Combining 2 images does not what I expect

Question

I am trying to scan images using twaindotnet, and as long as the user keeps scanning I want to add the scanned image to the already scanned image.

In other words, when the user does his first scan, he well see the scan appear in an PictureBox, and when he performs his second scan, he will see in the PictureBox the first scan and below that the second scan.
After the third scan he will see in the PictureBox the first scan, below that the second scan and below that the third scan.
And so on...

So in the PictureBox there should actual be one large image, that has the combined height of all scans.

So I tried this

        private void buttonScan_Click(object sender, EventArgs e)
        {
            _twain = new Twain(new WinFormsWindowMessageHook(this));

            _twain.ScanningComplete += _twain_ScanningComplete;
            _twain.TransferImage += _twain_TransferImage;
            _twain.SelectSource(_twain.SourceNames[0]);
            ScanSettings scanSettings = new ScanSettings();

            /// to do: figure out how to use scansettings
            //scanSettings.Page.Size = TwainDotNet.TwainNative.PageType.A4;
            //scanSettings.Resolution.Dpi = 800;

            _twain.StartScanning(scanSettings);
        }

        private void _twain_ScanningComplete(object sender, ScanningCompleteEventArgs e)
        {
            buttonNextStep.Enabled = true;

            _twain.ScanningComplete -= _twain_ScanningComplete;
            _twain.TransferImage -= _twain_TransferImage;
        }


        private void _twain_TransferImage(object sender, TransferImageEventArgs e)
        {
            if (e.Image != null)
            {
                if (pictureBox1.Image == null)
                {
                    pictureBox1.Image = e.Image;
                }
                else
                {
                    Bitmap bitmap = new Bitmap(pictureBox1.Image.Width, pictureBox1.Image.Height + e.Image.Height);
                    using (Graphics g = Graphics.FromImage(bitmap))
                    {
                        g.DrawImage(pictureBox1.Image, 0, 0);
                        g.DrawImage(e.Image, 0, pictureBox1.Image.Height);

                        pictureBox1.Image = bitmap;
                    }
                }
            }
        }

which I based on this answer

It does puts the new scan in the picturebox below the prior scan, but it also creates a large empty area. It looks like the height of the image is not doubled after the second scan, but is increased by 4 times the height (in stead of 2)

Is the Height property of the Image maybe different then the height of a bitmap ?
Or what else can cause this ?

In the gif below you see the result of 2 scans, I expect both scans to be above eachother without the gray space. Both are scanned from an A4 paper. When I scan the first paper there is no extra space at the end of the image, I already checked that.

Answer 1

Try forcing images .Width and .Height in g.DrawImage:

g.DrawImage(pictureBox1.Image, 0, 0, pictureBox1.Image.Width, pictureBox1.Image.Height);
g.DrawImage(e.Image, 0, pictureBox1.Image.Height, e.Image.Width, e.Image.Height);

Here is an explanation of this behavior: https://stackoverflow.com/a/47695040/2400834