Fix #2467 bmp encoding issue for BMP with 1 bit per pixel and more pixels per row than divisible by 8.

src/ImageSharp/Formats/Bmp/BmpEncoderCore.cs

@@ -668,7 +668,7 @@ private void Write1BitPixelData<TPixel>(Configuration configuration, Stream stre
 
             if (quantizedPixelRow.Length % 8 != 0)
             {
-                int startIdx = quantizedPixelRow.Length - 7;
+                int startIdx = quantizedPixelRow.Length - (quantizedPixelRow.Length % 8);
                 endIdx = quantizedPixelRow.Length;
                 Write1BitPalette(stream, startIdx, endIdx, quantizedPixelRow);
             }

tests/ImageSharp.Tests/Formats/Bmp/BmpEncoderTests.cs

@@ -370,6 +370,30 @@ public void Encode_WorksWithDiscontiguousBuffers<TPixel>(TestImageProvider<TPixe
         TestBmpEncoderCore(provider, bitsPerPixel);
     }
 
+    [Theory]
+    [WithFile(BlackWhitePalletDataMatrix, PixelTypes.Rgb24, BmpBitsPerPixel.Pixel1)]
+    public void Encode_Issue2467<TPixel>(TestImageProvider<TPixel> provider, BmpBitsPerPixel bitsPerPixel)
+        where TPixel : unmanaged, IPixel<TPixel>
+    {
+        using Image<TPixel> image = provider.GetImage();
+
+        using var reencodedStream = new MemoryStream();
+        var encoder = new BmpEncoder
+        {
+            BitsPerPixel = bitsPerPixel,
+            SupportTransparency = false,
+            Quantizer = KnownQuantizers.Octree
+        };
+        image.SaveAsBmp(reencodedStream, encoder);
+        reencodedStream.Seek(0, SeekOrigin.Begin);
+
+        using Image<TPixel> reencodedImage = Image.Load<TPixel>(reencodedStream);
+
+        reencodedImage.DebugSave(provider);
+
+        reencodedImage.CompareToOriginal(provider);
+    }
+
     private static void TestBmpEncoderCore<TPixel>(
         TestImageProvider<TPixel> provider,
         BmpBitsPerPixel bitsPerPixel,

tests/ImageSharp.Tests/TestImages.cs

@@ -407,6 +407,8 @@ public static class Bmp
         public const string Rgba321010102 = "Bmp/rgba32-1010102.bmp";
         public const string RgbaAlphaBitfields = "Bmp/rgba32abf.bmp";
 
+        public const string BlackWhitePalletDataMatrix = "Bmp/bit1datamatrix.bmp";
+
         public static readonly string[] BitFields =
         {
               Rgb32bfdef,