From f437114be0b141b242d63db81d3ffef13072cec1 Mon Sep 17 00:00:00 2001
From: Brion Vibber <brion@pobox.com>
Date: Mon, 20 Mar 2023 19:30:07 -0700
Subject: [PATCH] WIP median cut

---
 dither-image.js | 87 +++++++++++++++++++++++++++++++++++++++++--------
 1 file changed, 73 insertions(+), 14 deletions(-)

diff --git a/dither-image.js b/dither-image.js
index 2a9d5f0..e2927dd 100644
--- a/dither-image.js
+++ b/dither-image.js
@@ -4,6 +4,15 @@ import {
 
 import Jimp from 'Jimp';
 
+
+function zeroes(n) {
+    let arr = [];
+    for (let i = 0; i < n; i++) {
+        arr.push(0);
+    }
+    return arr;
+}
+
 function toLinear(val) {
     // use a 2.4 gamma approximation
     // this is BT.1886 compatible
@@ -392,7 +401,7 @@ let atariRGB = [
  * @param {number} n - target color count
  * @param {RGB[]} inputError
  * @param {number} y
- * @returns {{output: Uint8Array, palette: number[], error: RGB[]}}
+ * @returns {{output: number[], palette: number[], error: RGB[]}}
  */
 function decimate(input, palette, n, inputError, y) {
     // to brute-force, the possible palettes are:
@@ -425,7 +434,7 @@ function decimate(input, palette, n, inputError, y) {
 
     // Apply dithering with given palette and collect color usage stats
     let dither = (palette) => {
-        let fitness = new Float64Array(width);
+        let fitness = zeroes(width);
         let error = {
             cur: [],
             next: [],
@@ -435,8 +444,8 @@ function decimate(input, palette, n, inputError, y) {
             error.next[i] = new RGB(0, 0, 0);
         }
 
-        let output = new Uint8Array(width);
-        let popularity = new Int32Array(palette.length);
+        let output = zeroes(width);
+        let popularity = zeroes(palette.length);
         let distance2 = 0;
 
         let nextError = new RGB(0, 0, 0);
@@ -519,19 +528,11 @@ function decimate(input, palette, n, inputError, y) {
     }
     */
 
-    let keepers = new Uint8Array(256);
+    let keepers = zeroes(256);
     for (let i of reserved) {
         keepers[i & 0xfe] = 1; // drop that 0 luminance bit!
     }
 
-    let zeros = (n) => {
-        let arr = [];
-        for (let i = 0; i < n; i++) {
-            arr.push(0);
-        }
-        return arr;
-    };
-
     // this takes the top hues, and uses the brightest of each hue
     // needs tuning
     /*
@@ -567,6 +568,7 @@ function decimate(input, palette, n, inputError, y) {
 
     // popularity? not really working right
     // first, dither to the total atari palette
+    /*
     while (decimated.length > n) {
         //console.log(y);
 
@@ -605,6 +607,7 @@ function decimate(input, palette, n, inputError, y) {
         //decimated = a.slice(0, decimated.length - 1);
         //console.log(decimated);
     }
+    */
     //console.log('end', decimated);
 
     // old algo
@@ -652,7 +655,63 @@ function decimate(input, palette, n, inputError, y) {
         }
     }
     */
-                                                    
+
+    // Median cut!
+    // https://en.wikipedia.org/wiki/Median_cut
+    //let buckets = [input.slice()];
+    let initial = dither(palette);
+    let buckets = [initial.output.map((i) => atariRGB[palette[i]])];
+    let medianCut = (bucket, range) => {
+        // Sort by the channel with the greatest range,
+        // then cut the bucket in two at the median.
+        if (range.g >= range.r && range.g >= range.b) {
+            bucket.sort((a, b) => b.g - a.g);
+        } else if (range.r >= range.g && range.r >= range.b) {
+            bucket.sort((a, b) => b.r - a.r);
+        } else if (range.b >= range.g && range.b >= range.r) {
+            bucket.sort((a, b) => b.b - a.b);
+        }
+        let half = bucket.length >> 1;
+        return [bucket.slice(0, half), bucket.slice(half)];
+    };
+    while (buckets.length < n) {
+        // Find the bucket with the greatest range in any channel
+        let ranges = buckets.map((bucket) => {
+            let red   = bucket.map((rgb) => rgb.r);
+            let green = bucket.map((rgb) => rgb.g);
+            let blue  = bucket.map((rgb) => rgb.b);
+            return new RGB(
+                Math.max(...red)   - Math.min(...red),
+                Math.max(...green) - Math.min(...green),
+                Math.max(...blue)  - Math.min(...blue)
+            );
+        });
+        let topRanges = ranges.map((rgb) => Math.max(rgb.r, rgb.g, rgb.b));
+        let greatest = Math.max(...topRanges);
+        let index = topRanges.indexOf(greatest);
+        let [lo, hi] = medianCut(buckets[index], ranges[index]);
+        buckets.splice(index, 1, lo, hi);
+    }
+    decimated = buckets.map((bucket) => {
+        // Average the RGB colors in this chunk
+        let rgb = bucket
+            .reduce((acc, rgb) => acc.inc(rgb), new RGB(0, 0, 0))
+            .divide(bucket.length);
+
+        // Take the brightest color in the bucket
+        //let rgb = bucket[bucket.length - 1];
+
+        // And map into the Atari palette
+        let dists = palette.map((   i) => rgb.difference(atariRGB[i]).magnitude());
+        let closest = Math.min(...dists);
+        let index = dists.indexOf(closest);
+        return palette[index];
+    });
+    // hack
+    decimated.sort((a, b) => a - b);
+    console.log(decimated);
+    decimated[0] = 0;
+
     // Palette fits
     return dither(decimated);
 }