Problem with moveable viewports

[lizardperson]

Hi everyone.
I'm trying to use viewports as moveable objects to implement cameras which can be placed on the game scene and used as regular objects.
From what I've tried already the basic movement works fairly well, but I've found an unpleasant behaviour, which I can't fix - when the camera (viewport) reaches the screen border the objects inside it shift on 1-2 pixels in the opposite to the movement direction.
I've tried to apply various offsets in the sf::View:setCenter() manually to counter this effect, but it didn't do the trick.
Here's the minimal code example which reproduces the problem:

Code: [Select]

#include <SFML/Audio.hpp>
#include <SFML/Graphics.hpp>
int main()
{
    const sf::Vector2f windowSize(1280,720);
    sf::RenderWindow window(sf::VideoMode(static_cast<unsigned int>(windowSize.x),
                                          static_cast<unsigned int>(windowSize.y)), "SFML");

    sf::RectangleShape quad;
    quad.setFillColor(sf::Color::White);
    quad.setSize(sf::Vector2f(100,100));
    quad.setPosition(100,100);

    const sf::Vector2f cameraSize(640,480);
    sf::View camera;
    camera.setSize(cameraSize);
    sf::RectangleShape cameraBackground;
    cameraBackground.setFillColor(sf::Color(50,50,50,255));
    cameraBackground.setSize(cameraSize);


    sf::Clock clock;
    sf::Vector2f coord;
    const int speed = 100;

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

        sf::Time elapsed = clock.restart();
        if(sf::Keyboard::isKeyPressed(sf::Keyboard::Up))
            coord.y -= speed * elapsed.asSeconds();
        if(sf::Keyboard::isKeyPressed(sf::Keyboard::Down))
            coord.y += speed * elapsed.asSeconds();
        if(sf::Keyboard::isKeyPressed(sf::Keyboard::Left))
            coord.x -= speed * elapsed.asSeconds();
        if(sf::Keyboard::isKeyPressed(sf::Keyboard::Right))
            coord.x += speed * elapsed.asSeconds();

        camera.setCenter(coord + sf::Vector2f(cameraSize.x / 2, cameraSize.y / 2));
        camera.setViewport(sf::FloatRect(coord.x / windowSize.x,
                                         coord.y / windowSize.y,
                                         cameraSize.x / windowSize.x,
                                         cameraSize.y / windowSize.y));
        cameraBackground.setPosition(coord);

        window.clear();
        window.setView(camera);
        window.draw(cameraBackground);
        window.draw(quad);

        window.display();
    }
    return 0;
}
(Use direction arrows to move camera(grey square), when it crosses the screen's border, the object on camera (white square) will jitter slightly)


Any help would be appreciated.

[Hapax]

Just tested your code; it occurs when the viewport is attempting to draw outside of the window. Although I do not know if this is illegal in OpenGL (I suspect that the problem lies when it reaches negative numbers and maybe is truncating values or something), I would still say that it makes very little sense. A viewport, really, is which section of the window you wish to display the view; trying to display that view in a section not contained within the window seems wrong. Your approach, then, could be to reduce the size of the viewport and view to clip it to the screen. Of course, the centre would have to be adjusted too.

EDIT: added example below:

(click to show/hide)

#include <SFML/Audio.hpp>
#include <SFML/Graphics.hpp>
#include <algorithm>
int main()
{
        const sf::Vector2f windowSize(1280, 720);
        sf::RenderWindow window(sf::VideoMode(static_cast<unsigned int>(windowSize.x),
                static_cast<unsigned int>(windowSize.y)), "SFML");

        sf::RectangleShape quad;
        quad.setFillColor(sf::Color::White);
        quad.setSize(sf::Vector2f(100, 100));
        quad.setPosition(100, 100);

        const sf::Vector2f cameraSize(640, 480);
        sf::View camera;
        camera.setSize(cameraSize);
        sf::RectangleShape cameraBackground;
        cameraBackground.setFillColor(sf::Color(50, 50, 50, 255));
        cameraBackground.setSize(cameraSize);

        sf::Clock clock;
        sf::Vector2f coord;
        const int speed = 100;

        while (window.isOpen()) {
                sf::Event event;
                while (window.pollEvent(event)) {
                        if (event.type == sf::Event::Closed)
                                window.close();
                }
                sf::Time elapsed = clock.restart();
                const float speedMultiplier{ sf::Keyboard::isKeyPressed(sf::Keyboard::LShift) ? 5.f : (sf::Keyboard::isKeyPressed(sf::Keyboard::LControl) ? 0.2f : 1.f) }; // SHIFT = 5x faster, CTRL = 5x slower
                if (sf::Keyboard::isKeyPressed(sf::Keyboard::Up))
                        coord.y -= speed * elapsed.asSeconds() * speedMultiplier;
                if (sf::Keyboard::isKeyPressed(sf::Keyboard::Down))
                        coord.y += speed * elapsed.asSeconds() * speedMultiplier;
                if (sf::Keyboard::isKeyPressed(sf::Keyboard::Left))
                        coord.x -= speed * elapsed.asSeconds() * speedMultiplier;
                if (sf::Keyboard::isKeyPressed(sf::Keyboard::Right))
                        coord.x += speed * elapsed.asSeconds() * speedMultiplier;

                const sf::Vector2f overspill{ std::max(-coord.x, 0.f), std::max(-coord.y, 0.f) };
                const sf::Vector2f adjustedSize{ cameraSize - overspill };
                camera.setCenter(coord + sf::Vector2f(cameraSize.x / 2, cameraSize.y / 2) + (overspill / 2.f));
                camera.setSize(adjustedSize);
                camera.setViewport(sf::FloatRect(std::max(0.f, coord.x / windowSize.x),
                        std::max(0.f, coord.y / windowSize.y),
                        std::max(0.f, adjustedSize.x / windowSize.x),           // these are 'clipped' to zero to avoid negative sizes,
                        std::max(0.f, adjustedSize.y / windowSize.y)));         // which cause OpenGL errors
                cameraBackground.setPosition(coord);

                window.clear();
                window.setView(camera);
                window.draw(cameraBackground);
                window.draw(quad);

                window.display();
        }
        return 0;
}

Note that this only takes into account the top and left borders. If your viewports will pass the right and bottom borders of the window, you will need to expand this idea.

[lizardperson]

Thanks Hapax! I've actually tried this approach before but I managed to reproduce the same problem because of stupid mistake which lead me to think that solution lies somewhere else.
Anyway this really helped, thank you again!