Drawing on image and saving - image

Question

I guess I don't really understand how Graphics objects work in Visual C++ 2010 Express.
I am grabbing a frame from a webcam, and drawing a circle on it. It works great on the screen. I simply create a Graphics object, draw the image, and draw the ellipse.
In the pictureBox_paint function, I have
Graphics^ g = e->Graphics; // from the camera
System::Drawing::Rectangle destRect = System::Drawing::Rectangle(0,0,pbCameraMonitor->Size.Width,pbCameraMonitor->Size.Height);
double slitHeightToWidth = 3;
g->DrawImage(this->currentCamImage,destRect);
int circleX, circleY;
circleX = (int) (pbCameraMonitor->Size.Width - radius/slitHeightToWidth)/2;
circleY = (int) (pbCameraMonitor->Size.Height - radius)/2;
g->DrawEllipse(Pens::Red, circleX, circleY, (int) radius/slitHeightToWidth, (int) radius);
So far so good, my ellipse gets drawn on there nicely. The destRect bit makes sure it is scaled to the pictureBox size. I simply invalidate the pictureBox every time the camera reports a new image, and I have video.
Now, on a button click I want to save this image, with the red ellipse on it. However, I don't want the rescaled version shown on the screen, I want the full res version. So, I'll grab another frame into a Bitmap^ called "grabbedFrame" and do this:
String ^photofile = "Image_" + expRecord.timestamp.ToString("s") + ".jpg"; // get a unique filename
photofile = photofile->Replace(':', '_');
Graphics^ g = Graphics::FromImage(grabbedFrame);
g->DrawEllipse(Pens::Red, 20, 20, 20, 20); // circle size fixed just for demo
grabbedFrame->Save(photofile, System::Drawing::Imaging::ImageFormat::Jpeg);
When I do that, I get a save of the image without the red circle.
Does g->DrawEllipse actually modify the Bitmap? Or just contain the Bitmap + instructions to draw? If the latter, how does the pictureBox know the Bitmap has been modified? If the former, why doesn't my save contain the modification?
How can I save the modified Bitmap?

Answer 1

You need to draw the loaded image into a new Bitmap, make your modifications to that bitmap, and then save it.
Something like (pseudo-code'ish):
// create bitmap and get its graphics
Bitmap^ pBmp = gcnew Bitmap(grabbedFrame->Width, grabbedFrame->Height);
Graphics^ g = Graphics::FromImage(pBmp);
// draw grabbed frame into bitmap
g->DrawImage(grabbedFrame, 0, 0, grabbedFrame->Width, grabbedFrame->Height);
// draw other stuff
g->DrawEllipse(Pens::Red, 20, 20, 20, 20);
// save the result
pBmp->Save(photofile, System::Drawing::Imaging::ImageFormat::Jpeg);