From 737b22a2df0bf5986dbe72cd5d5ee072cf7662c7 Mon Sep 17 00:00:00 2001
From: Justin Mitchell <justin@jmitch.com>
Date: Fri, 12 Jun 2026 16:40:26 -0400
Subject: [PATCH] Fix XTC grayscale mapping for dark/light gray pixels

Swap the grayscale values for pixel values 1 and 2 to match the cover's kXthToBmp mapping. Previously 1 mapped to 170 (light gray) and 2 to 85 (dark gray), now corrected to 1->85 (dark gray) and 2->170 (light gray). Use lookup table for clarity.
---
 lib/Xtc/Xtc.cpp | 7 ++++---
 1 file changed, 4 insertions(+), 3 deletions(-)

diff --git a/lib/Xtc/Xtc.cpp b/lib/Xtc/Xtc.cpp
index 8052c88e..c75bae41 100644
--- a/lib/Xtc/Xtc.cpp
+++ b/lib/Xtc/Xtc.cpp
@@ -432,9 +432,10 @@ bool Xtc::generateThumbBmp(int height) const {
                 const uint8_t bit1 = (plane1[byteOffset] >> bitInByte) & 1;
                 const uint8_t bit2 = (plane2[byteOffset] >> bitInByte) & 1;
                 const uint8_t pixelValue = (bit1 << 1) | bit2;
-                // Convert 2-bit (0-3) to grayscale: 0=black, 3=white
-                // pixelValue: 0=white, 1=light gray, 2=dark gray, 3=black (XTC polarity)
-                grayValue = (3 - pixelValue) * 85;  // 0->255, 1->170, 2->85, 3->0
+                // pixelValue: 0=white, 1=dark gray, 2=light gray, 3=black —
+                // same semantics as the cover's kXthToBmp mapping above
+                static constexpr uint8_t kXthToGray[4] = {255, 85, 170, 0};
+                grayValue = kXthToGray[pixelValue];
               }
             }
           } else {