--- a/src/lib/MSPUBCollector.cpp	2026-10-27 15:18:02.834845369 +0100
+++ b/src/lib/MSPUBCollector.cpp	2026-10-27 15:18:02.846770072 +0100
@@ -786,7 +786,10 @@
                            static_cast<double>(obc.g) / 255.0, librevenge::RVNG_PERCENT);
     }
     if (bool(info.m_pictureBrightness))
-      graphicsProps.insert("draw:luminance", static_cast<double>(info.m_pictureBrightness.get() + 32768.0) / 65536.0, librevenge::RVNG_PERCENT);
+      // PubOpener: Publisher stores brightness as a signed value where 0 means
+      // "unchanged" and 32768 means +100%; ODF's draw:luminance also uses 0% for
+      // unchanged. The original formula turned "unchanged" into +50%.
+      graphicsProps.insert("draw:luminance", static_cast<double>(static_cast<int>(info.m_pictureBrightness.get())) / 32768.0, librevenge::RVNG_PERCENT);
     bool shadowPropsInserted = false;
     if (bool(info.m_shadow))
     {