How can I set gravity using this code? SFML/C++ [duplicate] - c++

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How can I set gravity using this code?
(1 answer)
Closed 8 years ago.
I am trying to make a game and am stuck on gravity..... In the following code a rectangle stands for a player and when I press up key it moves in y-axis but when I activate gravity on it (i.e resetting it's previous position) it does not animate (i.e. it does not jumps) instead it just stays in it's position.I know why it it happening. Because it is just staying in its position because when I press Up key it executes the code rectangle.setPosition(0, 350). Yeah I want it to do that but I also want to see my player in movement. I am using SFML library of C++. Please Help!
#include <SFML/Graphics.hpp>
int main(){
sf::RenderWindow window(sf::VideoMode(800, 600, 32), "Gravity");
sf::RectangleShape rectangle;
rectangle.setSize(sf::Vector2f(100, 100));
rectangle.setFillColor(sf::Color::Black);
rectangle.setPosition(sf::Vector2f(10, 350));
while(window.isOpen())
{
sf::Event Event;
while(window.pollEvent(Event))
{
if(Event.type == sf::Event::Closed)
{
window.close();
}
}
if(sf::Keyboard::isKeyPressed(sf::Keyboard::Up))
{
rectangle.move(0, -1);
}
if(rectangle.getPosition().y >= 350-1)
{
rectangle.setPosition(0, 350);
}
window.display();
window.clear(sf::Color::Cyan);
window.draw(rectangle);
}
}

Answer 1

Gravity is an acceleration: that is, the double derivative of displacement. So you can't directly set the displacement (as you're currently doing) to get a nice representation of gravity.
An approach would be to create an entity class of your own, adding members for velocity, acceleration; alongside sf::RectangleShape's internal displacement; then have member functions operate on initialization/every frame - a quick & dirty example (untested):
class SomeEntity {
public:
SomeEntity( float x_pos, float y_pos ) : position(x_pos, y_pos) {
m_shape.setSize(sf::Vector2f(100, 100));
m_shape.setFillColor(sf::Color::Black);
m_shape.setPosition(sf::Vector2f(x, y));
// Constants at the moment (initial velocity up, then gravity pulls down)
velocity = sf::Vector2f(0, -30);
acceleration = sf::Vector2f(0, 9.81); //Earth's acceleration
}
void Step() { // TODO: use delta time
if(sf::Keyboard::isKeyPressed(sf::Keyboard::Up)) {
velocity.y -= 1;
}
velocity.x += acceleration.x;
velocity.y += acceleration.y;
x += velocity.x;
y += velocity.y;
m_shape.setPosition(x, y);
}
void Draw(sf::RenderWindow &window) {
window.draw(m_shape);
}
private:
sf::RectangleShape m_shape;
sf::Vector2f position, velocity, acceleration;
}
Which also means you can rewrite your application so it's a little cleaner:
SomeEntity ent(360, 0);
while(window.isOpen()) {
sf::Event Event;
while(window.pollEvent(Event)) {
if(Event.type == sf::Event::Closed) {
window.close();
}
}
ent.Step();
window.display();
window.clear(sf::Color::Cyan);
ent.Draw();
}
Seeing as you're setting your rectangle to x = 0, y = 350 repeatedly, I'll work under the assumption that that's your 'ground plane'. To achieve that, you just want to check whether the entity is under the ground plane, and reset it's position to the ground plane if it is - either in the entity's 'Step' function or directly in your main loop. In the long run, you might be better off using an entity manager/third party physics engine to do this sort of thing (a la box2D, for example)

Answer 2

You can do something like this instead :
if(rectangle.getPosition().y > 350)
{
rectangle.move(0, 0.01);
}