Question

I know several methods for reading Bitmap pixels or to get a Graphic object from an image, but what I am trying to understand is how to know what pixels have been drawn by the user by means of a Graphics object. Example: the user draw a line (but it could be any possible shape) using something like:

surface.DrawLine(aPen, X0, Y0, X1, Y1);      

I need to know what pixels have been set by the user to perform some processing. This could be done quite easily for simple shapes using math (i.e. X = X0 + (X1-X0)*t) , but it seems to me possibly unefficient (specially for complex shapes). A solution I would like is to read a Bitmap looking for the pixels that have been set, but I do not know methods for getting a Bitmap image (or whatever relevant data structure) to work on from a Graphics object. Because this is an obvious need for any program allowing to the user do draw, I am for sure missing some points. Someone has a hint about this?

P.S. I am using Graphics object over a 8Bpp indexed Bitmap in a Windows Form and I need all the pixel coordinates and, possibly, the pixel values (they could be deduced from pixel coordinates, I guess)

Proposed solution

The best solution I can figure out after the contributions in this post is something similar to this (being sourceImage the image I want to draw on and surface the picturebox control where sourceImage is rendered):

         private void DrawOverlay()
        {
            using (var tmpImg = new Bitmap (SourceImage.Width,SourceImage.Height, PixelFormat .Format32bppArgb))
            {
                var g = Graphics.FromImage(tmpImg);
                g.DrawLine(pen1, Startx, Starty, Endx, Endy);
                tmpImg.MakeTransparent(Color.FromArgb(0, 0, 0));
                surface.DrawImage(tmpImg, new Point (0, 0));      

// process here the tmpImg  pixels drawn by the user 

            }                 
        }

If someone has better answer, please, if you like, let us know; otherwise I’ll close the post answering to my own question as above.

Answer 1

As far as I know the proposed solution draft in the question after the feedback received seems to be the an acceptable solution to have an easy access to the pixels drawn.