Fix combine for black & white

NAPS2.Images/Bitwise/BitwisePrimitives.cs

@@ -34,6 +34,7 @@ public static unsafe void Fill(BitwiseImageData data, byte value, int partStart
     {
         if (partStart == -1) partStart = 0;
         if (partEnd == -1) partEnd = data.h;
+        if (data.invertColorSpace) value = (byte) ~value;
 
         var longCount = data.stride / 8;
         var remainingStart = longCount * 8;

NAPS2.Images/Bitwise/CopyBitwiseImageOp.cs

@@ -59,8 +59,14 @@ protected override void ValidateCore(BitwiseImageData src, BitwiseImageData dst)
     protected override void PerformCore(BitwiseImageData src, BitwiseImageData dst, int partStart, int partEnd)
     {
         if (src.BitLayout == dst.BitLayout &&
-            (src.bytesPerPixel > 0 || (SourceXOffset % 8 == 0 && DestXOffset % 8 == 0)) &&
-            DestChannel == ColorChannel.All)
+            DestChannel == ColorChannel.All &&
+            (src.bytesPerPixel > 0 ||
+             // For Black & White images, to use the fast copy path, we must have that:
+             // 1. The offsets are to whole bytes
+             // 2a. Either we copy whole bytes, or
+             // 2b. We end at the far-right side of the destination (so any excess bits copied will be ignored)
+             (SourceXOffset % 8 == 0 && DestXOffset % 8 == 0 &&
+              (src.w % 8 == 0 || src.w + DestXOffset == dst.w))))
         {
             FastCopy(src, dst, partStart, partEnd);
         }
@@ -254,6 +260,7 @@ private unsafe void UnalignedBitCopy(BitwiseImageData src, BitwiseImageData dst,
                 var dstPixelIndex = j + DestXOffset;
                 var dstPtr = dstRow + dstPixelIndex / 8;
                 var dstByte = *dstPtr;
+                dstByte &= (byte) ~(1 << (7 - dstPixelIndex % 8));
                 dstByte |= (byte) (bit << (7 - dstPixelIndex % 8));
                 *dstPtr = dstByte;
             }

NAPS2.Images/Bitwise/FillColorImageOp.cs

@@ -15,12 +15,20 @@ public FillColorImageOp(byte r, byte g, byte b, byte a)
         _a = a;
     }
 
+    protected override void ValidateCore(BitwiseImageData data)
+    {
+    }
+
     protected override void PerformCore(BitwiseImageData data, int partStart, int partEnd)
     {
         if (data.bytesPerPixel is 1 or 3 or 4)
         {
             PerformRgba(data, partStart, partEnd);
         }
+        else if (data.bitsPerPixel == 1 && (_r, _g, _b, _a) is (0, 0, 0, 255) or (255, 255, 255, 255))
+        {
+            PerformBw(data, partStart, partEnd);
+        }
         else
         {
             throw new InvalidOperationException("Unsupported pixel format");
@@ -54,4 +62,10 @@ private unsafe void PerformRgba(BitwiseImageData data, int partStart, int partEn
             }
         }
     }
+
+    private void PerformBw(BitwiseImageData data, int partStart, int partEnd)
+    {
+        byte fill = (byte) (_r == 255 ? 0xFF : 0x00);
+        BitwisePrimitives.Fill(data, fill, partStart, partEnd);
+    }
 }

NAPS2.Sdk.Tests/ImageResources.resx

@@ -202,6 +202,9 @@
   <data name="skewed_bw" type="System.Resources.ResXFileRef, System.Windows.Forms">
     <value>Resources\skewed_bw.png;System.Byte[], mscorlib, Version=4.0.0.0, Culture=neutral, PublicKeyToken=b77a5c561934e089</value>
   </data>
+  <data name="cat" type="System.Resources.ResXFileRef, System.Windows.Forms">
+    <value>Resources\cat.jpg;System.Byte[], mscorlib, Version=4.0.0.0, Culture=neutral, PublicKeyToken=b77a5c561934e089</value>
+  </data>
   <data name="stock_cat" type="System.Resources.ResXFileRef, System.Windows.Forms">
     <value>Resources\stock-cat.jpeg;System.Byte[], mscorlib, Version=4.0.0.0, Culture=neutral, PublicKeyToken=b77a5c561934e089</value>
   </data>
@@ -304,4 +307,10 @@
   <data name="dog_exif" type="System.Resources.ResXFileRef, System.Windows.Forms">
     <value>Resources\dog_exif.jpg;System.Byte[], mscorlib, Version=4.0.0.0, Culture=neutral, PublicKeyToken=b77a5c561934e089</value>
   </data>
+  <data name="dog_cat_combined" type="System.Resources.ResXFileRef, System.Windows.Forms">
+    <value>Resources\dog_cat_combined.jpg;System.Byte[], mscorlib, Version=4.0.0.0, Culture=neutral, PublicKeyToken=b77a5c561934e089</value>
+  </data>
+  <data name="dog_cat_combined_bw" type="System.Resources.ResXFileRef, System.Windows.Forms">
+    <value>Resources\dog_cat_combined_bw.jpg;System.Byte[], mscorlib, Version=4.0.0.0, Culture=neutral, PublicKeyToken=b77a5c561934e089</value>
+  </data>
 </root>

NAPS2.Sdk.Tests/Images/TransformTests.cs

@@ -464,6 +464,32 @@ public void Thumbnail()
         AssertOwnership(original, transformed);
     }
 
+    [Fact]
+    public void Combine()
+    {
+        var first = LoadImage(ImageResources.dog);
+        var second = LoadImage(ImageResources.cat);
+        var expected = LoadImage(ImageResources.dog_cat_combined);
+
+        var transformed = MoreImageTransforms.Combine(first, second, CombineOrientation.Vertical);
+        Assert.Equal(ImagePixelFormat.RGB24, transformed.PixelFormat);
+
+        ImageAsserts.Similar(expected, transformed, ImageAsserts.GENERAL_RMSE_THRESHOLD);
+    }
+
+    [Fact]
+    public void CombineBlackAndWhite()
+    {
+        var first = LoadImage(ImageResources.dog).PerformTransform(new BlackWhiteTransform());
+        var second = LoadImage(ImageResources.cat).PerformTransform(new BlackWhiteTransform());
+        var expected = LoadImage(ImageResources.dog_cat_combined_bw).PerformTransform(new BlackWhiteTransform());
+
+        var transformed = MoreImageTransforms.Combine(first, second, CombineOrientation.Vertical);
+        Assert.Equal(ImagePixelFormat.BW1, transformed.UpdateLogicalPixelFormat());
+
+        ImageAsserts.Similar(expected, transformed, ImageAsserts.XPLAT_RMSE_THRESHOLD);
+    }
+
     private void AssertOwnership(IMemoryImage original, IMemoryImage transformed)
     {
         // The contract for a transform is that either it returns the original image or it disposes the original and