ocr_stabilizer

A real-time stabilization engine for live OCR overlays. Tracks text block identity across noisy captures, corrects positional drift, and provides spatial indexing for deduplication.

Built for Flutter. Designed for OCR pipelines where screenshots are captured at 1-2 Hz and translated overlays must remain stable as the user scrolls.

The Problem

Live OCR on scrollable content produces a stream of noisy, jittery observations. The same paragraph appears at slightly different positions each capture. Without a stabilization layer, overlays flicker, duplicate, and drift.

This is the same problem visual SLAM (Simultaneous Localization and Mapping) solves in robotics: associate noisy sensor observations to persistent landmarks, correct accumulated drift, and maintain a consistent map. ocr_stabilizer adapts SLAM techniques to the OCR domain.

Installation

dependencies:
  ocr_stabilizer: ^0.5.0

What's new in 0.5.0 — additive surface only; safe upgrade from 0.4.x: a typed BandPredicateException surfaces consumer-supplied predicate throws instead of swallowing them, a new rejectedTextBand counter makes the band funnel decomposable, and an internal assertConfidenceRange utility centralises the [0.0, 1.0] check across DefaultTrackedBlock, MergeResult, the engine guards, and the PositionConfidence.from / TextConfidence.from factories.

0.4.0 introduced the band-fallback path — see BandFallbackConfig below. Default BandFallbackMode.off keeps the upgrade backwards-compatible.

0.4.0 also tightened Confidence validation — stabilize(), merge(), and DefaultTrackedBlock's ctor now throw ArgumentError on NaN or out-of-[0.0, 1.0] confidences. Consumers going through .from() factories were already covered. See the CHANGELOG for migration details.

Getting Started

The fastest path is DefaultTrackedBlock<T> — a concrete reference implementation with documented defaults for every required field, including the load-bearing ones like carouselIdVotes: {-1: 1} that need careful initialization.

import 'package:ocr_stabilizer/ocr_stabilizer.dart';

final engine = StabilizationEngine<DefaultTrackedBlock<MyPayload>, MyPayload>(
  merger: (existing, fresh, merge) => existing.applyMerge(merge),
);

// Each capture:
final blocks = ocrResults.map((ocr) => DefaultTrackedBlock<MyPayload>(
  absoluteRect: ocr.absoluteRect,
  originalText: ocr.text,
  payload: ocr.payload,
  positionConfidence: PositionConfidence.from(ocr.posConf),
  textConfidence: TextConfidence.from(ocr.txtConf),
)).toList();

final result = engine.stabilize(blocks);
// stabilize() rebuilds engine.spatialIndex internally — no caller action.

See example/example.dart for a runnable version.

For app-specific block types not covered by DefaultTrackedBlock, implement TrackedBlock<T> directly — see the next section.

BandFallback: the band-relaxed matching path

OCR jitter — one character flipped or one ligature mis-segmented — can drop a stable block below the primary text-similarity floor for a single frame. BandFallbackConfig opens a relaxed second-pass match path so spatially- unambiguous blocks don't "blink off and back on."

final engine = StabilizationEngine<DefaultTrackedBlock<MyPayload>, MyPayload>(
  merger: (existing, fresh, merge) => existing.applyMerge(merge),
  // Opt in: start in observeOnly to read counters, then flip to admit.
  bandFallback: const BandFallbackConfig(mode: BandFallbackMode.observeOnly),
);

// After a few captures, inspect the counters before flipping to admit.
// Note: in admit mode, once a band candidate is locked for a fresh
// observation, subsequent candidates skip band evaluation — so
// candidatesConsidered is mode-variant (observeOnly will show a higher
// figure). The funnel terms (rejectedCandidateFloor + rejectedSpatial
// + rejectedTextBand + bandMatchesIdentified == candidatesConsidered)
// are themselves mode-invariant — every term ticks before the
// early-exit fires.
final s = engine.bandStats;
print('primary admits=${s.primaryMatchesAdmitted}, '
      'primary misses=${s.primaryMatchesRejected}, '
      'candidates considered=${s.candidatesConsidered}, '
      'band would-admit=${s.bandMatchesIdentified}, '
      'rejected obs-floor=${s.rejectedCandidateFloor}, '
      'rejected spatial=${s.rejectedSpatial}, '
      'rejected text-band=${s.rejectedTextBand}, '
      'matches admitted=${s.matchesAdmitted}');

Recommended adoption flow for callers that want band coverage: ship with off (the default — a ^0.5.0 upgrade is a no-op), switch to observeOnly to read the counters in production, then flip to admit once the ratios justify it. Staying on off permanently is also valid — it disables the band path entirely and pays no extra cost.

Core Components

TrackedBlock<T>

The engine's central interface. Every block the engine processes implements this.

class MyBlock implements TrackedBlock<MyPayload> {
  @override final AbsoluteRect absoluteRect;
  @override final String originalText;
  @override final ContainerId? containerId;
  @override final bool isViewportRelative;
  @override final bool isInnerScrollerChild;
  @override final double innerScrollerTop;
  @override final bool isHorizontalScrollChild;
  @override final ScrollContext scrollContext;
  @override final bool isFromStickyElement;
  @override final StickyFallback stickyFallback;
  @override final PositionConfidence positionConfidence;
  @override final TextConfidence textConfidence;
  @override final int sourceQuality;
  @override final MyPayload payload;  // opaque — engine carries but never reads
}

For the stabilization pipeline (vote accumulation, provisional state, SAR-merge history), implement ObservableBlock<T> instead — it extends TrackedBlock<T> with 8 more getters. Most integrators want DefaultTrackedBlock<T> rather than rolling their own.

The generic T carries app-specific data (translations, styles) without coupling the engine to your domain types.

DriftTracker

Tracks positional drift per coordinate-space region. OCR positions jitter between captures due to scroll timing, viewport changes, and sensor noise. DriftTracker accumulates observations and computes a robust median correction per region.

final drift = DriftTracker();

// Record a drift observation
drift.addObservation(block, measuredDrift);

// Query the correction for a region
final correction = drift.medianDriftForKey(spaceKey);

// Apply correction to a fresh observation
final corrected = DriftTracker.applyCorrectedPosition(rect, correction);

Key properties:

• Bounded corrections: Drift is clamped to the median block height per region — the engine can never shift a block farther than a typical line of text.
• Rolling window: Keeps the last 20 observations per region, so drift adapts to changing conditions.
• Submap isolation: Normal page-scroll and inner-scroller containers track drift independently via SpaceKey.

SpatialBlockIndex

Grid-cell spatial index for O(cells) overlap candidate lookup during deduplication. Blocks are indexed by their center position into adaptive grid cells.

final index = SpatialBlockIndex();
index.updateBucketSizes(viewportWidth: 1000, viewportHeight: 800);

index.add(block);
final nearby = index.candidates(queryBlock);
index.remove(block);

Three coordinate-space namespaces prevent cross-space false matches:

• Normal page-absolute blocks
• Viewport-relative (fixed/sticky) blocks (vr: prefix)
• Inner-scroller relative blocks (ic: prefix) — dual-indexed for both IC-to-normal and IC-to-IC comparisons.

HierarchyWeightX

Extension on TrackedBlock computing hierarchy weight from coordinate-space flags. Higher weight means more constrained coordinate space:

Tier	Weight	Meaning
Viewport-relative	40	Fixed/sticky — no scroll drift
Nested IC+carousel	30	Compound coordinate space
IC or carousel	20	Single-axis constraint
Normal	10	Unrestricted page scroll

Extension Types

Zero-cost compile-time wrappers for coordinate safety:

• AbsoluteRect — wraps Rect for world-space coordinates. Spatial operations (overlaps, expandToInclude) only accept other AbsoluteRect values, preventing accidental coordinate-space mixing.
• ContainerId — wraps String for stable container identity hashes.
• SpaceKey — wraps String with typed constructors (normal, ic, unknown) for drift observation coordinate spaces.

Six-Dimension Block Identity

A block's identity is a six-dimensional signature:

Dimension	What It Answers	Package Support
Textual	What does this text say?	originalText on TrackedBlock
Spatial	Where is it in the page?	absoluteRect, confidence scores
Relative	Which coordinate space?	SpaceKey, ContainerId
Semantic	What kind of element?	hierarchyWeight (extension)
Temporal	How much evidence?	observationCount (ObservableBlock)
Contextual	What context was it in?	ContextualInvalidationCheck (callback)

API Reference

Interfaces

Type	Purpose
TrackedBlock<T>	Core block contract (14 getters including the opaque payload)
ObservableBlock<T>	Extends TrackedBlock; adds observation history (8 getters: counts, votes, provisional state)
ClassificationInput	Platform-agnostic viewport geometry
CarouselInput	Carousel-specific geometry
SubmapMembership	Strategy for coordinate-space partitioning
ContextualInvalidationCheck	Callback for context-change detection

Components

Type	Purpose
StabilizationEngine<T, P>	SAR-merge, intra-batch dedup, contradiction detection
DriftTracker	Regional drift correction with submap isolation
SpatialBlockIndex	Grid-cell spatial index for overlap queries
BlockClassifierService	Classifies blocks into fixed / sticky / carousel / IC / normal
OverlapResolver	Spatial NMS with language-aware thresholds
BlockKeyGenerator	Position + text dedup keys with fuzzy neighbor matching
CssSubmapMembership	Default WebView submap partitioning
RobustStats	Robust statistics (median, MAD, IQR)
IqrOutlier	Tukey-fence outlier detection
TextDedupUtils	Levenshtein, Jaccard, CJK detection helpers

BandFallback (v0.4.0+)

Type	Purpose
BandFallbackConfig	Configures the band-relaxed matching path. Default mode: off.
BandFallbackMode	off (no band loop) / observeOnly (counters only) / admit (production).
BandFallbackStats	Read-only per-capture telemetry exposed via engine.bandStats.
BandSpatialPredicate	Optional bool Function(TrackedBlock fresh, TrackedBlock candidate) injection. null → engine substitutes a drift-aware overlapRatio >= 0.80 closure.
BandPredicateException	Typed wrapper for consumer-predicate throws (v0.5.0+) — caught and rewrapped by the engine so failures surface with a typed shape, never swallowed. Original predicate stack lives on predicateStackTrace.

Reference Implementations

Type	Purpose
DefaultTrackedBlock<T>	Concrete ObservableBlock<T> with documented defaults, copyWith, and applyMerge(MergeResult) — the fastest path for new integrators

Result Types

Type	Purpose
StabilizationResult<T>	Output of engine.stabilize() — stable blocks + bookkeeping
MergeResult	Exhaustive engine-computed delta passed to BlockMerger
ClassificationResult	Output of BlockClassifierService

Value Types

Type	Purpose
ScrollContext	Scroll offsets and carousel identity at capture time
StickyFallback	Fallback coordinate context for demoted sticky elements
TextVote	Accumulated confidence evidence for one text variant

Extension Types

Type	Wraps	Purpose
AbsoluteRect	Rect	World-space coordinate safety
ContainerId	String	Stable container identity
SpaceKey	String	Typed drift observation keys
PositionConfidence	double	Position-accuracy confidence in [0, 1]
TextConfidence	double	OCR-text confidence in [0, 1]

Platform Support

The package depends on dart:ui (for Rect, Offset) and therefore requires the Flutter SDK. It has no platform-specific code — it works on Android, iOS, macOS, Windows, Linux, and Web.

The SubmapMembership and ClassificationInput interfaces allow the engine to support different input sources:

Platform	SubmapMembership	ClassificationInput
WebView	CssSubmapMembership (default)	CaptureSnapshotAdapter (app-side)
PDF	Custom (page-based submaps)	Custom (page geometry)
Camera	Custom (frame regions)	Custom (camera frame)

Contributing

See CONTRIBUTING.md for dev setup, conventions, and the release flow.