Question

Someone could recommend a better way to detect if a bitmap has Alpha channel and if it’s used? This method works for me, but could be inefficient if the image is very large, because it loops all the pixels in the worst case:

private static bool IsAlphaBitmap(Bitmap bmp, out BitmapData bmpData)
{
    Rectangle bmpBounds = new Rectangle(0, 0, bmp.Width, bmp.Height);

    bmpData = bmp.LockBits(bmpBounds, ImageLockMode.ReadOnly, bmp.PixelFormat);

    try
    {
        for (int y = 0; y <= bmpData.Height - 1; y++)
        {
            for (int x = 0; x <= bmpData.Width - 1; x++)
            {
                Color pixelColor = Color.FromArgb(
                    Marshal.ReadInt32(
                       bmpData.Scan0, (bmpData.Stride * y) + (4 * x)));

                if (pixelColor.A > 0 & pixelColor.A < 255)
                {
                    return true;
                }
            }
        }
    }
    finally
    {
        bmp.UnlockBits(bmpData);
    }

    return false;
}

1. Is this the way to proceed or is there any optimal solution?
2. Is this solution quick enough with very large files?

Thank you very much

Answer 1

Didn’t try this, but the idea is to work with pointers:

unsafe
{
   byte* ptrAlpha = ((byte*)bmpData.Scan0.ToPointer()) + 3;
   for (int i = bmpData.Width * bmpData.Height; i > 0; --i)  // prefix-- should be faster
   {
      if ( *ptrAlpha < 255 )
          return true;

      ptrAlpha += 4;
   }
}