From e3e1dd7c6e999b5ba113f4b973c7a799aebd6b10 Mon Sep 17 00:00:00 2001
From: Brion Vibber <brion@pobox.com>
Date: Sat, 5 Nov 2022 19:38:31 -0700
Subject: [PATCH] whee 2.4 gamma

---
 dither4.js | 41 ++++++++++++++++++-----------------------
 1 file changed, 18 insertions(+), 23 deletions(-)

diff --git a/dither4.js b/dither4.js
index a110b70..2641aa1 100644
--- a/dither4.js
+++ b/dither4.js
@@ -1,20 +1,15 @@
-function sRGBToLinear(val) {
+function toLinear(val) {
+    // use a 2.4 gamma approximation
+    // this is BT.1886 compatible
+    // and simpler than sRGB
     let unit = val / 255;
-    if (unit < 0.04045) {
-        unit /= 12.92;
-    } else {
-        unit = ((unit + 0.055) / 1.055) ** 2.4;
-    }
+    unit **= 2.4;
     return unit * 255;
 }
 
-function linearTosRGB(val) {
+function fromLinear(val) {
     let unit = val / 255;
-    if (unit < 0.0031308) {
-        unit *= 12.92;
-    } else {
-        unit = 1.055 * unit ** (1 / 2.4) - 0.055;
-    }
+    unit **= (1 / 2.4);
     return unit * 255;
 }
 
@@ -32,19 +27,19 @@ class RGB {
         return new RGB(r,g,b);
     }
 
-    sRGBToLinear() {
+    toLinear() {
         return new RGB(
-            sRGBToLinear(this.r),
-            sRGBToLinear(this.g),
-            sRGBToLinear(this.b)
+            toLinear(this.r),
+            toLinear(this.g),
+            toLinear(this.b)
         );
     }
 
-    linearTosRGB() {
+    fromLinear() {
         return new RGB(
-            linearTosRGB(this.r),
-            linearTosRGB(this.g),
-            linearTosRGB(this.b)
+            fromLinear(this.r),
+            fromLinear(this.g),
+            fromLinear(this.b)
         );
     }
 
@@ -357,7 +352,7 @@ let palette256 = [
     0xf6e46f,
     0xfffa84,
     0xffff99,
-].map((hex) => RGB.fromHex(hex).sRGBToLinear());
+].map((hex) => RGB.fromHex(hex).toLinear());
 
 function decimate(input, palette, n) {
     // to brute-force, the possible palettes are:
@@ -535,7 +530,7 @@ 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();
+            ).toLinear();
             if (nextError) {
                 rgb.r += nextError.red[i];
                 rgb.g += nextError.green[i];
@@ -549,7 +544,7 @@ function convert(source, sink) {
         nextError = error;
 
         for (let x = 0; x < width; x++) {
-            let rgb = palette[output[x >> 1]].linearTosRGB();
+            let rgb = palette[output[x >> 1]].fromLinear();
             line[x * 4 + 0] = rgb.r;
             line[x * 4 + 1] = rgb.g;
             line[x * 4 + 2] = rgb.b;