Fast way to compare 2 byte arrays

Question

I am uploading jpeg images as fast as i can to a web service (it is the requirement I have been given).

I am using async call to the web service and I calling it within a timer.

I am trying to optimise as much as possible and tend to use an old laptop for testing. On a normal/reasonable build PC all is OK. On the laptop I get high RAM usage.

I know I will get a higher RAM usage using that old laptop but I want to know the lowest spec PC the app will work on.

As you can see in the code below I am converting the jpeg image into a byte array and then I upload the byte array.

If I can reduce/compress/zip the bye array then I am hoping this will be 1 of the ways of improving memory usage.

I know jpegs are already compressed but if I compare the current byte array with the previous byre array then uploading the difference between this byte arrays I could perhaps compress it even more on the basis that some of the byte values will be zero.

If I used a video encoder (which would do the trick) I would not be real time as much I would like.

Is there an optimum way of comparing 2 byte arrays and outputting to a 3rd byte array? I have looked around but could not find an answer that I liked.

This is my code on the client:

bool _uploaded = true;
private void tmrLiveFeed_Tick(object sender, EventArgs e)
{
  try
  {
      if (_uploaded)
      {
        _uploaded = false;
          _live.StreamerAsync(Shared.Alias, imageToByteArray((Bitmap)_frame.Clone()), Guid.NewGuid().ToString()); //web service being called here
      }
  }
  catch (Exception _ex)
  {
      //do some thing but probably time out error here
  }
}

//web service has finished the client invoke 
void _live_StreamerCompleted(object sender, AsyncCompletedEventArgs e)
{
 _uploaded = true; //we are now saying we start to upload the next byte array
}

private wsLive.Live _live = new wsLive.Live(); //web service 
private byte[] imageToByteArray(Image imageIn)
{
  MemoryStream ms = new MemoryStream();
  imageIn.Save(ms,System.Drawing.Imaging.ImageFormat.Jpeg);  //convert image to best image compression
  imageIn.Dispose();
  return ms.ToArray();
}

thanks...

Answer 1

If your goal is to find out whether two byte arrays contain exactly the same data, you can create an MD5 hash and compare these as others have suggested. However in your question you mention you want to upload the difference which means the result of the comparison must be more than a simple yes/no.

As JPEGs are already compressed, the smallest change to the image could lead to a large difference in the binary data. I don't think any two JPEGs contain binary data similar enough to easily compare.

For BMP files you may find that changing a single pixel affects only one or a few bytes, and more importantly, the data for the pixel at a certain offset in the image is located at the same position in both binary files (given that both images are of equal size and color depth). So for BMPs the difference in binary data directly relates to the difference in the images.

In short, I don't think obtaining the binary difference between JPEG files will improve the size of the data to be sent.

Answer 2

As C.Evenhuis said - JPEG files are compressed, and changing even few pixels results in complettly differrent file. So - comparing resulting JPEG files is useless.

BUT you can compare your Image objects - quick search results in finding this:

unsafe Bitmap PixelDiff(Bitmap a, Bitmap b)
{
    Bitmap output = new Bitmap(a.Width, a.Height, PixelFormat.Format32bppArgb);
    Rectangle rect = new Rectangle(Point.Empty, a.Size);
    using (var aData = a.LockBitsDisposable(rect, ImageLockMode.ReadOnly, PixelFormat.Format32bppArgb))
    using (var bData = b.LockBitsDisposable(rect, ImageLockMode.ReadOnly, PixelFormat.Format32bppArgb))
    using (var outputData = output.LockBitsDisposable(rect, ImageLockMode.ReadWrite, PixelFormat.Format32bppArgb))
    {
        byte* aPtr = (byte*)aData.Scan0;
        byte* bPtr = (byte*)bData.Scan0;
        byte* outputPtr = (byte*)outputData.Scan0;
        int len = aData.Stride * aData.Height;
        for (int i = 0; i < len; i++)
        {
            // For alpha use the average of both images (otherwise pixels with the same alpha won't be visible)
            if ((i + 1) % 4 == 0)
                *outputPtr = (byte)((*aPtr  + *bPtr) / 2);
            else
                *outputPtr = (byte)~(*aPtr ^ *bPtr);

            outputPtr++;
            aPtr++;
            bPtr++;
        }
    }
    return output;
}