Edge Analysis

(no submodules)

Purpose

Analyse colour transitions between horizontally and vertically adjacent pixels to determine whether an image has sharp edges (pixel art) or soft edges (photographic). Edge metrics are a critical input to the complexity classifier, providing a signal that is orthogonal to pool distribution. Two images may have the same number of pools but very different edge characteristics.

Features

Examines every right and bottom neighbour pair
Counts hard edges (perceptually distinct) and soft edges (nearly identical)
Computes sharpness ratio and mean edge distance
Configurable hard/soft thresholds

Usage: Basic Example

import { analyseEdges } from "@suaveplan/image-quantisation/analysis/edges";

const edges = analyseEdges(pixels, 32, 32);
console.log(edges.sharpnessRatio); // 0.0 – 1.0

Usage: Advanced Example

import { analyseEdges } from "@suaveplan/image-quantisation/analysis/edges";

// Stricter thresholds for high-contrast artwork
const edges = analyseEdges(pixels, 64, 64, 20, 5);

if (edges.sharpnessRatio > 0.5) {
    console.log("High-contrast pixel art detected");
}

API Reference

`analyseEdges(pixels, width, height, hardThreshold?, softThreshold?): EdgeAnalysis`

pixels — RGBA Uint8Array
width, height — image dimensions
hardThreshold — default 30
softThreshold — default 10

Returns EdgeAnalysis with totalEdges, hardEdges, softEdges, sharpnessRatio, meanEdgeDistance.

Implementation Notes

The function uses weightedDistance (ITU-R BT.601) for perceptually fair comparisons. Pairs are counted once each (right and bottom neighbours only) to avoid double counting. A solid-colour image produces zero hard edges and zero mean distance, while a checkerboard produces a very high sharpness ratio. The thresholds can be adjusted for different content types: stricter thresholds treat more edges as hard, which is useful for high-contrast artwork, while lenient thresholds classify most edges as soft, which is better for smooth gradients.

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