how to get base64 string from canvas

Stream stream = new MemoryStream(pict6);

// Stopwatch stopwatch = new Stopwatch();
// stopwatch.Start();
string path = context.Server.MapPath("~/Image/watermark_with_whiteoutline_Stroke2with35%_1200X950.png");

Bitmap _watermarkImage = new Bitmap(path); // watermark image
Bitmap _baseImage = new Bitmap(stream);
Bitmap bmpToSave = new Bitmap(1200, 950);
using (Graphics g = Graphics.FromImage(bmpToSave))
{
g.DrawImage(_baseImage, 0, 0, 1200, 950);
g.Save();
g.Dispose();
}

//Draw the watermark offset 10 pixels from the left and 10 pixels from the top of the base image.
Graphics canvas = Graphics.FromImage(bmpToSave);
canvas.DrawImage(_watermarkImage, new Point(50, 50));
canvas.Save();

bmpToSave.Save(context.Response.OutputStream, ImageFormat.Jpeg);


//clean up

bmpToSave.Dispose();
canvas.Dispose();
_baseImage.Dispose();