logo_test/logo_detection_test_methodology.md

Logo Detection Test Methodology

This document describes how the logo detection test framework works and the various techniques implemented to improve detection accuracy.

Overview

The system uses a two-stage pipeline:

1. DETR (DEtection TRansformer) - Detects potential logo regions in images
2. CLIP (Contrastive Language-Image Pre-training) - Extracts feature embeddings for matching

Test Framework (test_logo_detection.py)

Test Flow

1. Sample Reference Logos: Randomly select N logos from the database, with multiple reference images per logo
2. Compute Reference Embeddings: Generate CLIP embeddings for all reference logo images
3. Build Test Set: For each sampled logo, select:
   • Positive samples: Images known to contain the logo
   • Negative samples: Images known NOT to contain the logo
4. Run Detection: Process each test image through DETR to find logo regions
5. Match Against References: Compare detected regions against reference embeddings using margin-based matching
6. Calculate Metrics: Compute precision, recall, and F1 score

Configurable Parameters

General Parameters

Parameter	Default	Description
--num-logos	10	Number of reference logos to sample
--refs-per-logo	3	Reference images per logo
--positive-samples	5	Positive test images per logo
--negative-samples	20	Negative test images per logo
--threshold	0.7	CLIP similarity threshold for matching
--detr-threshold	0.5	DETR detection confidence threshold
--seed	None	Random seed for reproducibility

Matching Method Selection

Parameter	Default	Description
--matching-method	margin	Matching method: margin or multi-ref
--margin	0.05	Required margin between best and second-best match (applies to both methods)

Multi-Ref Method Parameters (when --matching-method multi-ref)

Parameter	Default	Description
--min-matching-refs	1	Minimum references that must match above threshold
--use-max-similarity	False	Use max similarity instead of mean across references

Cache Control

Parameter	Default	Description
--no-cache	False	Disable embedding cache
--clear-cache	False	Clear cache before running

Metrics

• True Positives: Detected logo correctly matches expected logo
• False Positives: Detected logo matches wrong logo or image has no logo
• False Negatives: Expected logo not detected/matched
• Precision: TP / (TP + FP) - How many detections were correct
• Recall: TP / Total Expected - How many logos were found
• F1 Score: Harmonic mean of precision and recall

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Accuracy Improvement Techniques

1. Non-Maximum Suppression (NMS)

Location: logo_detection_detr.py:214-268

Problem: DETR may produce multiple overlapping bounding boxes for the same logo.

Solution: NMS removes redundant detections by:

1. Sorting detections by confidence score (descending)
2. Keeping the highest-scoring box
3. Removing any remaining boxes with IoU > threshold (default 0.5)
4. Repeating until no boxes remain

IoU (Intersection over Union) = Area of Overlap / Area of Union

Configuration: nms_iou_threshold parameter (default: 0.5)

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2. Minimum Box Size Filtering

Location: logo_detection_detr.py:187-191

Problem: Very small detections are often noise or partial logo fragments.

Solution: Filter out detections where width OR height is below a minimum threshold.

Configuration: min_box_size parameter (default: 20 pixels)

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3. Confidence Threshold Filtering

Location: logo_detection_detr.py:177-179

Problem: Low-confidence DETR detections are unreliable.

Solution: Only keep detections with confidence score >= threshold.

Configuration: detr_threshold parameter (default: 0.5)

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4. Multiple Reference Images Per Logo

Location: logo_detection_detr.py:397-457 (find_best_match_multi_ref)

Problem: A single reference image may not capture all variations of a logo (different angles, lighting, scales).

Solution: Use multiple reference images per logo and aggregate their similarity scores:

• Calculate similarity to each reference embedding
• Count how many references match above threshold
• Use mean or max similarity as the aggregate score
• Require a minimum number of references to match

Configuration:

• refs_per_logo: Number of reference images (default: 3)
• min_matching_refs: Minimum references that must match
• use_mean_similarity: Use mean vs max aggregation

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5. Margin-Based Matching

Location: logo_detection_detr.py:459-505 (find_best_match_with_margin)

Problem: When multiple logos have similar embeddings, the best match may not be significantly better than alternatives, leading to false positives.

Solution: Require the best match to exceed the second-best match by a minimum margin:

Match only if: best_similarity - second_best_similarity >= margin

This ensures confident matches and reduces ambiguous classifications.

Configuration: --margin parameter (default: 0.05)

Example:

• Best match: Logo A with similarity 0.82
• Second best: Logo B with similarity 0.79
• Margin required: 0.05
• Result: No match (0.82 - 0.79 = 0.03 < 0.05)

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6. Embedding Caching

Location: test_logo_detection.py:49-82 (EmbeddingCache class)

Problem: Computing CLIP embeddings is computationally expensive. Re-running tests would reprocess the same images.

Solution: Cache embeddings to disk using pickle:

• Reference embeddings keyed by ref:{filename}
• Detection results keyed by det:{filename}
• Cache persists between runs (.embedding_cache.pkl)

Configuration:

• --no-cache: Disable caching entirely
• --clear-cache: Clear cache before running

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7. Normalized Embeddings for Cosine Similarity

Location: logo_detection_detr.py:334-335

Problem: Raw CLIP embeddings have varying magnitudes, which can affect similarity calculations.

Solution: L2-normalize all embeddings before comparison:

features = F.normalize(features, dim=-1)

This ensures cosine similarity is computed correctly and scores fall in the range [-1, 1].

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Matching Methods Summary

Method	Test Script Option	Key Feature
find_best_match	N/A (library only)	Returns highest similarity above threshold
find_best_match_with_margin	--matching-method margin	Requires margin over second-best match
find_best_match_multi_ref	--matching-method multi-ref	Aggregates scores across reference images

The test script supports both margin and multi-ref matching methods via the --matching-method parameter.

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Detection Pipeline Summary

Input Image
    │
    ▼
┌─────────────────────────────────────┐
│  DETR Object Detection              │
│  - Identifies potential logo regions│
│  - Returns bounding boxes + scores  │
└─────────────────────────────────────┘
    │
    ▼
┌─────────────────────────────────────┐
│  Confidence Filtering               │
│  - Remove detections < threshold    │
└─────────────────────────────────────┘
    │
    ▼
┌─────────────────────────────────────┐
│  Size Filtering                     │
│  - Remove boxes < min_box_size      │
└─────────────────────────────────────┘
    │
    ▼
┌─────────────────────────────────────┐
│  CLIP Embedding Extraction          │
│  - Crop each detected region        │
│  - Generate normalized embedding    │
└─────────────────────────────────────┘
    │
    ▼
┌─────────────────────────────────────┐
│  Non-Maximum Suppression            │
│  - Remove overlapping detections    │
│  - Keep highest confidence boxes    │
└─────────────────────────────────────┘
    │
    ▼
┌─────────────────────────────────────┐
│  Matching (selectable method)       │
│  ┌───────────────┬────────────────┐ │
│  │ margin        │ multi-ref      │ │
│  ├───────────────┼────────────────┤ │
│  │ Require margin│ Aggregate      │ │
│  │ over 2nd best │ across refs    │ │
│  │ match         │ (mean or max)  │ │
│  └───────────────┴────────────────┘ │
└─────────────────────────────────────┘
    │
    ▼
Matched Logo Labels

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Tuning Recommendations

For Margin-Based Matching (--matching-method margin)

Goal	Adjustments
Reduce false positives	Increase --threshold, increase --margin
Reduce false negatives	Decrease --threshold, decrease --margin

For Multi-Ref Matching (--matching-method multi-ref)

Goal	Adjustments
Reduce false positives	Increase --threshold, increase --margin, increase --min-matching-refs, use mean similarity
Reduce false negatives	Decrease --threshold, decrease --margin, decrease --min-matching-refs, use --use-max-similarity

General Tuning

Goal	Adjustments
Faster processing	Decrease --refs-per-logo, use caching
More robust detection	Increase --refs-per-logo, decrease --detr-threshold
Higher precision	Increase --detr-threshold, use margin method with high margin
Higher recall	Decrease --detr-threshold, use multi-ref with low --min-matching-refs

---

Example Usage

# Default margin-based matching
python test_logo_detection.py -n 20 --threshold 0.75 --margin 0.05

# Multi-ref matching with margin (recommended for reducing false positives)
python test_logo_detection.py -n 20 --matching-method multi-ref \
    --refs-per-logo 5 --min-matching-refs 2 --threshold 0.70 --margin 0.05

# Multi-ref matching with max similarity (more lenient)
python test_logo_detection.py -n 20 --matching-method multi-ref \
    --refs-per-logo 5 --min-matching-refs 1 --use-max-similarity --margin 0.03

# Reproducible test with seed
python test_logo_detection.py -n 50 --seed 42 --clear-cache