Drawing line from vertices not visible when x = 0

[Neil]

I am trying to draw a vertical line where x=0 but I can't see it. I have to use x=1 to see a line. However, for horizontal lines, I can see lines where y=0. I don't understand the difference in behavior.

sf::RenderWindow window(sf::VideoMode(640, 480), "fghjnufhg");

{

                        const std::array<sf::Vertex, 2> line =
                        {
                                sf::Vertex(sf::Vector2f(0, 0), sf::Color::Red),
                                sf::Vertex(sf::Vector2f(0, 100), sf::Color::Red)
                        };

                        window.draw(line.data(), line.size(), sf::Lines, sf::RenderStates::Default);
                }

                {

                        const std::array<sf::Vertex, 2> line =
                        {
                                sf::Vertex(sf::Vector2f(1, 0), sf::Color::Green),
                                sf::Vertex(sf::Vector2f(1, 100), sf::Color::Green)
                        };

                        window.draw(line.data(), line.size(), sf::Lines, sf::RenderStates::Default);
                }

                {

                        const std::array<sf::Vertex, 2> line =
                        {
                                sf::Vertex(sf::Vector2f(0, 0), sf::Color::Yellow),
                                sf::Vertex(sf::Vector2f(100, 0), sf::Color::Yellow)
                        };

                        window.draw(line.data(), line.size(), sf::Lines, sf::RenderStates::Default);
                }

                {

                        const std::array<sf::Vertex, 2> line =
                        {
                                sf::Vertex(sf::Vector2f(0, 1), sf::Color::Blue),
                                sf::Vertex(sf::Vector2f(100, 1), sf::Color::Blue)
                        };

                        window.draw(line.data(), line.size(), sf::Lines, sf::RenderStates::Default);
                }

                window.display();
 

[Laurent]

This question is asked quite frequently. Have you first tried to use the search engine?

[Neil]

I found this: https://en.sfml-dev.org/forums/index.php?topic=20500.msg147365#msg147365

Adding 0.5 to my vertex coordinates seems to work, though it seems strange to me the different directions would behave differently.

Do I need to add 0.5 to other drawable objects? That post says "You should add 0.5 to all your coordinates", but when I draw a sf::RectangleShape at (0.5, 0.5), it is at the left edge but overcorrected down 1 pixel. (0, 0) works as expected. How can I know when I need this correction or not?

[Laurent]

though it seems strange to me the different directions would behave differently

It's because SFML reverts the Y axis (OpenGL initially has it from bottom to top), so I guess that rasterization rules still round towards the bottom for Y values.

Do I need to add 0.5 to other drawable objects?

Only for lines. They are a special case: to rasterize them, OpenGL fills pixels crossed by the line. So you easily understand that if the line is exactly between two pixel rows or columns, it's an edge case, and OpenGL may choose one or the other depending on internal rounding rules that may not be clear to the end user.

To understand this problem, the best is to take square paper and try to rasterize (fill whole squares -- "pixels") these different cases yourself, as well as other primitives such as rectangles.