From 16c2caba95c3991077610695aabeb2a1466b7bf9 Mon Sep 17 00:00:00 2001
From: Brion Vibber <brion@pobox.com>
Date: Sat, 5 Nov 2022 19:34:31 -0700
Subject: [PATCH] nice

---
 dither4.js | 42 +++++++++++++++++++++++++++++++++++++++---
 1 file changed, 39 insertions(+), 3 deletions(-)

diff --git a/dither4.js b/dither4.js
index 00978fd..a110b70 100644
--- a/dither4.js
+++ b/dither4.js
@@ -1,3 +1,23 @@
+function sRGBToLinear(val) {
+    let unit = val / 255;
+    if (unit < 0.04045) {
+        unit /= 12.92;
+    } else {
+        unit = ((unit + 0.055) / 1.055) ** 2.4;
+    }
+    return unit * 255;
+}
+
+function linearTosRGB(val) {
+    let unit = val / 255;
+    if (unit < 0.0031308) {
+        unit *= 12.92;
+    } else {
+        unit = 1.055 * unit ** (1 / 2.4) - 0.055;
+    }
+    return unit * 255;
+}
+
 class RGB {
     constructor(r, g, b) {
         this.r = r;
@@ -12,6 +32,22 @@ class RGB {
         return new RGB(r,g,b);
     }
 
+    sRGBToLinear() {
+        return new RGB(
+            sRGBToLinear(this.r),
+            sRGBToLinear(this.g),
+            sRGBToLinear(this.b)
+        );
+    }
+
+    linearTosRGB() {
+        return new RGB(
+            linearTosRGB(this.r),
+            linearTosRGB(this.g),
+            linearTosRGB(this.b)
+        );
+    }
+
     cap() {
         if (this.r < 0) {
             this.r = 0;
@@ -321,7 +357,7 @@ let palette256 = [
     0xf6e46f,
     0xfffa84,
     0xffff99,
-].map((hex) => RGB.fromHex(hex));
+].map((hex) => RGB.fromHex(hex).sRGBToLinear());
 
 function decimate(input, palette, n) {
     // to brute-force, the possible palettes are:
@@ -499,7 +535,7 @@ function convert(source, sink) {
                 (line[x * 4 + 0] + line[x * 4 + 4]) / 2,
                 (line[x * 4 + 1] + line[x * 4 + 5]) / 2,
                 (line[x * 4 + 2] + line[x * 4 + 6]) / 2
-            );
+            ).sRGBToLinear();
             if (nextError) {
                 rgb.r += nextError.red[i];
                 rgb.g += nextError.green[i];
@@ -513,7 +549,7 @@ function convert(source, sink) {
         nextError = error;
 
         for (let x = 0; x < width; x++) {
-            let rgb = palette[output[x >> 1]];
+            let rgb = palette[output[x >> 1]].linearTosRGB();
             line[x * 4 + 0] = rgb.r;
             line[x * 4 + 1] = rgb.g;
             line[x * 4 + 2] = rgb.b;