Fix Decoding interlaced grayscale (#831)

Author: brianpopow

src/ImageSharp/Formats/Png/PngScanlineProcessor.cs

@@ -145,9 +145,9 @@ public static void ProcessInterlacedGrayscaleScanline<TPixel>(
             {
                 byte scaledLuminanceTrans = (byte)(luminanceTrans.PackedValue * scaleFactor);
                 Rgba32 rgba32 = default;
-                for (int x = pixelOffset; x < header.Width; x += increment)
+                for (int x = pixelOffset, o = 0; x < header.Width; x += increment, o++)
                 {
-                    byte luminance = (byte)(Unsafe.Add(ref scanlineSpanRef, x) * scaleFactor);
+                    byte luminance = (byte)(Unsafe.Add(ref scanlineSpanRef, o) * scaleFactor);
                     rgba32.R = luminance;
                     rgba32.G = luminance;
                     rgba32.B = luminance;

tests/ImageSharp.Tests/Formats/Png/PngDecoderTests.cs

@@ -67,10 +67,12 @@ public partial class PngDecoderTests
             TestImages.Png.GrayTrns16BitInterlaced
         };
 
-        public static readonly string[] TestImagesGrayAlpha8Bit =
+        public static readonly string[] TestImagesGray8BitInterlaced =
             {
-                TestImages.Png.GrayAlpha8Bit,
-                TestImages.Png.GrayAlpha8Bit2
+                TestImages.Png.GrayAlpha1BitInterlaced,
+                TestImages.Png.GrayAlpha2BitInterlaced,
+                TestImages.Png.Gray4BitInterlaced,
+                TestImages.Png.GrayAlpha8BitInterlaced
             };
 
         public static readonly TheoryData<string, int, int, PixelResolutionUnit> RatioFiles =
@@ -130,8 +132,8 @@ public void Decode_64Bpp<TPixel>(TestImageProvider<TPixel> provider)
         }
 
         [Theory]
-        [WithFileCollection(nameof(TestImagesGrayAlpha8Bit), PixelTypes.Rgba32)]
-        public void Decoder_Gray8bitWithAlpha<TPixel>(TestImageProvider<TPixel> provider)
+        [WithFileCollection(nameof(TestImagesGray8BitInterlaced), PixelTypes.Rgba32)]
+        public void Decoder_Gray8bitInterlaced<TPixel>(TestImageProvider<TPixel> provider)
             where TPixel : struct, IPixel<TPixel>
         {
             using (Image<TPixel> image = provider.GetImage(new PngDecoder()))
@@ -166,8 +168,8 @@ public void Decode_GrayAlpha16Bit<TPixel>(TestImageProvider<TPixel> provider)
         }
 
         [Theory]
-        [WithFile(TestImages.Png.Splash, PixelTypes)]
-        public void Decoder_IsNotBoundToSinglePixelType<TPixel>(TestImageProvider<TPixel> provider)
+        [WithFile(TestImages.Png.GrayAlpha8BitInterlaced, PixelTypes)]
+        public void Decoder_CanDecodeGrey8bitWithAlpha<TPixel>(TestImageProvider<TPixel> provider)
             where TPixel : struct, IPixel<TPixel>
         {
             using (Image<TPixel> image = provider.GetImage(new PngDecoder()))
@@ -178,8 +180,8 @@ public void Decoder_IsNotBoundToSinglePixelType<TPixel>(TestImageProvider<TPixel
         }
 
         [Theory]
-        [WithFile(TestImages.Png.GrayAlpha8Bit2, PixelTypes)]
-        public void Decoder_CanDecodeGrey8bitWithAlpha<TPixel>(TestImageProvider<TPixel> provider)
+        [WithFile(TestImages.Png.Splash, PixelTypes)]
+        public void Decoder_IsNotBoundToSinglePixelType<TPixel>(TestImageProvider<TPixel> provider)
             where TPixel : struct, IPixel<TPixel>
         {
             using (Image<TPixel> image = provider.GetImage(new PngDecoder()))

tests/ImageSharp.Tests/TestImages.cs

@@ -29,7 +29,10 @@ public static class Png
             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 GrayAlpha8BitInterlaced = "Png/rollsroyce.png";
+            public const string GrayAlpha1BitInterlaced = "Png/iftbbn0g01.png";
+            public const string GrayAlpha2BitInterlaced = "Png/iftbbn0g02.png";
+            public const string Gray4BitInterlaced = "Png/iftbbn0g04.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";