PackBits bug fix

src/ImageSharp/Formats/Tiff/Compression/Compressors/PackBitsCompressor.cs

@@ -3,8 +3,6 @@
 
 using System;
 using System.IO;
-using SixLabors.ImageSharp.Formats.Experimental.Tiff.Compression.Compressors;
-using SixLabors.ImageSharp.Formats.Experimental.Tiff.Constants;
 using SixLabors.ImageSharp.Memory;
 
 namespace SixLabors.ImageSharp.Formats.Experimental.Tiff.Compression.Compressors
@@ -22,16 +20,23 @@ namespace SixLabors.ImageSharp.Formats.Experimental.Tiff.Compression.Compressors
 
         public override void Initialize(int rowsPerStrip)
         {
-            int additionalBytes = (this.BytesPerRow / 127) + 1;
+            int additionalBytes = ((this.BytesPerRow + 126) / 127) + 1;
-            this.pixelData = this.Allocator.AllocateManagedByteBuffer((this.BytesPerRow + additionalBytes) * rowsPerStrip);
+            this.pixelData = this.Allocator.AllocateManagedByteBuffer(this.BytesPerRow + additionalBytes);
         }
 
         public override void CompressStrip(Span<byte> rows, int height)
         {
+            DebugGuard.IsTrue(rows.Length % height == 0, "Invalid height");
+            DebugGuard.IsTrue(this.BytesPerRow == rows.Length / height, "The widths must match");
+
             this.pixelData.Clear();
             Span<byte> span = this.pixelData.GetSpan();
-            int size = PackBitsWriter.PackBits(rows, span);
+            for (int i = 0; i < height; i++)
-            this.Output.Write(span.Slice(0, size));
+            {
+                Span<byte> row = rows.Slice(i * this.BytesPerRow, this.BytesPerRow);
+                int size = PackBitsWriter.PackBits(row, span);
+                this.Output.Write(span.Slice(0, size));
+            }
         }
 
         protected override void Dispose(bool disposing) => this.pixelData?.Dispose();

src/ImageSharp/Formats/Tiff/Writers/TiffBiColorWriter.cs

@@ -51,18 +51,19 @@ namespace SixLabors.ImageSharp.Formats.Experimental.Tiff.Writers
 
             if (compressor.Method == TiffEncoderCompression.CcittGroup3Fax || compressor.Method == TiffEncoderCompression.ModifiedHuffman)
             {
+                // special case for T4BitCompressor
                 compressor.CompressStrip(pixelAsGraySpan, height);
             }
             else
             {
+                int bytesPerStrip = this.BytesPerRow * height;
                 if (this.bitStrip == null)
                 {
-                    int bytesPerRow = this.BytesPerRow * height;
+                    this.bitStrip = this.MemoryAllocator.AllocateManagedByteBuffer(bytesPerStrip);
-                    this.bitStrip = this.MemoryAllocator.AllocateManagedByteBuffer(bytesPerRow);
                 }
 
-                this.bitStrip.Clear();
+                Span<byte> rows = this.bitStrip.Slice(0, bytesPerStrip);
-                Span<byte> rows = this.bitStrip.GetSpan();
+                rows.Clear();
 
                 int xx = 0;
                 for (int s = 0; s < height; s++)