Various review comments

src/ImageSharp/Formats/Pbm/PbmEncoderCore.cs

@@ -88,6 +88,7 @@ namespace SixLabors.ImageSharp.Formats.Pbm
             if (this.colorType != PbmColorType.BlackAndWhite)
             {
                 this.maxPixelValue = this.options.MaxPixelValue ?? metadata.MaxPixelValue;
+                this.maxPixelValue = Math.Max(this.maxPixelValue, PbmConstants.MaxLength);
             }
         }
 

src/ImageSharp/Formats/Pbm/PlainEncoder.cs

@@ -76,7 +76,7 @@ namespace SixLabors.ImageSharp.Formats.Pbm
             MemoryAllocator allocator = configuration.MemoryAllocator;
             using IMemoryOwner<L8> row = allocator.Allocate<L8>(width);
             Span<L8> rowSpan = row.GetSpan();
-            IMemoryOwner<byte> plainMemory = allocator.Allocate<byte>(width * MaxCharsPerPixelGrayscale);
+            using IMemoryOwner<byte> plainMemory = allocator.Allocate<byte>(width * MaxCharsPerPixelGrayscale);
             Span<byte> plainSpan = plainMemory.GetSpan();
 
             for (int y = 0; y < height; y++)
@@ -108,7 +108,7 @@ namespace SixLabors.ImageSharp.Formats.Pbm
             MemoryAllocator allocator = configuration.MemoryAllocator;
             using IMemoryOwner<L16> row = allocator.Allocate<L16>(width);
             Span<L16> rowSpan = row.GetSpan();
-            IMemoryOwner<byte> plainMemory = allocator.Allocate<byte>(width * MaxCharsPerPixelGrayscaleWide);
+            using IMemoryOwner<byte> plainMemory = allocator.Allocate<byte>(width * MaxCharsPerPixelGrayscaleWide);
             Span<byte> plainSpan = plainMemory.GetSpan();
 
             for (int y = 0; y < height; y++)
@@ -140,7 +140,7 @@ namespace SixLabors.ImageSharp.Formats.Pbm
             MemoryAllocator allocator = configuration.MemoryAllocator;
             using IMemoryOwner<Rgb24> row = allocator.Allocate<Rgb24>(width);
             Span<Rgb24> rowSpan = row.GetSpan();
-            IMemoryOwner<byte> plainMemory = allocator.Allocate<byte>(width * MaxCharsPerPixelRgb);
+            using IMemoryOwner<byte> plainMemory = allocator.Allocate<byte>(width * MaxCharsPerPixelRgb);
             Span<byte> plainSpan = plainMemory.GetSpan();
 
             for (int y = 0; y < height; y++)
@@ -178,7 +178,7 @@ namespace SixLabors.ImageSharp.Formats.Pbm
             MemoryAllocator allocator = configuration.MemoryAllocator;
             using IMemoryOwner<Rgb48> row = allocator.Allocate<Rgb48>(width);
             Span<Rgb48> rowSpan = row.GetSpan();
-            IMemoryOwner<byte> plainMemory = allocator.Allocate<byte>(width * MaxCharsPerPixelRgbWide);
+            using IMemoryOwner<byte> plainMemory = allocator.Allocate<byte>(width * MaxCharsPerPixelRgbWide);
             Span<byte> plainSpan = plainMemory.GetSpan();
 
             for (int y = 0; y < height; y++)
@@ -216,7 +216,7 @@ namespace SixLabors.ImageSharp.Formats.Pbm
             MemoryAllocator allocator = configuration.MemoryAllocator;
             using IMemoryOwner<L8> row = allocator.Allocate<L8>(width);
             Span<L8> rowSpan = row.GetSpan();
-            IMemoryOwner<byte> plainMemory = allocator.Allocate<byte>(width * MaxCharsPerPixelBlackAndWhite);
+            using IMemoryOwner<byte> plainMemory = allocator.Allocate<byte>(width * MaxCharsPerPixelBlackAndWhite);
             Span<byte> plainSpan = plainMemory.GetSpan();
 
             for (int y = 0; y < height; y++)

tests/ImageSharp.Tests/Formats/Pbm/PbmDecoderTests.cs

@@ -84,6 +84,7 @@ namespace SixLabors.ImageSharp.Tests.Formats.Pbm
             where TPixel : unmanaged, IPixel<TPixel>
         {
             using Image<TPixel> image = provider.GetImage();
+            image.DebugSave(provider);
 
             image.CompareToReferenceOutput(provider, grayscale: isGrayscale);
         }

tests/ImageSharp.Tests/Formats/Pbm/RoundTripTests.cs → tests/ImageSharp.Tests/Formats/Pbm/PbmRoundTripTests.cs

@@ -11,8 +11,28 @@ using static SixLabors.ImageSharp.Tests.TestImages.Pbm;
 namespace SixLabors.ImageSharp.Tests.Formats.Pbm
 {
     [Trait("Format", "Pbm")]
-    public class RoundTripTests
+    public class PbmRoundTripTests
     {
+        [Theory]
+        [InlineData(BlackAndWhiteBinary)]
+        [InlineData(GrayscalePlain)]
+        [InlineData(GrayscaleBinary)]
+        public void PbmGrayscaleImageCanRoundTrip(string imagePath)
+        {
+            // Arrange
+            var testFile = TestFile.Create(imagePath);
+            using var stream = new MemoryStream(testFile.Bytes, false);
+
+            // Act
+            using var originalImage = Image.Load(stream);
+            Image<Rgb24> colorImage = originalImage.CloneAs<Rgb24>();
+            using Image<Rgb24> encodedImage = this.RoundTrip(colorImage);
+
+            // Assert
+            Assert.NotNull(encodedImage);
+            ImageComparer.Exact.VerifySimilarity(colorImage, encodedImage);
+        }
+
         [Theory]
         [InlineData(RgbPlain)]
         [InlineData(RgbBinary)]
@@ -37,7 +57,7 @@ namespace SixLabors.ImageSharp.Tests.Formats.Pbm
             using var decodedStream = new MemoryStream();
             originalImage.SaveAsPbm(decodedStream);
             decodedStream.Seek(0, SeekOrigin.Begin);
-            var encodedImage = (Image<TPixel>)Image.Load(decodedStream);
+            var encodedImage = Image.Load<TPixel>(decodedStream);
             return encodedImage;
         }
     }

tests/ImageSharp.Tests/Formats/Pbm/PbmTestUtils.cs

@@ -1,65 +0,0 @@
-// Copyright (c) Six Labors.
-// Licensed under the Apache License, Version 2.0.
-
-using System;
-using System.IO;
-using ImageMagick;
-using SixLabors.ImageSharp.PixelFormats;
-using SixLabors.ImageSharp.Tests.TestUtilities.ImageComparison;
-using Xunit;
-
-namespace SixLabors.ImageSharp.Tests.Formats.Pbm
-{
-    public static class PbmTestUtils
-    {
-        public static void CompareWithReferenceDecoder<TPixel>(
-            TestImageProvider<TPixel> provider,
-            Image<TPixel> image,
-            bool useExactComparer = true,
-            float compareTolerance = 0.01f)
-            where TPixel : unmanaged, ImageSharp.PixelFormats.IPixel<TPixel>
-        {
-            string path = TestImageProvider<TPixel>.GetFilePathOrNull(provider);
-            if (path == null)
-            {
-                throw new InvalidOperationException("CompareToOriginal() works only with file providers!");
-            }
-
-            var testFile = TestFile.Create(path);
-            Image<Rgba32> magickImage = DecodeWithMagick<Rgba32>(Configuration.Default, new FileInfo(testFile.FullPath));
-            if (useExactComparer)
-            {
-                ImageComparer.Exact.VerifySimilarity(magickImage, image);
-            }
-            else
-            {
-                ImageComparer.Tolerant(compareTolerance).VerifySimilarity(magickImage, image);
-            }
-        }
-
-        public static Image<TPixel> DecodeWithMagick<TPixel>(Configuration configuration, FileInfo fileInfo)
-            where TPixel : unmanaged, ImageSharp.PixelFormats.IPixel<TPixel>
-        {
-            using (var magickImage = new MagickImage(fileInfo))
-            {
-                magickImage.AutoOrient();
-                var result = new Image<TPixel>(configuration, magickImage.Width, magickImage.Height);
-
-                Assert.True(result.TryGetSinglePixelSpan(out Span<TPixel> resultPixels));
-
-                using (IUnsafePixelCollection<ushort> pixels = magickImage.GetPixelsUnsafe())
-                {
-                    byte[] data = pixels.ToByteArray(PixelMapping.RGBA);
-
-                    PixelOperations<TPixel>.Instance.FromRgba32Bytes(
-                        configuration,
-                        data,
-                        resultPixels,
-                        resultPixels.Length);
-                }
-
-                return result;
-            }
-        }
-    }
-}