Fix for B&W pbm images with width not dividable by 8

src/ImageSharp/Formats/Pbm/BinaryDecoder.cs

@@ -152,7 +152,6 @@ private static void ProcessBlackAndWhite<TPixel>(Configuration configuration, Bu
     {
         int width = pixels.Width;
         int height = pixels.Height;
-        int startBit = 0;
         MemoryAllocator allocator = configuration.MemoryAllocator;
         using IMemoryOwner<L8> row = allocator.Allocate<L8>(width);
         Span<L8> rowSpan = row.GetSpan();
@@ -162,23 +161,12 @@ private static void ProcessBlackAndWhite<TPixel>(Configuration configuration, Bu
             for (int x = 0; x < width;)
             {
                 int raw = stream.ReadByte();
-                int bit = startBit;
-                startBit = 0;
-                for (; bit < 8; bit++)
+                int stopBit = Math.Min(8, width - x);
+                for (int bit = 0; bit < stopBit; bit++)
                 {
                     bool bitValue = (raw & (0x80 >> bit)) != 0;
                     rowSpan[x] = bitValue ? black : white;
                     x++;
-                    if (x == width)
-                    {
-                        startBit = (bit + 1) & 7; // Round off to below 8.
-                        if (startBit != 0)
-                        {
-                            stream.Seek(-1, System.IO.SeekOrigin.Current);
-                        }
-
-                        break;
-                    }
                 }
             }
 

src/ImageSharp/Formats/Pbm/BinaryEncoder.cs

@@ -33,25 +33,40 @@ public static void WritePixels<TPixel>(Configuration configuration, Stream strea
             {
                 WriteGrayscale(configuration, stream, image);
             }
-            else
+            else if (componentType == PbmComponentType.Short)
             {
                 WriteWideGrayscale(configuration, stream, image);
             }
+            else
+            {
+                throw new ImageFormatException("Component type not supported for Grayscale PBM.");
+            }
         }
         else if (colorType == PbmColorType.Rgb)
         {
             if (componentType == PbmComponentType.Byte)
             {
                 WriteRgb(configuration, stream, image);
             }
-            else
+            else if (componentType == PbmComponentType.Short)
             {
                 WriteWideRgb(configuration, stream, image);
             }
+            else
+            {
+                throw new ImageFormatException("Component type not supported for Color PBM.");
+            }
         }
         else
         {
-            WriteBlackAndWhite(configuration, stream, image);
+            if (componentType == PbmComponentType.Bit)
+            {
+                WriteBlackAndWhite(configuration, stream, image);
+            }
+            else
+            {
+                throw new ImageFormatException("Component type not supported for Black & White PBM.");
+            }
         }
     }
 
@@ -164,8 +179,6 @@ private static void WriteBlackAndWhite<TPixel>(Configuration configuration, Stre
         using IMemoryOwner<L8> row = allocator.Allocate<L8>(width);
         Span<L8> rowSpan = row.GetSpan();
 
-        int previousValue = 0;
-        int startBit = 0;
         for (int y = 0; y < height; y++)
         {
             Span<TPixel> pixelSpan = pixelBuffer.DangerousGetRowSpan(y);
@@ -177,28 +190,19 @@ private static void WriteBlackAndWhite<TPixel>(Configuration configuration, Stre
 
             for (int x = 0; x < width;)
             {
-                int value = previousValue;
-                for (int i = startBit; i < 8; i++)
+                int value = 0;
+                int stopBit = Math.Min(8, width - x);
+                for (int i = 0; i < stopBit; i++)
                 {
                     if (rowSpan[x].PackedValue < 128)
                     {
                         value |= 0x80 >> i;
                     }
 
                     x++;
-                    if (x == width)
-                    {
-                        previousValue = value;
-                        startBit = (i + 1) & 7; // Round off to below 8.
-                        break;
-                    }
                 }
 
-                if (startBit == 0)
-                {
-                    stream.WriteByte((byte)value);
-                    previousValue = 0;
-                }
+                stream.WriteByte((byte)value);
             }
         }
     }

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

@@ -81,6 +81,7 @@ public void ImageLoadRgb24CanDecode(string imagePath)
     [Theory]
     [WithFile(BlackAndWhitePlain, PixelTypes.L8, "pbm")]
     [WithFile(BlackAndWhiteBinary, PixelTypes.L8, "pbm")]
+    [WithFile(Issue2477, PixelTypes.L8, "pbm")]
     [WithFile(GrayscalePlain, PixelTypes.L8, "pgm")]
     [WithFile(GrayscalePlainNormalized, PixelTypes.L8, "pgm")]
     [WithFile(GrayscaleBinary, PixelTypes.L8, "pgm")]

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

@@ -26,6 +26,7 @@ public class PbmEncoderTests
         {
             { BlackAndWhiteBinary, PbmColorType.BlackAndWhite },
             { BlackAndWhitePlain, PbmColorType.BlackAndWhite },
+            { Issue2477, PbmColorType.BlackAndWhite },
             { GrayscaleBinary, PbmColorType.Grayscale },
             { GrayscaleBinaryWide, PbmColorType.Grayscale },
             { GrayscalePlain, PbmColorType.Grayscale },
@@ -96,6 +97,11 @@ public void PbmEncoder_P1_Works<TPixel>(TestImageProvider<TPixel> provider)
     public void PbmEncoder_P4_Works<TPixel>(TestImageProvider<TPixel> provider)
         where TPixel : unmanaged, IPixel<TPixel> => TestPbmEncoderCore(provider, PbmColorType.BlackAndWhite, PbmEncoding.Binary);
 
+    [Theory]
+    [WithFile(Issue2477, PixelTypes.Rgb24)]
+    public void PbmEncoder_P4_Irregular_Works<TPixel>(TestImageProvider<TPixel> provider)
+        where TPixel : unmanaged, IPixel<TPixel> => TestPbmEncoderCore(provider, PbmColorType.BlackAndWhite, PbmEncoding.Binary);
+
     [Theory]
     [WithFile(GrayscalePlainMagick, PixelTypes.Rgb24)]
     public void PbmEncoder_P2_Works<TPixel>(TestImageProvider<TPixel> provider)

tests/ImageSharp.Tests/TestImages.cs

@@ -1038,5 +1038,6 @@ public static class Pbm
         public const string RgbPlain = "Pbm/rgb_plain.ppm";
         public const string RgbPlainNormalized = "Pbm/rgb_plain_normalized.ppm";
         public const string RgbPlainMagick = "Pbm/rgb_plain_magick.ppm";
+        public const string Issue2477 = "Pbm/issue2477.pbm";
     }
 }