Fix bug in PngFromPdfImageFactory where softmask is wrongly referenced.

Author: xufeng

src/UglyToad.PdfPig.Tests/Images/Files/Pdf/indexed-png-with-mask.pdf

@@ -103,6 +103,30 @@ public void CanGeneratePngFromIndexedImageData8bpc()
             Assert.True(ImageHelpers.ImagesAreEqual(LoadImage("3x3.png"), bytes));
         }
 
+        [Fact]
+        public void CanExtractPngFromPdfWithIndexedImageData8bpc()
+        {
+            var documetFolder = Path.GetFullPath(Path.Combine(AppDomain.CurrentDomain.BaseDirectory, "..", "..", "..", "Images", "Files"));
+
+            var pdfPath = Path.Combine(documetFolder, "Pdf",  "indexed-png-with-mask.pdf");
+
+            using (PdfDocument document = PdfDocument.Open(pdfPath))
+            {
+                var page = document.GetPage(1);
+
+                var img = page.GetImages().First();
+
+                var result = img.TryGetPng(out var bytes);
+
+                if (result)
+                {
+                    var outputPath = Path.Combine(documetFolder, "Pdf", "indexed-png-with-mask.png");
+
+                    File.WriteAllBytes(outputPath, bytes);
+                }
+            }
+        }
+
         [Fact]
         public void CanGeneratePngFromIndexedImageData1bpc()
         {

src/UglyToad.PdfPig.Tests/Images/Files/Pdf/indexed-png-with-mask.png

@@ -7,36 +7,40 @@
 
     internal static class PngFromPdfImageFactory
     {
-        private static bool TryGenerateSoftMask(IPdfImage image, [NotNullWhen(true)] out ReadOnlySpan<byte> bytes)
+        private static bool TryGenerateSoftMask(IPdfImage image, [NotNullWhen(true)] out ReadOnlySpan<byte> maskBytes)
         {
-            bytes = ReadOnlySpan<byte>.Empty;
+            maskBytes = ReadOnlySpan<byte>.Empty;
 
-            if (image.MaskImage is null)
+            var mask = image.MaskImage;
+
+            if (mask is null)
             {
                 return false;
             }
 
             // Because we cannot resize images directly in PdfPig, we only
             // apply the mask if it has the same size as the image
-            if (image.HeightInSamples != image.MaskImage.HeightInSamples ||
-                image.WidthInSamples != image.MaskImage.WidthInSamples)
+            if (image.HeightInSamples != mask.HeightInSamples ||
+                image.WidthInSamples != mask.WidthInSamples)
             {
                 return false;
             }
 
-            if (!image.TryGetBytesAsMemory(out var imageMemory))
+            if (!mask.TryGetBytesAsMemory(out var maskMemory))
             {
                 return false;
             }
 
             try
             {
-                bytes = ColorSpaceDetailsByteConverter.Convert(image.ColorSpaceDetails!,
-                    imageMemory.Span,
-                    image.BitsPerComponent,
-                    image.WidthInSamples,
-                    image.HeightInSamples);
-                return IsCorrectlySized(image, bytes);
+                maskBytes = ColorSpaceDetailsByteConverter.Convert(
+                    mask.ColorSpaceDetails!,
+                    maskMemory.Span,
+                    mask.BitsPerComponent,
+                    mask.WidthInSamples,
+                    mask.HeightInSamples);
+
+                return IsCorrectlySized(mask, maskBytes);
             }
             catch (Exception)
             {