From fb9e23020bf0882956ef9c509ba9dccbb09cd2f9 Mon Sep 17 00:00:00 2001
From: Brian Popow <38701097+brianpopow@users.noreply.github.com>
Date: Tue, 5 Feb 2019 02:41:26 +0100
Subject: [PATCH] Fix Decoding 8-Bit grayscale png's with alpha (#830)

---
 .../Formats/Png/PngScanlineProcessor.cs       |  3 +-
 .../Formats/Png/PngDecoderTests.cs            | 30 +++++++++++++++++++
 tests/ImageSharp.Tests/TestImages.cs          |  1 +
 tests/Images/Input/Png/rollsroyce.png         |  3 ++
 4 files changed, 36 insertions(+), 1 deletion(-)
 create mode 100644 tests/Images/Input/Png/rollsroyce.png

diff --git a/src/ImageSharp/Formats/Png/PngScanlineProcessor.cs b/src/ImageSharp/Formats/Png/PngScanlineProcessor.cs
index 4242b2d55..40c284117 100644
--- a/src/ImageSharp/Formats/Png/PngScanlineProcessor.cs
+++ b/src/ImageSharp/Formats/Png/PngScanlineProcessor.cs
@@ -240,9 +240,9 @@ namespace SixLabors.ImageSharp.Formats.Png
             else
             {
                 Rgba32 rgba32 = default;
+                int offset = 0;
                 for (int x = pixelOffset; x < header.Width; x += increment)
                 {
-                    int offset = x * bytesPerPixel;
                     byte luminance = Unsafe.Add(ref scanlineSpanRef, offset);
                     byte alpha = Unsafe.Add(ref scanlineSpanRef, offset + bytesPerSample);
                     rgba32.R = luminance;
@@ -252,6 +252,7 @@ namespace SixLabors.ImageSharp.Formats.Png
 
                     pixel.FromRgba32(rgba32);
                     Unsafe.Add(ref rowSpanRef, x) = pixel;
+                    offset += bytesPerPixel;
                 }
             }
         }
diff --git a/tests/ImageSharp.Tests/Formats/Png/PngDecoderTests.cs b/tests/ImageSharp.Tests/Formats/Png/PngDecoderTests.cs
index f51f9b6c5..1452f233b 100644
--- a/tests/ImageSharp.Tests/Formats/Png/PngDecoderTests.cs
+++ b/tests/ImageSharp.Tests/Formats/Png/PngDecoderTests.cs
@@ -67,6 +67,12 @@ namespace SixLabors.ImageSharp.Tests.Formats.Png
             TestImages.Png.GrayTrns16BitInterlaced
         };
 
+        public static readonly string[] TestImagesGrayAlpha8Bit =
+            {
+                TestImages.Png.GrayAlpha8Bit,
+                TestImages.Png.GrayAlpha8Bit2
+            };
+
         public static readonly TheoryData<string, int, int, PixelResolutionUnit> RatioFiles =
         new TheoryData<string, int, int, PixelResolutionUnit>
         {
@@ -123,6 +129,18 @@ namespace SixLabors.ImageSharp.Tests.Formats.Png
             }
         }
 
+        [Theory]
+        [WithFileCollection(nameof(TestImagesGrayAlpha8Bit), PixelTypes.Rgba32)]
+        public void Decoder_Gray8bitWithAlpha<TPixel>(TestImageProvider<TPixel> provider)
+            where TPixel : struct, IPixel<TPixel>
+        {
+            using (Image<TPixel> image = provider.GetImage(new PngDecoder()))
+            {
+                image.DebugSave(provider);
+                image.CompareToOriginal(provider, ImageComparer.Exact);
+            }
+        }
+
         [Theory]
         [WithFileCollection(nameof(TestImagesGray16Bit), PixelTypes.Rgb48)]
         public void Decode_Gray16Bit<TPixel>(TestImageProvider<TPixel> provider)
@@ -159,6 +177,18 @@ namespace SixLabors.ImageSharp.Tests.Formats.Png
             }
         }
 
+        [Theory]
+        [WithFile(TestImages.Png.GrayAlpha8Bit2, PixelTypes)]
+        public void Decoder_CanDecodeGrey8bitWithAlpha<TPixel>(TestImageProvider<TPixel> provider)
+            where TPixel : struct, IPixel<TPixel>
+        {
+            using (Image<TPixel> image = provider.GetImage(new PngDecoder()))
+            {
+                image.DebugSave(provider);
+                image.CompareToOriginal(provider, ImageComparer.Exact);
+            }
+        }
+
         [Fact]
         public void Decode_IgnoreMetadataIsFalse_TextChunckIsRead()
         {
diff --git a/tests/ImageSharp.Tests/TestImages.cs b/tests/ImageSharp.Tests/TestImages.cs
index d83fe4907..92773df7e 100644
--- a/tests/ImageSharp.Tests/TestImages.cs
+++ b/tests/ImageSharp.Tests/TestImages.cs
@@ -29,6 +29,7 @@ namespace SixLabors.ImageSharp.Tests
             public const string Gray4Bpp = "Png/gray_4bpp.png";
             public const string Gray16Bit = "Png/gray-16.png";
             public const string GrayAlpha8Bit = "Png/gray-alpha-8.png";
+            public const string GrayAlpha8Bit2 = "Png/rollsroyce.png";
             public const string GrayAlpha16Bit = "Png/gray-alpha-16.png";
             public const string GrayTrns16BitInterlaced = "Png/gray-16-tRNS-interlaced.png";
             public const string Rgb24BppTrans = "Png/rgb-8-tRNS.png";
diff --git a/tests/Images/Input/Png/rollsroyce.png b/tests/Images/Input/Png/rollsroyce.png
new file mode 100644
index 000000000..c2b9199ac
--- /dev/null
+++ b/tests/Images/Input/Png/rollsroyce.png
@@ -0,0 +1,3 @@
+version https://git-lfs.github.com/spec/v1
+oid sha256:90017d23999f41be7ac525648d8f435dcf8d1481ab98ebfd01a9ce87241e534f
+size 25400