Merge pull request #2643 from SixLabors/bp/Issue2629

TGA: Treat 32 bit True Color images always as transparent

Author: brianpopow

src/ImageSharp/Formats/Tga/TgaDecoderCore.cs

@@ -937,7 +937,9 @@ private TgaImageOrigin ReadFileHeader(BufferedReadStream stream)
         this.tgaMetadata = this.metadata.GetTgaMetadata();
         this.tgaMetadata.BitsPerPixel = (TgaBitsPerPixel)this.fileHeader.PixelDepth;
 
-        int alphaBits = this.fileHeader.ImageDescriptor & 0xf;
+        // TrueColor images with 32 bits per pixel are assumed to always have 8 bit alpha channel,
+        // because some encoders do not set correctly the alpha bits in the image descriptor.
+        int alphaBits = this.IsTrueColor32BitPerPixel(this.tgaMetadata.BitsPerPixel) ? 8 : this.fileHeader.ImageDescriptor & 0xf;
         if (alphaBits is not 0 and not 1 and not 8)
         {
             TgaThrowHelper.ThrowInvalidImageContentException("Invalid alpha channel bits");
@@ -949,4 +951,8 @@ private TgaImageOrigin ReadFileHeader(BufferedReadStream stream)
         // Bits 4 and 5 describe the image origin.
         return (TgaImageOrigin)((this.fileHeader.ImageDescriptor & 0x30) >> 4);
     }
+
+    private bool IsTrueColor32BitPerPixel(TgaBitsPerPixel bitsPerPixel) => bitsPerPixel == TgaBitsPerPixel.Pixel32 &&
+                                                                           (this.fileHeader.ImageType == TgaImageType.TrueColor ||
+                                                                            this.fileHeader.ImageType == TgaImageType.RleTrueColor);
 }

src/ImageSharp/Formats/Tga/TgaFileHeader.cs

@@ -131,10 +131,7 @@ public TgaFileHeader(
     /// </summary>
     public byte ImageDescriptor { get; }
 
-    public static TgaFileHeader Parse(Span<byte> data)
-    {
-        return MemoryMarshal.Cast<byte, TgaFileHeader>(data)[0];
-    }
+    public static TgaFileHeader Parse(Span<byte> data) => MemoryMarshal.Cast<byte, TgaFileHeader>(data)[0];
 
     public void WriteTo(Span<byte> buffer)
     {

tests/ImageSharp.Tests/Formats/Tga/TgaDecoderTests.cs

@@ -1,7 +1,6 @@
 // Copyright (c) Six Labors.
 // Licensed under the Six Labors Split License.
 
-using System.Runtime.InteropServices;
 using System.Runtime.Intrinsics.X86;
 using Microsoft.DotNet.RemoteExecutor;
 using SixLabors.ImageSharp.Formats;
@@ -724,10 +723,8 @@ public void TgaDecoder_CanDecode_WhenAlphaBitsNotSet<TPixel>(TestImageProvider<T
     {
         using (Image<TPixel> image = provider.GetImage(TgaDecoder.Instance))
         {
-            // Using here the reference output instead of the the reference decoder,
-            // because the reference decoder does not ignore the alpha data here.
             image.DebugSave(provider);
-            image.CompareToReferenceOutput(ImageComparer.Exact, provider);
+            ImageComparingUtils.CompareWithReferenceDecoder(provider, image);
         }
     }
 
@@ -771,6 +768,19 @@ public void TgaDecoder_Decode_Resize<TPixel>(TestImageProvider<TPixel> provider)
             appendPixelTypeToFileName: false);
     }
 
+    // https://github.com/SixLabors/ImageSharp/issues/2629
+    [Theory]
+    [WithFile(Issue2629, PixelTypes.Rgba32)]
+    public void TgaDecoder_CanDecode_Issue2629<TPixel>(TestImageProvider<TPixel> provider)
+        where TPixel : unmanaged, IPixel<TPixel>
+    {
+        using (Image<TPixel> image = provider.GetImage(TgaDecoder.Instance))
+        {
+            image.DebugSave(provider);
+            ImageComparingUtils.CompareWithReferenceDecoder(provider, image);
+        }
+    }
+
     [Theory]
     [WithFile(Bit16BottomLeft, PixelTypes.Rgba32)]
     [WithFile(Bit24BottomLeft, PixelTypes.Rgba32)]

tests/ImageSharp.Tests/TestImages.cs

@@ -617,6 +617,8 @@ public static class Tga
 
         public const string Github_RLE_legacy = "Tga/Github_RLE_legacy.tga";
         public const string WhiteStripesPattern = "Tga/whitestripes.png";
+
+        public const string Issue2629 = "Tga/issues/Issue2629.tga";
     }
 
     public static class Webp

tests/Images/External/ReferenceOutput/TgaDecoderTests/TgaDecoder_CanDecode_WhenAlphaBitsNotSet_Rgba32_32bit_no_alphabits.png

@@ -1,3 +1,3 @@
 version https://git-lfs.github.com/spec/v1
-oid sha256:0aea1128a1bd7477dfa0d007a1eba25907be24847284c48a5f9fbd61bcea3cf0
-size 61522
+oid sha256:019315f9dcbe4516ecb15426a45c210d437e9ad152c8e1a0e80abe9449177e12
+size 235218

tests/Images/External/ReferenceOutput/TgaDecoderTests/TgaDecoder_CanDecode_WhenAlphaBitsNotSet_Rgba32_32bit_rle_no_alphabits.png

@@ -1,3 +1,3 @@
 version https://git-lfs.github.com/spec/v1
-oid sha256:98a198392bd527523f8649d6126af81e5a588ad7265dc3d48a1da7b5a6cb6ff7
-size 230578
+oid sha256:33954ae93b4c7d57f52965a9028e97119c546db1da255100c2903a2760c7479e
+size 76870

tests/Images/Input/Tga/32bit_no_alphabits.tga

@@ -0,0 +1,3 @@
+version https://git-lfs.github.com/spec/v1
+oid sha256:defc1396481f426a74e8af51ed57f65cbed932f932673ce5a87fa12ea9b460f8
+size 32786