Add sf::Shape::getGeometricCenter()

[TedLyngmo]

This adds

virtual Vector2f sf::Shape::getGeometricCenter() const = 0;

as discussed here:
https://en.sfml-dev.org/forums/index.php?topic=29011

One practical use can be to set a rotation point in the middle of a circle, rectangle or convex shape.

Example:

#include <SFML/Graphics.hpp>

#include <initializer_list>

class Boulder : public sf::ConvexShape {
public:
    inline Boulder(std::initializer_list<sf::Vector2f> pnts) :
        sf::ConvexShape(pnts.size())
    {
        for(std::size_t idx = 0; idx < pnts.size(); ++idx)
        {
            setPoint(idx, pnts.begin()[idx]);
        }

        // Set the rotation point at the geometrical center of the convex shape:
        setOrigin(getGeometricCenter());
    }
};

int main()
{
    sf::RenderWindow window(sf::VideoMode({1280, 720}), "Minimal, complete and verifiable example");
    window.setFramerateLimit(60);

    Boulder boulder1({{20,0}, {60,0}, {80,40}, {70,60}, {30,80}, {10,60}, {0,30}});
    boulder1.setPosition({1280/3, 720/3});

    Boulder boulder2({{20,0}, {60,0}, {180,40}, {70,60}, {30,80}, {10,60}, {0,30}});
    boulder2.setPosition({1280*2/3, 720/3});

    sf::RectangleShape rectangle({100,50});
    rectangle.setOrigin(rectangle.getGeometricCenter());
    rectangle.setPosition({1280/2, 720*2/3});

    sf::CircleShape circle1(40); // center slightly off to wobble a bit:
    circle1.setOrigin(circle1.getGeometricCenter() + sf::Vector2(5.f,5.f));
    circle1.setPosition({1280/3, 720*2/3});

    sf::CircleShape circle2(40,3);
    circle2.setOrigin(circle1.getGeometricCenter());
    circle2.setPosition({1280*2/3, 720*2/3});

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

        boulder1.rotate(sf::degrees(2));
        boulder2.rotate(sf::degrees(3));
        rectangle.rotate(sf::degrees(4));
        circle1.rotate(sf::degrees(5));
        circle2.rotate(sf::degrees(5));

        window.clear();
        window.draw(boulder1);
        window.draw(boulder2);
        window.draw(rectangle);
        window.draw(circle1);
        window.draw(circle2);
        window.display();
    }
}

• Has this change been discussed on the forum or in an issue before?
• Does the code follow the SFML Code Style Guide?
• Have you provided some example/test code for your changes?
• If you have additional steps which need to be performed list them as tasks!

---

Tasks

• Tested on Linux
• Tested on Windows
• Tested on macOS
• Tested on iOS
• Tested on Android

[codecov]

Codecov Report

Merging #2536 (8e391ca) into master (97c00d4) will increase coverage by 0.11%.
The diff coverage is 96.87%.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #2536      +/-   ##
==========================================
+ Coverage   27.15%   27.26%   +0.11%     
==========================================
  Files         228      228              
  Lines       19637    19669      +32     
  Branches     4710     4716       +6     
==========================================
+ Hits         5332     5363      +31     
- Misses      13826    13847      +21     
+ Partials      479      459      -20     

Impacted Files	Coverage Δ
include/SFML/Graphics/CircleShape.hpp	100.00% <ø> (ø)
include/SFML/Graphics/ConvexShape.hpp	100.00% <ø> (ø)
include/SFML/Graphics/RectangleShape.hpp	100.00% <ø> (ø)
include/SFML/Graphics/Shape.hpp	100.00% <ø> (ø)
src/SFML/Graphics/ConvexShape.cpp	97.72% <95.45%> (-2.28%)	⬇️
src/SFML/Graphics/CircleShape.cpp	100.00% <100.00%> (ø)
src/SFML/Graphics/RectangleShape.cpp	97.05% <100.00%> (+0.39%)	⬆️

... and 3 files with indirect coverage changes

---

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[vittorioromeo]

Indifferent on whether we should add this to SFML or not in general

[vittorioromeo]

Not too sure about this one. It's impossible to differentiate between a polygon whose geometrical center is in 0,0 and a polygon with no points. Maybe this function should return std::optional<Vector2f>?

[TedLyngmo]

That's an option or it could throw an exception or leave the case UB

[Bromeon]

Or assert.

I'm not sure about optional here, the original problem is that sf::ConvexShape can be in a "degenerate" state -- it would just make users dereference the optional all the time, possibly causing more UB than assert (depending on whether optional::operator* has also an assert).

[TedLyngmo]

@Bromeon Sounds like a good idea. What to do in NDEBUG builds? std::terminate or throw std::out_of_range?

[vittorioromeo]

What's the significance of special-casing for 3 points?

[TedLyngmo]

@vittorioromeo It's the one case where the geometric center is not @ Vector2f(m_radius, m_radius).

[TedLyngmo]

@vittorioromeo & @Bromeon Would it perhaps be better to place the generic algorithm in Shape instead of ConvexShape?

[ChrisThrasher]

Which PR do you want to keep open. This one or #2537? I don't think we need both.

[TedLyngmo]

Which PR do you want to keep open. This one or #2537? I don't think we need both.

I wasn't sure if you'd be ok with adding the implementation to the Shape class so I made two separate patches - but it seems we all agree that the other patch is better so I'll close this.