Create methodology.md (#1)

- Create methodology.md (a59d17b86fc7007fd6b71a9f237dbe362539f63e)


Co-authored-by: Mandar Garud <mandarmgd-03@users.noreply.huggingface.co>

methodology.md

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+# Methodology
+
+## Problem Framing
+
+Classify an input image into one of {rust, zinc, normal} using fast, interpretable color cues. We rely on CIELab chroma tendencies:
+
+Rustish surfaces skew a* positive (reddish/brownish).
+
+Zincish surfaces skew b* positive (yellowish).
+
+The method is heuristic (no material guarantees) but is fast, parameter-tunable, and explainable.
+
+## Pipeline Overview
+
+Input → Preprocess → Color Analysis → Heuristic Ratios → Rule-based Classification → (Optional) Palette Visualization
+
+Read & (optional) Resize
+
+Load as BGR (cv.imread / bytes→cv.imdecode).
+
+If max(H, W) > resize_max, downscale with INTER_AREA to speed up.
+
+Color Space Transforms
+
+BGR → RGB, BGR → HSV, BGR → Lab (OpenCV 8-bit Lab).
+
+Compute per-space statistics (channel means/stds) for explainability.
+
+Dominant Colors (K-Means)
+
+Flatten pixels to N×3 (BGR float32).
+
+Run cv.kmeans with KMEANS_PP_CENTERS, termination criteria (eps + max_iter).
+
+Sort clusters by frequency to produce a palette (with shares) and a palette image.
+
+Heuristic Indicators (Lab)
+
+Split Lab: L*, a*, b*.
+
+Compute medians: a_med, b_med.
+
+Define thresholds:
+a_thr = a_med + Δ, b_thr = b_med + Δ where Δ = lab_delta (default 6.0).
+
+Ratios:
+rustish_ratio = mean(a* > a_thr)
+zincish_ratio = mean(b* > b_thr)
+
+Rule-based Classification
+
+If zincish_ratio > zinc_thr ⇒ zinc
+
+Else if rustish_ratio > rust_thr ⇒ rust
+
+Else ⇒ normal
+
+Outputs
+
+Class label, rustish_ratio, zincish_ratio
+
+Top k RGB colors and their shares
+
+(CLI) JSON report + palette image
+
+## Parameters & Defaults
+
+k (clusters): 3 — palette granularity vs. speed.
+
+lab_delta (Δ): 6.0 — sensitivity around median; higher = stricter.
+
+rust_thr: 0.01 — minimum fraction of above-threshold a* pixels.
+
+zinc_thr: 0.02 — minimum fraction of above-threshold b* pixels.
+
+resize_max: 1200 — longest side cap for speed (CLI).
+
+Tuning tips:
+• Increase lab_delta to reduce false positives.
+• Decrease rust_thr/zinc_thr to increase sensitivity.
+• Keep k=3..5 unless you need very fine palettes.
+
+## Implementation Mapping
+
+FastAPI (api.py): /classify/ accepts an image + query params → returns JSON with classification, ratios, and palette.
+
+Gradio (gradio_app.py): Interactive UI with sliders for k, thresholds, and lab_delta.
+
+CLI (reports):
+
+color.py / main.py → stats + palette + heuristics (+ classification in color.py).
+
+classify.py → reads a *_color_report.json and applies thresholds to output a label.
+
+All core logic is shared: color conversions, K-Means, Lab heuristics, and the rule-based decision.
+
+## Complexity & Performance
+
+K-Means: ~O(N × k × iters), where N = pixels after optional resize.
+
+Heuristics: O(N) on the Lab channels.
+
+Typical runtime: Sub-second to a few seconds on commodity CPUs for ≤2MP images with k=3.
+
+## Evaluation Protocol (Recommended)
+
+Dataset: Curate labeled images for rust, zinc, normal under varied lighting/backgrounds.
+
+Split: Train-free method; still use dev set to tune lab_delta, rust_thr, zinc_thr; hold out a test set.
+
+Metrics: Accuracy, per-class precision/recall/F1; confusion matrix to catch “rust↔zinc” confusions.
+
+Stress Tests: Vary illumination, white balance, exposure; add clutter/background metals; scale/resize sensitivity.
+
+Ablations:
+
+Different lab_delta values
+
+Median vs. mean baselines for a*/b* thresholds
+
+Removing K-Means (should not affect the classification, only explainability)
+
+## Assumptions & Limitations
+
+Heuristic, not material science: Color alone can’t guarantee presence of oxides/plating.
+
+Lighting-dependent: Harsh color casts and shadows can skew ratios.
+
+Background leakage: Non-metal regions may influence medians and ratios.
+
+Specular highlights: Can wash out chroma; consider controlled lighting for best results.
+
+Mitigations:
+
+Encourage consistent, diffuse illumination and controlled background.
+
+Optionally crop/segment to the object of interest.
+
+Adjust lab_delta and thresholds per environment.
+
+## Reproducibility
+
+Dependencies: pinned in requirements.txt (use opencv-python-headless).
+
+Determinism: OpenCV k-means with KMEANS_PP_CENTERS is typically stable; for strict reproducibility fix random seeds if you wrap k-means calls accordingly.
+
+Reports: JSON artifacts and palette images are saved for auditability.
+
+## Deployment Notes
+
+API: uvicorn api:app --host 0.0.0.0 --port 8000; OpenAPI docs at /docs.
+
+UI: python gradio_app.py for operator testing/threshold tuning.
+
+Batch: Use color.py/main.py to generate reports; classify.py to re-classify with updated thresholds.
+
+## Ethical Use
+
+Intended for screening/triage in industrial contexts, not definitive corrosion/plating certification.
+
+Users should verify with appropriate physical/chemical tests when decisions carry risk.