A simple example on how one could render concave shapes such as tetrominos using SFML's `sf::VertexArray`

tetrominos.cpp

#include <SFML/Graphics.hpp>

int main() {
    sf::RenderWindow window(sf::VideoMode(800, 600), "Tetrominos");

    // The quads approach is easiest to understand
    // (Note this is not supported on mobile targets and you'd have to define triangles instead)
    sf::VertexArray quads(sf::Quads);

    // Left piece
    quads.append({{0, 0}, sf::Color::White});
    quads.append({{95, 0}, sf::Color::Red});
    quads.append({{95, 95}, sf::Color::Green});
    quads.append({{0, 95}, sf::Color::Blue});

    // Center piece
    quads.append({{100, 0}, sf::Color::White});
    quads.append({{195, 0}, sf::Color::Red});
    quads.append({{195, 95}, sf::Color::Green});
    quads.append({{100, 95}, sf::Color::Blue});

    // Right piece
    quads.append({{200, 0}, sf::Color::White});
    quads.append({{295, 0}, sf::Color::Red});
    quads.append({{295, 95}, sf::Color::Green});
    quads.append({{200, 95}, sf::Color::Blue});

    // Bottom piece
    quads.append({{100, 100}, sf::Color::White});
    quads.append({{195, 100}, sf::Color::Red});
    quads.append({{195, 195}, sf::Color::Green});
    quads.append({{100, 195}, sf::Color::Blue});

    // Triangle fan makes it shorter, but a bit more complex to understandd
    sf::VertexArray triangleFan(sf::TriangleFan);
    // Starting point is the bottom left corner of the center piece
    triangleFan.append({{100, 100}, sf::Color::White});
    triangleFan.append({{0, 100}, sf::Color::Red});
    triangleFan.append({{0, 0}, sf::Color::Green});
    triangleFan.append({{300, 0}, sf::Color::Blue});
    triangleFan.append({{300, 100}, sf::Color::Magenta});
    // The next point creates an invisible triangle just to move to the center piece
    triangleFan.append({{200, 100}, sf::Color::Black});
    triangleFan.append({{200, 200}, sf::Color::Cyan});
    triangleFan.append({{100, 200}, sf::Color::Yellow});

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

        window.clear();
        sf::Transform transform;
        transform.translate({50, 50});
        window.draw(quads, transform);
        transform.translate({0, 300});
        window.draw(triangleFan, transform);
        window.display();
    }

    return 0;
}