CODEWITHSUNDEEP
c++renderinggame-enginesfmlgame-development
Prakhar
Prakhar

Reputation: 1

Why is the screen space coordinate system for my sfml rendered app inverted?

I am learning C++ and I thought I'd make the original asteroids game with a fresh coat of paint using the SFML graphics library. However, for my player sprite, while the origin is at the top left corner of the screen, to the right of it is the negative x axis and downwards is negative y axis (opposite of what it's supposed to be in both cases). Also, no matter what object or rotation, invoking the setRotation function always rotates any object about the top left corner of the screen even if, for that object, I have set the origin to the object's center.

#include<SFML\Graphics.hpp>

using namespace sf;

const int W{ 1200 }, H{ 800 };
const float degToRad = 0.017453f;

int main() {
    float x{ -600 }, y{ -400 };
    float dx{}, dy{}, angle{};
    bool thrust;

    RenderWindow app(VideoMode(W, H), "Asteroids!");
    app.setFramerateLimit(60);

    Texture t1, t2;
    t1.loadFromFile("images/spaceship.png");
    t2.loadFromFile("images/background.jpg");

    Sprite sPlayer(t1), sBackground(t2);
    sPlayer.setTextureRect(IntRect(40, 0, 40, 40));
    sPlayer.setOrigin(-600, -400);

    while (app.isOpen())
    {
        app.clear();
        app.draw(sPlayer);
        app.display();
    }
    return 0;
}

The above code draws the player (spaceship.png) to the center of my rendered window (app) but notice how I have had to put in negative coordinates. Also, if I further put in the code for taking keyboard inputs and call the setRotation function, instead of rotating my sPlayer sprite about its center (i.e. (-600,-400)), it rotates the sprite about the top left corner of the screen which is (0,0). I can't find any explanation for this in the SFML online documentation. What should I do?

As I mentioned I have tried reading the documentation. I've watched online tutorials but to no avail.

Upvotes: 0

Views: 120

Answers (1)

Revolver_Ocelot
Revolver_Ocelot

Reputation: 8785

Origin is the point on sprite where you "hold" it. Position is the point on screen where you put Origin of the sprite. In short, you take your sprite by Origin and put it so Origin is on Position.

By default, both Origin and Position are (0, 0), so top left of your sprite is put at top left of the screen. What you did was to say "take this point on sprite, which is way to the upper-left that actual visible part of the sprite is and put it to the top left of the screen". This had an effect of moving your sprite to the bottom right.

You probably want something like:

// This is will make sure that Origin, i.e. point which defines rotation and other transformations center is at center of the ship
sPlayer.setOrigin(sprite_width / 2, sprite_height / 2);

// This will put Origin (center of the ship) at center of the screen
sPlayer.setPosition(screen_width / 2, screen_height / 2);

Upvotes: 1

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