Code explanation for bitmap conversion - c#-4.0

Question

https://stackoverflow.com/a/2574798/159072
public static Bitmap BitmapTo1Bpp(Bitmap img)
{
int w = img.Width;
int h = img.Height;
//
Bitmap bmp = new Bitmap(w, h, PixelFormat.Format1bppIndexed);
BitmapData data = bmp.LockBits(new Rectangle(0, 0, w, h), ImageLockMode.ReadWrite, PixelFormat.Format1bppIndexed);
Why this addition and division?
byte[] scan = new byte[(w + 7) / 8];
for (int y = 0; y < h; y++)
{
for (int x = 0; x < w; x++)
{////Why this condition check?
if (x % 8 == 0)
//Why divide by 8?
scan[x / 8] = 0;
Color c = img.GetPixel(x, y);
//Why this condition check?
if (c.GetBrightness() >= 0.5)
{
// What is going on here?
scan[x / 8] |= (byte)(0x80 >> (x % 8));
}
}
// Why Martial.Copy() called here?
Marshal.Copy(scan, 0, (IntPtr)((long)data.Scan0 + data.Stride * y), scan.Length);
}
bmp.UnlockBits(data);
return bmp;
}

Answer 1

The code uses some basic bit-hacking techniques, required because it needs to set bits and the minimum storage element you can address in C# is a byte. I intentionally avoided using the BitArray class.
int w = img.Width;
I copy the Width and Height properties of the bitmap into a local variable to speed up the code, the properties are too expensive. Keep in mind that w are the number of pixels across the bitmap, it represents the number of bits in the final image.
byte[] scan = new byte[(w + 7) / 8];
The scan variable stores the pixels in one scan line of the bitmap. The 1bpp format uses 1 bit per pixel so the total number of bytes in a scan line is w / 8. I add 7 to ensure the value is rounded up, necessary because integer division always truncates. w = 1..7 requires 1 byte, w = 8..15 requires 2 bytes, etcetera.
if (x % 8 == 0) scan[x / 8] = 0;
The x % 8 expression represents the bit number, x / 8 is the byte number. This code sets all the pixels to Black when it progresses to the next byte in the scan line. Another way to do it would be re-allocating the byte[] in the outer loop or resetting it back to 0 with a for-loop.
if (c.GetBrightness() >= 0.5)
The pixel should be set to White when the source pixel is bright enough. Otherwise it leaves it at Black. Using Color.Brightness is a simple way to avoid dealing with the human eye's non-linear perception of brightness (luminance ~= 0.299 * red + 0.587 * green + 0.114 * blue).
scan[x / 8] |= (byte)(0x80 >> (x % 8));
Sets a bit to White in the scan line. As noted x % 8 is the bit number, it shifts 0x80 to the right by the bit number, they are stored in reverse order in this pixel format.