Fix bug in avifToLinearIEC61966

CHANGELOG.md

@@ -27,6 +27,7 @@ The changes are relative to the previous release, unless the baseline is specifi
   fails to allocate the destination gain map image.
 * avifenc: reject mismatched --depth for Y4M input
 * Use libaom AOMD_SET_FRAME_SIZE_LIMIT if available
+* Fix bug in transfer function 11 (used for gain map creation/tone mapping)
 
 ## [1.4.1] - 2026-03-20
 

src/colr.c

@@ -317,7 +317,7 @@ static float avifToGammaLog100Sqrt10(float linear)
 static float avifToLinearIEC61966(float gamma)
 {
     if (gamma < -4.5f * 0.018053968510807f) {
-        return powf((-gamma + 0.09929682680944f) / -1.09929682680944f, 1.0f / 0.45f);
+        return -powf((gamma - 0.09929682680944f) / -1.09929682680944f, 1.0f / 0.45f);
     } else if (gamma < 4.5f * 0.018053968510807f) {
         return gamma / 4.5f;
     } else {

tests/gtest/avifcolrtest.cc

@@ -75,6 +75,29 @@ TEST(TransferCharacteristicsTest, RoundTrip) {
   }
 }
 
+TEST(TransferCharacteristicsTest, NegativeRoundTrip) {
+  // Transfer functions that accept negative values.
+  const avifTransferCharacteristics tcs[] = {
+      AVIF_TRANSFER_CHARACTERISTICS_IEC61966,
+      AVIF_TRANSFER_CHARACTERISTICS_BT1361,
+  };
+  for (const avifTransferCharacteristics tc : tcs) {
+    SCOPED_TRACE("transfer characteristics: " + std::to_string(tc));
+
+    const avifTransferFunction to_linear =
+        avifTransferCharacteristicsGetGammaToLinearFunction(tc);
+    const avifTransferFunction to_gamma =
+        avifTransferCharacteristicsGetLinearToGammaFunction(tc);
+
+    const float test_values[] = {-0.1f, -0.05f, -0.2f};
+    for (const float v : test_values) {
+      // Check round trips.
+      EXPECT_NEAR(to_linear(to_gamma(v)), v, 0.00001f);
+      EXPECT_NEAR(to_gamma(to_linear(v)), v, 0.00001f);
+    }
+  }
+}
+
 // Check that the liner->gamma function has the right shape, i.e. it's mostly
 // above the y=x diagonal.
 // This detects bugs where the linear->gamma and