---
phase: 03-diagnostic-features
plan: 01
subsystem: processing
tags: [pixel-compare, delta-highlight, idle-compress, gif-export, processing-pipeline]
dependency_graph:
  requires: [grid-compiler, timestamp-overlay, types]
  provides: [pixel-compare, delta-highlighter, idle-compressor, gif-exporter]
  affects: [03-02-mcp-integration]
tech_stack:
  added: ["@skyra/gifenc@^1.0.1"]
  patterns: [sequential-pair-processing, RGB-euclidean-distance, frame-compression]
key_files:
  created:
    - src/processing/pixel-compare.ts
    - src/processing/delta-highlighter.ts
    - src/processing/idle-compressor.ts
    - src/processing/gif-exporter.ts
    - src/processing/pixel-compare.test.ts
    - src/processing/delta-highlighter.test.ts
    - src/processing/idle-compressor.test.ts
    - src/processing/gif-exporter.test.ts
  modified:
    - package.json
    - package-lock.json
decisions:
  - "Overlay resize for small frames: when timestamp overlay exceeds frame dimensions, scale down to fit"
  - "GIF encoder stream order: createReadStream before start/addFrame/finish per @skyra/gifenc API"
metrics:
  duration: "4m 46s"
  completed: "2026-04-12T16:23:25Z"
  tasks_completed: 3
  tasks_total: 3
  tests_added: 19
  tests_total: 42
---

# Phase 03 Plan 01: Diagnostic Processing Modules Summary

Four processing modules for pixel comparison, delta highlighting, idle frame compression, and animated GIF export with 19 new tests and zero regressions.

## What Was Done

### Task 1: Pixel comparison utility, idle compressor, and @skyra/gifenc install
- **Commit:** `575ea86`
- Created `pixel-compare.ts` with `extractRawPixels` (sharp ensureAlpha + raw) and `compareFrames` (RGB Euclidean distance, threshold default 25)
- Created `idle-compressor.ts` with `compressIdleFrames` that collapses consecutive identical frames into single frames labeled "unchanged N.Ns"
- Installed `@skyra/gifenc@^1.0.1` dependency
- 10 tests covering extraction, comparison, thresholds, compression, pass-through

### Task 2: Delta highlighter module
- **Commit:** `cdbfe88`
- Created `delta-highlighter.ts` with `applyDeltaHighlights` that overlays semi-transparent red (R=255, G=0, B=0, A=100) on changed regions
- First frame passes through unchanged per D-04
- Sequential pair processing for memory control
- 5 tests covering pass-through, overlay detection, length and metadata preservation

### Task 3: GIF exporter module
- **Commit:** `88eb5d5`
- Created `gif-exporter.ts` with `exportGif` using @skyra/gifenc GifEncoder
- Per-frame delays from elapsedMs differences, clamped to minimum 20ms
- Frames resized to max 800px wide maintaining aspect ratio
- GIF89a format with infinite loop (repeat=0)
- 4 tests covering magic bytes, multi-frame sizing, maxWidth, timing

## Deviations from Plan

### Auto-fixed Issues

**1. [Rule 1 - Bug] Overlay exceeds small frame dimensions**
- **Found during:** Task 1 (idle compressor tests)
- **Issue:** Pango text overlay from `createTimestampOverlay` could be larger than the frame (e.g., 20x20 test frames), causing sharp "Image to composite must have same dimensions or smaller" error
- **Fix:** Added overlay resize logic in `createCollapsedFrame` to scale overlay down when it exceeds frame dimensions
- **Files modified:** src/processing/idle-compressor.ts
- **Commit:** 575ea86

## Verification

- All 19 new processing module tests pass
- Full test suite: 42 tests, 0 failures, 0 regressions
- `@skyra/gifenc` installed and resolved: `node -e "require.resolve('@skyra/gifenc')"`

## Pipeline Order Enforced

Per D-12, the processing pipeline order is: compress idle -> delta highlight -> grid compile / gif export. This is enforced by the module design:
- `compressIdleFrames` operates on raw CaptureFrame arrays
- `applyDeltaHighlights` operates on (potentially compressed) CaptureFrame arrays
- `compileGrid` and `exportGif` operate on the final processed frames

Plan 03-02 will wire these into the MCP tools in the correct order.

## Self-Check: PASSED

- All 8 created files exist on disk
- All 3 task commits verified: 575ea86, cdbfe88, 88eb5d5
- Full test suite: 42/42 pass