My image is cut when writing to a bitmap!

[Sebastian12]

Hello everyone!

I currently write a program which indexes an image to 16 colors, correcting its background-color and correcting the image itself afterwards. To "write pixels" I coded a little method:

void SetPixel(Bitmap bmp, int x, int y, Color c)
            int i;
            int min = int.MaxValue;
            int minI = 0;
            for (i = 0; i < 256; i++)
            {
                Color cPal = bmp.Palette.Entries[i];
                int r = c.R - cPal.R;
                int g = c.G - cPal.G;
                int b = c.B - cPal.B;
                int d = r * r + g * g + b * b;
                if (d == 0) break;
                if (d < min)
                {
                    min = d;
                    minI = i;
                }
            }
            if (i == 256) i = minI;
            BitmapData bmpData = bmp.LockBits(new Rectangle(x, y, 1, 1),
                                              ImageLockMode.ReadOnly,
                                              bmp.PixelFormat);
            Marshal.WriteByte(bmpData.Scan0, (byte)i);
            bmp.UnlockBits(bmpData);

The output is this:

It works with 8bpp images but not with 4bpp! help :(
As you can see, the image is kind of cut every 2nd pixel.
(Note: Left one is INPUT; right is OUTPUT)