Add test making sure not always RLE packets are written

src/ImageSharp/Formats/Tga/TgaEncoderCore.cs

@@ -301,7 +301,7 @@ namespace SixLabors.ImageSharp.Formats.Tga
                 TPixel nextPixel = pixelRow[x];
                 if (currentPixel.Equals(nextPixel))
                 {
-                    return (byte)Math.Max(0, unEqualPixelCount - 1);
+                    return unEqualPixelCount;
                 }
 
                 unEqualPixelCount++;
@@ -314,7 +314,7 @@ namespace SixLabors.ImageSharp.Formats.Tga
                 currentPixel = nextPixel;
             }
 
-            return (byte)Math.Max(0, unEqualPixelCount - 1);
+            return unEqualPixelCount;
         }
 
         private IMemoryOwner<byte> AllocateRow(int width, int bytesPerPixel)

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

@@ -56,12 +56,8 @@ namespace SixLabors.ImageSharp.Tests.Formats.Tga
         [MemberData(nameof(TgaBitsPerPixelFiles))]
         public void TgaEncoder_WithCompression_PreserveBitsPerPixel(string imagePath, TgaBitsPerPixel bmpBitsPerPixel)
         {
-            var options = new TgaEncoder()
-                          {
-                              Compression = TgaCompression.RunLength
-                          };
-
-            TestFile testFile = TestFile.Create(imagePath);
+            var options = new TgaEncoder() { Compression = TgaCompression.RunLength };
+            var testFile = TestFile.Create(imagePath);
             using (Image<Rgba32> input = testFile.CreateRgba32Image())
             {
                 using (var memStream = new MemoryStream())
@@ -121,6 +117,24 @@ namespace SixLabors.ImageSharp.Tests.Formats.Tga
         public void TgaEncoder_Bit32_WithRunLengthEncoding_Works<TPixel>(TestImageProvider<TPixel> provider, TgaBitsPerPixel bitsPerPixel = TgaBitsPerPixel.Pixel32)
             where TPixel : unmanaged, IPixel<TPixel> => TestTgaEncoderCore(provider, bitsPerPixel, TgaCompression.RunLength);
 
+        [Theory]
+        [WithFile(WhiteStripesPattern, PixelTypes.Rgba32, 2748)]
+        public void TgaEncoder_DoesNotAlwaysUseRunLengthPackets<TPixel>(TestImageProvider<TPixel> provider, int expectedBytes)
+            where TPixel : unmanaged, IPixel<TPixel>
+        {
+            // The test image has alternating black and white pixels, which should make using always RLE data inefficient.
+            using (Image<TPixel> image = provider.GetImage())
+            {
+                var options = new TgaEncoder() { Compression = TgaCompression.RunLength };
+                using (var memStream = new MemoryStream())
+                {
+                    image.Save(memStream, options);
+                    byte[] imageBytes = memStream.ToArray();
+                    Assert.Equal(expectedBytes, imageBytes.Length);
+                }
+            }
+        }
+
         [Theory]
         [WithFile(Bit32BottomLeft, PixelTypes.Rgba32, TgaBitsPerPixel.Pixel32)]
         [WithFile(Bit24BottomLeft, PixelTypes.Rgba32, TgaBitsPerPixel.Pixel24)]

tests/ImageSharp.Tests/TestImages.cs

@@ -546,6 +546,7 @@ namespace SixLabors.ImageSharp.Tests
             public const string NoAlphaBits32BitRle = "Tga/32bit_rle_no_alphabits.tga";
 
             public const string Github_RLE_legacy = "Tga/Github_RLE_legacy.tga";
+            public const string WhiteStripesPattern = "Tga/whitestripes.png";
         }
 
         public static class Webp

tests/Images/Input/Tga/whitestripes.png

@@ -0,0 +1,3 @@
+version https://git-lfs.github.com/spec/v1
+oid sha256:2bc5d67ce368d2a40fb99df994c6973287fca2d8c8cff78227996f9acb5c6e1e
+size 127