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openspec/specs/image-quantisation/spec.md
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openspec/specs/image-quantisation/spec.md
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<!-- markdownlint-disable MD024 -->
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# Capability Spec Delta: image-quantisation
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**Package**: `@suaveplan/image-quantisation`
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**Status**: PROPOSED
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> Every requirement below carries a `**Verified by:**` line at authoring time. No exceptions.
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## ADDED Requirements
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### Requirement: Package provides image complexity analysis
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The package SHALL analyse raw RGBA pixel buffers and classify image complexity into one of four tiers: pixel-art, vector-clean, vector-aa, or photographic.
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#### Scenario: Solid-colour image classified as pixel-art
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- **WHEN** a solid-colour 32×32 buffer is passed to `analyseComplexity`
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- **THEN** the returned `tier` is `"pixel-art"`, `uniqueColours` is 1, and `recommendedColours` ≤ 32
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**Verified by:** `src/analysis/classify/classify.test.ts::"classifies solid colour as pixel-art"`
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#### Scenario: Gradient classified as photographic
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- **WHEN** a smooth gradient 64×64 buffer is passed to `analyseComplexity`
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- **THEN** the returned `tier` is `"photographic"`, `uniqueColours` > 100, and `recommendDithering` is `true`
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**Verified by:** `src/analysis/classify/classify.test.ts::"classifies gradient as photographic"`
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#### Scenario: Vector-clean tier detected
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- **WHEN** a 32×32 image with 64 colours in 4×4 blocks is analysed
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- **THEN** the returned `tier` is `"vector-clean"`
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**Verified by:** `src/analysis/classify/classify.test.ts::"classifies vector-clean for moderate colours with medium pools"`
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#### Scenario: Vector-aa tier detected
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- **WHEN** a 64×64 image with 256 colours in 4×4 blocks is analysed
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- **THEN** the returned `tier` is `"vector-aa"`
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**Verified by:** `src/analysis/classify/classify.test.ts::"classifies vector-aa for high colour count with structure"`
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---
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### Requirement: Package provides pool detection and distribution
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The package SHALL detect contiguous same-colour regions via 4-connected flood fill and compute pool size distribution statistics.
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#### Scenario: Single pool for solid colour
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- **WHEN** `detectPools` is called on an 8×8 solid red buffer
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- **THEN** it returns exactly one pool with `size` 64 and colour `{r:255,g:0,b:0,a:255}`
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**Verified by:** `src/analysis/pools/pools.test.ts::"finds 1 pool for a solid colour"`
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#### Scenario: Pool distribution computed correctly
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- **WHEN** `poolDistribution` is called on pools from a checkerboard
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- **THEN** `singletonRatio` is 1 and `mean` is 1
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**Verified by:** `src/analysis/pools/pools.test.ts::"detects high singleton ratio in checkerboard"`
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---
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### Requirement: Package provides edge analysis
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The package SHALL analyse edges between adjacent pixels and return sharpness metrics.
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#### Scenario: Solid image has zero hard edges
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- **WHEN** `analyseEdges` is called on a solid-colour buffer
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- **THEN** `hardEdges` is 0 and `meanEdgeDistance` is 0
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**Verified by:** `src/analysis/edges/edges.test.ts::"solid image has zero hard edges"`
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#### Scenario: 1×1 image has zero edges
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- **WHEN** `analyseEdges` is called on a 1×1 buffer
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- **THEN** `totalEdges` is 0, `sharpnessRatio` is 0, and `meanEdgeDistance` is 0
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**Verified by:** `src/analysis/edges/edges.test.ts::"1x1 image has zero edges"`
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---
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### Requirement: Package provides colour quantisation
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The package SHALL quantise RGBA pixel buffers to a target palette size using wrapped `image-q` algorithms.
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#### Scenario: Gradient reduced to target colours
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- **WHEN** `quantise` is called on a 32×32 gradient with `colours: 16`
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- **THEN** `paletteSize` ≤ 16
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**Verified by:** `src/quantise/quantise.test.ts::"reduces a gradient to target colour count"`
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#### Scenario: All three algorithms supported
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- **WHEN** `quantise` is called with `algorithm: "wuquant"`, `"neuquant"`, and `"rgbquant"`
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- **THEN** each result has `paletteSize` ≤ 8
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**Verified by:** `src/quantise/quantise.test.ts::"supports different algorithms"`
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#### Scenario: Invalid dimensions throw structured error
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- **WHEN** `quantise` is called with `width: 0`
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- **THEN** it throws `QuantisationError` with code `QUANTISATION.INVALID_DIMENSIONS`
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**Verified by:** `src/quantise/quantise.test.ts::"throws on invalid dimensions"`
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---
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### Requirement: Package provides spatial AA cleanup
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The package SHALL merge small pools into nearest spatial neighbours below a colour distance threshold.
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#### Scenario: Near-colour AA pixels merged
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- **WHEN** `spatialCleanup` is called on an AA edge buffer with default options
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- **THEN** the unique colour count drops from 3 to 2
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**Verified by:** `src/spatial/cleanup/cleanup.test.ts::"merges near-colour AA pixels into spatial neighbours"`
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#### Scenario: Pool size threshold respected
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- **WHEN** `spatialCleanup` is called with `maxPoolSize: 0`
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- **THEN** no merge occurs and colour count stays 3
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**Verified by:** `src/spatial/cleanup/cleanup.test.ts::"does not merge pools above maxPoolSize"`
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---
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### Requirement: Package provides adaptive pipeline
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The package SHALL orchestrate analysis → spatial cleanup → quantisation in a single call, selecting strategy based on complexity tier.
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#### Scenario: Pixel-art skips quantisation
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- **WHEN** `adaptivePipeline` is called on a solid-colour buffer
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- **THEN** `quantisation` is `null` and `finalColours` is 1
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**Verified by:** `src/pipeline/pipeline.test.ts::"skips quantisation for pixel-art"`
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#### Scenario: Photographic images are quantised
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- **WHEN** `adaptivePipeline` is called on a gradient
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- **THEN** `quantisation` is not `null` and `finalColours` ≤ 256
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**Verified by:** `src/pipeline/pipeline.test.ts::"quantises photographic images"`
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#### Scenario: Spatial cleanup applied for vector-aa
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- **WHEN** `adaptivePipeline` is called on a vector-aa image
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- **THEN** `spatialCleanupApplied` is `true`
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**Verified by:** `src/pipeline/pipeline.test.ts::"applies spatial cleanup for vector-aa images"`
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---
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### Requirement: Package validates options via Zod schemas
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All public entry points that accept options SHALL validate them through exported Zod schemas before processing.
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#### Scenario: Invalid options rejected by quantise
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- **WHEN** `quantise` is called with `colours: 1`
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- **THEN** it throws `QuantisationError` with code `QUANTISATION.PROCESSING_FAILED`
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**Verified by:** `src/quantise/quantise.test.ts::"throws on invalid options"`
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---
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### Requirement: Package emits OpenTelemetry instruments
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The package SHALL emit a counter and histogram for `quantise` and `adaptivePipeline` invocations.
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#### Scenario: Instruments exist and are lazily initialised
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- **WHEN** `quantise` is called
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- **THEN** internal `getCounter()` and `getDuration()` functions return valid OTel instruments
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**Verified by:** `src/quantise/quantise.test.ts::"reduces a gradient to target colour count"` (implicit via execution path)
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---
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### Requirement: Package defines stable error codes
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The package SHALL declare error codes via `defineCodes("QUANTISATION", [...])` and throw `QuantisationError` with those codes.
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#### Scenario: Every code is reachable
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- **WHEN** invalid inputs are provided to `quantise` and `adaptivePipeline`
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- **THEN** each of `INVALID_PIXEL_BUFFER`, `INVALID_DIMENSIONS`, and `PROCESSING_FAILED` is thrown at least once
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**Verified by:** `src/quantise/quantise.test.ts::"throws on invalid dimensions"` AND `src/quantise/quantise.test.ts::"throws on mismatched buffer size"` AND `src/pipeline/pipeline.test.ts::"throws on invalid options"`
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---
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### Requirement: Package provides property-based tests
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The package SHALL include fast-check property tests that verify invariants across random inputs.
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#### Scenario: Palette size invariant
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- **WHEN** random RGBA buffers and colour counts are quantised
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- **THEN** `result.paletteSize` ≤ requested colours
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**Verified by:** `src/property.test.ts::"quantise palette size <= requested colours"`
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#### Scenario: Output dimensions invariant
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- **WHEN** random RGBA buffers are quantised
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- **THEN** output `width`, `height`, and `pixels.length` match input
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**Verified by:** `src/property.test.ts::"quantise output has same dimensions as input"`
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---
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### Requirement: Package includes benchmarks
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The package SHALL include mitata benchmarks for `quantise`, `analyseComplexity`, and `adaptivePipeline`.
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#### Scenario: Benchmark file exists and compiles
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- **WHEN** the benchmark file is inspected
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- **THEN** `src/__benchmarks__/quantisation.bench.ts` exists and imports the public API
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**Verified by:** manual inspection of `src/__benchmarks__/quantisation.bench.ts`
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