How to create Bitmap object from a Graphics object ?

Question

How to create Bitmap object from a Graphics object ? I would like to read pixels from my Graphics object. for example, like, System.Drawing.BitMap.GetPixel().

I am trying to find out empty area (all white, or of any colour) inside a pdf document, to write some graphics / image. I have tried like this, but it is not working. why the following code is not working as expected ?

//
// System.Drawing.Bitmap
// System.Drawing.Graphics
//
Bitmap b = new Bitmap(width, height, graphics);

//
// In this case, for any (i, j) values, Bitmap.GetPixel returns 0
//
int rgb = b.GetPixel(i, j).ToArgb();

( posting this question again in .net-only context, removing other library dependencies )

Answer 1

Ideally you want to avoid GetPixel/SetPixel and use unsafe access methods to the bitmap for some speed.

System.Drawing.Graphics graphics = System.Drawing.Graphics.FromImage(bitmap);

then use the graphics instance. If I recall, modifying the graphics object modifies the bitmap.

Answer 2

Firstly you should create bitmap, then create graphics from this bitmap, work with graphics and after that save (or work with it) bitmap.

Your code will be like this:

using (Bitmap image = new Bitmap(X, Y))
{
    using (Graphics gr = Graphics.FromImage(image))
    {
        // work with graphics, Draw objects
    }
    image.Save("YourPathToFile"); // Or GetPixel, if you want
}

Your code is not working as you excepted because constructor of Bitmap gets Graphics only for resolution. MSDN tells Initializes a new instance of the Bitmap class with the specified size and with the resolution of the specified Graphics object.