What's an efficient way to tell if a bitmap is entirely black?

Question

I\'m wondering if there\'s a super-efficient way of confirming that an Image object references an entirely black image, so every pixel within the bitmap is ARGB(255, 0, 0, 0).

Answer 1

I'd recommend you to lock the bitmap in the memory using the LockBits method of the System.Drawing.Bitmap type. This method returns the BitmapData type, from which you can receive a pointer to the locked memory region. Then iterate through the memory, searching for the non-zero bytes (really, faster by scanning for the Int32 or even Int64 values, depending on the platform you use). Code will look like this:

// Lock the bitmap's bits.  
Rectangle rect = new Rectangle(0, 0, bmp.Width, bmp.Height);
BitmapData bmpData =bmp.LockBits(rect, ImageLockMode.ReadWrite, bmp.PixelFormat);

// Get the address of the first line.
IntPtr ptr = bmpData.Scan0;

// Declare an array to hold the bytes of the bitmap.
int bytes  = bmpData.Stride * bmp.Height;
byte[] rgbValues = new byte[bytes];

// Copy the RGB values into the array.
Marshal.Copy(ptr, rgbValues, 0, bytes);

// Scanning for non-zero bytes
bool allBlack = true;
for (int index = 0; index < rgbValues.Length; index++)
    if (rgbValues[index] != 0) 
    {
       allBlack = false;
       break;
    }
// Unlock the bits.
bmp.UnlockBits(bmpData);

Consider using the unsafe code and direct memory access (using pointers) to improve performance.