Question

I am making an application in VB.NET that allows a user to highlight some text in images of documents so that the highlights could be saved for future reference, but the original images would not be modified. I figured I can achieve it by creating a graphics out of an image file and allowing the user to draw on that graphics. The problem is – the graphics can not be created from indexed image files, which many of mine are. I am wondering: is it possible to create a transparent graphics surface above an image in a PictureBox control, so that the user could draw on that surface, but it would appear as if he is drawing directly on an image?

Thank you!

Answer 1

(following assumes WinForms)

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Actually you can.
To display image, use .BackgroundImage property of PictureBox, not .Image.
Don’t forget to set correct value for .BackgroundImageLayout to disable default tiling and keep it in sync with .SizeMode property, so that overlay image is properly aligned.
After you set image, create an overlay image (assume your PictureBox is PictureBox1:

Dim image = PictureBox1.BackgroundImage
Dim overlay As New Bitmap(image.Width, image.Height, System.Drawing.Imaging.PixelFormat.Format32bppArgb)
Dim g As Graphics = Graphics.FromImage(overlay)
g.Clear(Color.Transparent)
'draw something just to show this is working:'
g.DrawEllipse(Pens.Aqua, New Rectangle(0, 0, image.Width, image.Height))
g.Dispose()
PictureBox1.Image = overlay

Now you can draw on the overlay image.
Alternatively, you can copy your image (draw it to the overlay for example).