From e460fcefb0543315eb738ed7e8c93b9b9b779c02 Mon Sep 17 00:00:00 2001
From: Brion Vibber <brion@pobox.com>
Date: Sun, 4 Dec 2022 15:03:36 -0800
Subject: [PATCH] wip dither tweaks

---
 dither-image.js | 38 +++++++++++++++++++-------------------
 1 file changed, 19 insertions(+), 19 deletions(-)

diff --git a/dither-image.js b/dither-image.js
index 717f1ef..141fa12 100644
--- a/dither-image.js
+++ b/dither-image.js
@@ -78,14 +78,18 @@ class RGB {
         return this;
     }
 
-    add(other) {
+    static add(a, b) {
         return new RGB(
-            this.r + other.r,
-            this.g + other.g,
-            this.b + other.b
+            a.r + b.r,
+            a.g + b.g,
+            a.b + b.b
         );
     }
 
+    add(other) {
+        return RGB.add(this, other);
+    }
+
     difference(other) {
         return new RGB(
             this.r - other.r,
@@ -407,10 +411,11 @@ function decimate(input, palette, n, inputError) {
     let dither = (palette) => {
         let fitness = new Float64Array(width);
         let error = {
-            right: new RGB(0, 0, 0),
+            cur: [],
             next: [],
         };
         for (let i = 0; i < width; i++) {
+            error.cur[i] = new RGB(0, 0, 0);
             error.next[i] = new RGB(0, 0, 0);
         }
 
@@ -421,8 +426,7 @@ function decimate(input, palette, n, inputError) {
 
         // Try dithering with this palette.
         for (let x = 0; x < width; x++) {
-            let rgb = input[x];
-            rgb = rgb.add(error.right);
+            let rgb = RGB.add(input[x], error.cur[x]);
             if (inputError) {
                 rgb.inc(inputError[x]);
             }
@@ -445,18 +449,14 @@ function decimate(input, palette, n, inputError) {
             output[x] = pick;
             popularity[pick]++;
 
-            if (x == width - 1) {
-                let half = nextError.divide(4);
-                error.next[x - 1].inc(half);
-                error.next[x].inc(half);
-                // drop half the error on the floor at the edge ;_;
-            } else {
-                let quarter = nextError.divide(4);
-                error.right = quarter;
-                error.next[x - 1]?.inc(quarter); // @fixme should we change the amount?
-                error.next[x].inc(quarter);
-                error.next[x + 1].inc(quarter);
-            }
+            let shares = 8;
+            let single = nextError.divide(shares);
+            let double = nextError.divide(shares / 2);
+            error.cur[x + 1]?.inc(double);
+            error.cur[x + 2]?.inc(single);
+            error.next[x - 1]?.inc(double);
+            error.next[x]?.inc(double);
+            error.next[x + 1]?.inc(single);
 
             // 442 is the 3d distance across the rgb cube
             //fitness[x] = 442 - (nextError.magnitude());