Use scaled luminance trns comparison.

src/ImageSharp/Formats/Png/PngScanlineProcessor.cs

@@ -80,6 +80,7 @@ namespace SixLabors.ImageSharp.Formats.Png
             }
             else
             {
+                byte scaledLuminanceTrans = (byte)(luminanceTrans * scaleFactor);
                 Rgba32 rgba32 = default;
                 for (int x = 0; x < header.Width; x++)
                 {
@@ -87,7 +88,7 @@ namespace SixLabors.ImageSharp.Formats.Png
                     rgba32.R = luminance;
                     rgba32.G = luminance;
                     rgba32.B = luminance;
-                    rgba32.A = luminance.Equals(luminanceTrans) ? byte.MinValue : byte.MaxValue;
+                    rgba32.A = luminance.Equals(scaledLuminanceTrans) ? byte.MinValue : byte.MaxValue;
 
                     pixel.PackFromRgba32(rgba32);
                     Unsafe.Add(ref rowSpanRef, x) = pixel;
@@ -163,6 +164,7 @@ namespace SixLabors.ImageSharp.Formats.Png
             }
             else
             {
+                byte scaledLuminanceTrans = (byte)(luminanceTrans * scaleFactor);
                 Rgba32 rgba32 = default;
                 for (int x = pixelOffset; x < header.Width; x += increment)
                 {
@@ -170,7 +172,7 @@ namespace SixLabors.ImageSharp.Formats.Png
                     rgba32.R = luminance;
                     rgba32.G = luminance;
                     rgba32.B = luminance;
-                    rgba32.A = luminance.Equals(luminanceTrans) ? byte.MinValue : byte.MaxValue;
+                    rgba32.A = luminance.Equals(scaledLuminanceTrans) ? byte.MinValue : byte.MaxValue;
 
                     pixel.PackFromRgba32(rgba32);
                     Unsafe.Add(ref rowSpanRef, x) = pixel;

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

@@ -40,7 +40,8 @@ namespace SixLabors.ImageSharp.Tests.Formats.Png
             TestImages.Png.VimImage2,
 
             TestImages.Png.Rgb24BppTrans,
-            TestImages.Png.GrayAlpha8Bit
+            TestImages.Png.GrayAlpha8Bit,
+            TestImages.Png.Gray1BitTrans
         };
 
         public static readonly string[] TestImages48Bpp =

tests/ImageSharp.Tests/TestImages.cs

@@ -45,6 +45,7 @@ namespace SixLabors.ImageSharp.Tests
             public const string Kaboom = "Png/kaboom.png";
             public const string PDSrc = "Png/pd-source.png";
             public const string PDDest = "Png/pd-dest.png";
+            public const string Gray1BitTrans = "Png/gray-1-trns.png";
 
             // Filtered test images from http://www.schaik.com/pngsuite/pngsuite_fil_png.html
             public const string Filter0 = "Png/filter0.png";

tests/Images/Input/Png/gray-1-trns.png

@@ -0,0 +1,3 @@
+version https://git-lfs.github.com/spec/v1
+oid sha256:cca88b334de7037abfacd9d87c2cb8eec7391f355c1d6b4a694fc4240f358ee1
+size 366