize (min-side 736, cap the longer side at det_max_side = 1600 by default vs PaddleOCR's 4000, round to ×32), normalize (BGR mean/std), run the DB detector. Lower detector resolution reduces work but can miss small text; recognition still crops from the full-res image.
2. DB post-process — threshold 0.2, connected components, minAreaRect, box score ≥ 0.4, unclip ratio 1.4, rescale to source coordinates.
3. Sort boxes top-to-bottom / left-to-right; crop each via perspective warp.
4. Recognition — resize each crop to H=48, normalize, batch, run the CTC recognizer (6,906 output classes for tiny, 18,710 for small), greedy CTC decode.
5. Reconstruct reading-order text with dynamic column/line detection.

Detection matches PaddlePaddle at 96 % IoU>0.5 with 0.93 character-level similarity on the recognized text; the residual difference is ONNX-Runtime vs PaddlePaddle floating-point numerics, not the algorithm.

The bundled tiny and small PP-OCRv6 models were exported with paddle2onnx.

Building from source

pip install maturin
maturin develop --release      # build + install into the current environment
# or
maturin build --release        # produce a wheel in target/wheels/

Requires a Rust toolchain. ONNX Runtime is fetched automatically by the ort crate at build time and linked into the extension.

Tests

cargo test --release                 # Rust unit tests (geometry, resize, CTC)
maturin develop --release            # then the Python integration tests:
python -m pytest tests -q

The tests cover geometry equivalence, fused resize/padding, recognition caps, known text, both small models, batch ordering and transforms, buffer reuse, concurrent calls, error recovery, CPU affinity/budgets, and preprocessing. CI runs Rust and Python tests before publishing. Install pytest and pillow for the Python suite. Performance comparisons live under benchmarks/; the small bundled test corpus is not a comprehensive OCR accuracy benchmark.

License

MIT