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

The first answer to this post is Awesome. I modified the code to more generically determine if the image is all one color(all black, all white, all magenta, etc...). Assuming you have a bitmap with 4 part color values ARGB, compare each color to the color in the top left if any is different then the image isn't all one color.

private bool AllOneColor(Bitmap bmp)
{
    // 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.

    System.Runtime.InteropServices.Marshal.Copy(ptr, rgbValues, 0, bytes);

    bool AllOneColor = true;
    for (int index = 0; index < rgbValues.Length; index++)
    {
        //compare the current A or R or G or B with the A or R or G or B at position 0,0.
        if (rgbValues[index] != rgbValues[index % 4])
        {
            AllOneColor= false;
            break;
        }
    }
    // Unlock the bits.
    bmp.UnlockBits(bmpData);
    return AllOneColor;
}