From 1ca18c457508a3294f3eb35626348adfc662fa9d Mon Sep 17 00:00:00 2001
From: jpirnay <jens@pirnay.com>
Date: Thu, 7 May 2026 20:25:57 +0200
Subject: [PATCH] Revert to same quantization thresholds as in upstream

---
 lib/GfxRenderer/BitmapHelpers.cpp |  5 +----
 lib/GfxRenderer/BitmapHelpers.h   | 32 +++++++++++++++----------------
 2 files changed, 17 insertions(+), 20 deletions(-)

diff --git a/lib/GfxRenderer/BitmapHelpers.cpp b/lib/GfxRenderer/BitmapHelpers.cpp
index 4bc3b039..dd11675d 100644
--- a/lib/GfxRenderer/BitmapHelpers.cpp
+++ b/lib/GfxRenderer/BitmapHelpers.cpp
@@ -52,11 +52,8 @@ int adjustPixel(int gray) {
 
   return gray;
 }
-// Simple quantization without dithering — evenly-spaced midpoint thresholds for factory LUT
+// Simple quantization without dithering — thresholds tuned to X4 display brightness
 uint8_t quantizeSimple(int gray) {
-  // Midpoint thresholds for 4 levels would be: 0-63=0, 64-127=1, 128-191=2, 192-255=3
-  // Crosspoint thresholds for 4 levels: 0-44=0, 45-69=1, 70-139=2, 140-255=3
-  // (slightly brighter to compensate for quantization)
   if (gray < 45) {
     return 0;
   } else if (gray < 70) {
diff --git a/lib/GfxRenderer/BitmapHelpers.h b/lib/GfxRenderer/BitmapHelpers.h
index bd76cefd..04abce39 100644
--- a/lib/GfxRenderer/BitmapHelpers.h
+++ b/lib/GfxRenderer/BitmapHelpers.h
@@ -127,21 +127,21 @@ class AtkinsonDitherer {
     if (adjusted < 0) adjusted = 0;
     if (adjusted > 255) adjusted = 255;
 
-    // Quantize to 4 levels — evenly-spaced midpoint thresholds for factory LUT
+    // Quantize to 4 levels — thresholds tuned to X4 display brightness
     uint8_t quantized;
     int quantizedValue;
-    if (adjusted < 43) {
+    if (adjusted < 30) {
       quantized = 0;
-      quantizedValue = 0;
-    } else if (adjusted < 128) {
+      quantizedValue = 15;
+    } else if (adjusted < 50) {
       quantized = 1;
-      quantizedValue = 85;
-    } else if (adjusted < 213) {
+      quantizedValue = 30;
+    } else if (adjusted < 140) {
       quantized = 2;
-      quantizedValue = 170;
+      quantizedValue = 80;
     } else {
       quantized = 3;
-      quantizedValue = 255;
+      quantizedValue = 210;
     }
 
     // Calculate error (only distribute 6/8 = 75%)
@@ -215,21 +215,21 @@ class FloydSteinbergDitherer {
     if (adjusted < 0) adjusted = 0;
     if (adjusted > 255) adjusted = 255;
 
-    // Quantize to 4 levels — evenly-spaced midpoint thresholds for factory LUT
+    // Quantize to 4 levels — thresholds tuned to X4 display brightness
     uint8_t quantized;
     int quantizedValue;
-    if (adjusted < 43) {
+    if (adjusted < 30) {
       quantized = 0;
-      quantizedValue = 0;
-    } else if (adjusted < 128) {
+      quantizedValue = 15;
+    } else if (adjusted < 50) {
       quantized = 1;
-      quantizedValue = 85;
-    } else if (adjusted < 213) {
+      quantizedValue = 30;
+    } else if (adjusted < 140) {
       quantized = 2;
-      quantizedValue = 170;
+      quantizedValue = 80;
     } else {
       quantized = 3;
-      quantizedValue = 255;
+      quantizedValue = 210;
     }
 
     // Calculate error