SFML: keep a RenderWindow's partial region unrefreshed

Question

I am looking to get the same effect as the GraphicsPath from Winforms which allows to keep some particular areas of myForm unrefreshed. fi:

myForm.Invalidate(new Region(graphicsPath));

My final goal is to draw things at the unrefreshed location, using a HDC (Device context handle) that I will provide to an external application. (This currently works using winforms).

I am using SFML.Net 2.4 and I create my window this way:

 SFML.Graphics.RenderWindow  mySfmlWindow = new RenderWindow(myForm.Handle, settings);

I can still create the HDC on myForm, however, even without calling :

mySfmlWindow.Clear(color);

, the things drawn by the external application are still instantly cleared.

Answer 1

Manual approach

You can draw your desired background yourself. I've got an example, where I draw a half of the window background manually, while the other half is not cleared.

The left half is "cleared" in gray just to show the point.

In the code, I use a sf::RectangleShape to clear the window, but you can use a sf::VertexArray if your shape is more complex.

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Full Code

int main(){
    sf::RenderWindow win(sf::VideoMode(640, 480), "SFML Test");

    sf::RectangleShape r1;
    r1.setOrigin(sf::Vector2f(25, 25));
    r1.setPosition(50, 50);
    r1.setSize(sf::Vector2f(50, 50));
    r1.setFillColor(sf::Color::Red);

    sf::RectangleShape r2;
    r2.setOrigin(sf::Vector2f(25, 25));
    r2.setPosition(400, 50);
    r2.setSize(sf::Vector2f(50, 50));
    r2.setFillColor(sf::Color::Blue);


    sf::RectangleShape updatedRegion;
    updatedRegion.setPosition(0, 0);
    updatedRegion.setSize(sf::Vector2f(320, 480));  // Half window
    updatedRegion.setFillColor(sf::Color(30,30,30));    // Dark gray just for the sake of the example

    while (win.isOpen())
    {
        sf::Event event;
        while (win.pollEvent(event))
        {
            if (event.type == sf::Event::Closed)
            {
                win.close();
            }
        }

        //win.clear();              // We skip the clearing process
        win.draw(updatedRegion);    // And do our own "clear"
        win.draw(r1);
        win.draw(r2);
        win.display();

        // Just some movement to test the concept
        r1.rotate(0.01);
        r2.rotate(0.01);

    }

    return 0;
}