From 4cb915897c0712813c72caf564c79a3f4f9c87aa Mon Sep 17 00:00:00 2001
From: Eunchurn Park <eunchurn.park@gmail.com>
Date: Thu, 15 Jan 2026 00:52:17 +0900
Subject: [PATCH] Fix XTC 1-bit thumb BMP polarity inversion

The thumb BMP generation for 1-bit XTC files had inverted polarity,
causing the Continue Reading cover image to appear with inverted colors.

- Fix: Change `grayValue = pixelBit ? 0 : 255` to `grayValue = pixelBit ? 255 : 0`
  to match the cover BMP generation logic where bit=0 is black and bit=1 is white
- Update misleading comment about XTC polarity
---
 lib/Xtc/Xtc.cpp | 6 +++---
 1 file changed, 3 insertions(+), 3 deletions(-)

diff --git a/lib/Xtc/Xtc.cpp b/lib/Xtc/Xtc.cpp
index 7205ffb..c79421d 100644
--- a/lib/Xtc/Xtc.cpp
+++ b/lib/Xtc/Xtc.cpp
@@ -203,7 +203,7 @@ bool Xtc::generateCoverBmp() const {
   coverBmp.write(reinterpret_cast<const uint8_t*>(&colorsImportant), 4);
 
   // Color palette (2 colors for 1-bit)
-  // XTC uses inverted polarity: 0 = black, 1 = white
+  // XTC 1-bit polarity: 0 = black, 1 = white (standard BMP palette order)
   // Color 0: Black (text/foreground in XTC)
   uint8_t black[4] = {0x00, 0x00, 0x00, 0x00};
   coverBmp.write(black, 4);
@@ -506,8 +506,8 @@ bool Xtc::generateThumbBmp() const {
             // Bounds check for buffer access
             if (byteIdx < bitmapSize) {
               const uint8_t pixelBit = (pageBuffer[byteIdx] >> bitIdx) & 1;
-              // XTC polarity: 1=black, 0=white
-              grayValue = pixelBit ? 0 : 255;
+              // XTC 1-bit polarity: 0=black, 1=white (same as BMP palette)
+              grayValue = pixelBit ? 255 : 0;
             }
           }