How to copy image via drawImage

Question

I need to make a copy of an PNG image on my disk. In fact I need more complicated think, but this is a part of it. So,

I'm doing this:

Bitmap oldbmp = new Bitmap( filename );

Bitmap newbmp = new Bitmap(oldbmp.Size.Width, oldbmp.Size.Height, oldbmp.PixelFormat);

// if I don't do this, destination image looks bigger and clipped
newbmp1.SetResolution(oldbmp.HorizontalResolution, oldbmp.VerticalResolution);

Graphics graphics = Graphics.FromImage(newbmp);
graphics.DrawImageUnscaled(oldbmp,0,0);

newbmp.Save( filename2 );

What I have is the second file has different filesize (sometimes more, sometimes less then the first one).

I guess something wrong with encoding, but cannot understand what exactly. What must I change or add?

P.S. Bitmap.Clone( ... ) doesn't do what I need.

Update:

Ok, my actual task is:

1. Cut file to several smaller parts.

2. Save parts to disk.

3. Read parts from disk.

4. Assemble parts back to full image.

5. Save full image to disk.

First file is PNG, last file is PNG, they must be of equall size if parts haven't been changed.

Different size matters, because some files a really large and difference can be more than 2 MBs!

UPDATE2:

The first image. After executing the code the size of file grows.

Answer 1

        Bitmap source = new Bitmap("input.png");

        Rectangle rec = new Rectangle(0, 0, 80, 144);


        Bitmap[] parts = new Bitmap[10];

        for (int i = 0; i < 10; i++)
        {
            rec.Y = 144 * i;

            Bitmap b = (Bitmap)cropImage(source, rec);

            b.Save(String.Format("{0}.png", i), ImageFormat.Png);
            parts[i] = b;
        }

        Bitmap output = new Bitmap(source.Width, source.Height);
        Graphics g = Graphics.FromImage(output);

        for(int i=0; i<10; i++)
        {
            Bitmap b = parts[i];

            g.DrawImageUnscaled(b, new Point(0, i * 144));
        }

        output.MakeTransparent(Color.Transparent); // test if the image view don't changes, use this to reduce the overhead, in my test it reduced output size by 10 percent without changing the view

        output.Save("output.png", ImageFormat.Png);

Answer 2

I think this is due to a lack of control over the .Net PNG Encoder. You can confirm this by running the split combine again on the result file and in my test the file size stayed the same the second time, The second test I did was a load/save using Paint.Net and in my own code without the splitting/combining and this also increased the filesize. The other thing I noticed in my hastily constructed splitter/combiner was a subtly different file at the result due to rounding errors if the number of splits wasn't a factor of the width of the image, which of course affects the filesize.

I think the only thing you can do to get the same file size is to use the same encoder the file was originally constructed with. Depending on how you produce the images this might be possible, but more than likely you're stuck with this increase.

Note: using the test file above (originally 103,191 bytes) my result was 121,715 bytes