Fix TIFF decoding of legacy 8-bit color maps

src/ImageSharp/Formats/Tiff/PhotometricInterpretation/PaletteTiffColor{TPixel}.cs

@@ -26,8 +26,6 @@ internal class PaletteTiffColor<TPixel> : TiffBaseColorDecoder<TPixel>
     private readonly bool hasAlpha;
     private Color[]? paletteColors;
 
-    private const float InvMax = 1f / 65535f;
-
     /// <summary>
     /// Initializes a new instance of the <see cref="PaletteTiffColor{TPixel}"/> class.
     /// </summary>
@@ -42,8 +40,15 @@ public PaletteTiffColor(TiffBitsPerSample bitsPerSample, ushort[] colorMap, Tiff
 
         int colorCount = 1 << this.bitsPerSample0;
 
+        // Match libtiff's legacy palette detection: use 8-bit scaling only when every RGB entry
+        // fits in 0-255. The runtime vectorizes this range check without allocating.
+        // Normalize by the selected channel range, not the observed maximum, to preserve brightness.
+        float colorScale = colorMap.AsSpan().ContainsAnyExceptInRange((ushort)0, (ushort)255)
+            ? 1f / 65535f
+            : 1f / 255f;
+
         // TIFF PaletteColor uses ColorMap (tag 320 / 0x0140) which is RGB-only (no alpha).
-        this.vectorPallete = GenerateVectorPalette(colorMap, colorCount);
+        this.vectorPallete = GenerateVectorPalette(colorMap, colorCount, colorScale);
 
         // ExtraSamples (tag 338 / 0x0152) describes extra per-pixel samples stored in the image data stream.
         // For PaletteColor, any alpha is per pixel (stored alongside the index), not per palette entry.
@@ -61,7 +66,7 @@ public PaletteTiffColor(TiffBitsPerSample bitsPerSample, ushort[] colorMap, Tiff
         else
         {
             // Pre-generate pixel palette for non-alpha case for performance.
-            this.pixelPalette = GeneratePixelPalette(colorMap, colorCount);
+            this.pixelPalette = GeneratePixelPalette(colorMap, colorCount, colorScale);
         }
     }
 
@@ -131,7 +136,14 @@ public override void Decode(ReadOnlySpan<byte> data, Buffer2D<TPixel> pixels, in
         }
     }
 
-    private static Vector4[] GenerateVectorPalette(ushort[] colorMap, int colorCount)
+    /// <summary>
+    /// Converts the RGB color map to normalized vectors with opaque alpha.
+    /// </summary>
+    /// <param name="colorMap">The planar RGB color map.</param>
+    /// <param name="colorCount">The number of palette colors.</param>
+    /// <param name="colorScale">The reciprocal of the detected channel range maximum.</param>
+    /// <returns>The normalized vector palette.</returns>
+    private static Vector4[] GenerateVectorPalette(ushort[] colorMap, int colorCount, float colorScale)
     {
         Vector4[] palette = new Vector4[colorCount];
 
@@ -141,16 +153,23 @@ private static Vector4[] GenerateVectorPalette(ushort[] colorMap, int colorCount
 
         for (int i = 0; i < palette.Length; i++)
         {
-            float r = colorMap[rOffset + i] * InvMax;
-            float g = colorMap[gOffset + i] * InvMax;
-            float b = colorMap[bOffset + i] * InvMax;
+            float r = colorMap[rOffset + i] * colorScale;
+            float g = colorMap[gOffset + i] * colorScale;
+            float b = colorMap[bOffset + i] * colorScale;
             palette[i] = new Vector4(r, g, b, 1f);
         }
 
         return palette;
     }
 
-    private static TPixel[] GeneratePixelPalette(ushort[] colorMap, int colorCount)
+    /// <summary>
+    /// Converts the RGB color map to pixels with opaque alpha.
+    /// </summary>
+    /// <param name="colorMap">The planar RGB color map.</param>
+    /// <param name="colorCount">The number of palette colors.</param>
+    /// <param name="colorScale">The reciprocal of the detected channel range maximum.</param>
+    /// <returns>The pixel palette.</returns>
+    private static TPixel[] GeneratePixelPalette(ushort[] colorMap, int colorCount, float colorScale)
     {
         TPixel[] palette = new TPixel[colorCount];
 
@@ -160,9 +179,9 @@ private static TPixel[] GeneratePixelPalette(ushort[] colorMap, int colorCount)
 
         for (int i = 0; i < palette.Length; i++)
         {
-            float r = colorMap[rOffset + i] * InvMax;
-            float g = colorMap[gOffset + i] * InvMax;
-            float b = colorMap[bOffset + i] * InvMax;
+            float r = colorMap[rOffset + i] * colorScale;
+            float g = colorMap[gOffset + i] * colorScale;
+            float b = colorMap[bOffset + i] * colorScale;
             palette[i] = TPixel.FromUnassociatedScaledVector4(new Vector4(r, g, b, 1f));
         }
 

tests/ImageSharp.Tests/Formats/Tiff/TiffDecoderTests.cs

@@ -202,6 +202,17 @@ public void TiffDecoder_CanDecode_Planar_128Bit<TPixel>(TestImageProvider<TPixel
     public void TiffDecoder_CanDecode_WithPalette<TPixel>(TestImageProvider<TPixel> provider)
         where TPixel : unmanaged, IPixel<TPixel> => TestTiffDecoder(provider);
 
+    /// <summary>
+    /// Palette TIFFs with LZW compression and horizontal prediction preserve colors for both palette ranges.
+    /// </summary>
+    /// <typeparam name="TPixel">The pixel type.</typeparam>
+    /// <param name="provider">The TIFF image provider.</param>
+    [Theory]
+    [WithFile(Issue3182ColorMap16Bit, PixelTypes.Rgba64)]
+    [WithFile(Issue3182ColorMap8Bit, PixelTypes.Rgba32)]
+    public void TiffDecoder_CanDecode_PaletteWithLzwAndPredictor<TPixel>(TestImageProvider<TPixel> provider)
+        where TPixel : unmanaged, IPixel<TPixel> => TestTiffDecoder(provider);
+
     [Theory]
     [WithFile(Rgb4BitPalette, PixelTypes.Rgba32)]
     [WithFile(Flower4BitPalette, PixelTypes.Rgba32)]

tests/ImageSharp.Tests/TestImages.cs

@@ -1210,6 +1210,8 @@ public static class Tiff
 
         public const string Issue2909 = "Tiff/Issues/Issue2909.tiff";
         public const string Issue2983 = "Tiff/Issues/Issue2983.tiff";
+        public const string Issue3182ColorMap8Bit = "Tiff/Issues/Issue3182ColorMap8Bit.tiff";
+        public const string Issue3182ColorMap16Bit = "Tiff/Issues/Issue3182ColorMap16Bit.tiff";
 
         public static readonly string[] Multiframes = [MultiframeDeflateWithPreview, MultiframeLzwPredictor /*, MultiFrameDifferentSize, MultiframeDifferentSizeTiled, MultiFrameDifferentVariants,*/
         ];