C++ OpenGL - How to make a 2D square move on screen? - c++

Question

How do I make a 2D square move on the screen? I try to move it but it just stays there.
int x = 100;
int y = 100;
int width = 50;
int height = 50;
x += 1;
glBegin(GL_QUADS);
glColor3f(r, g, b);
glVertex2f(x, y);
glVertex2f(x + width, y);
glVertex2f(x + width, y + height);
glVertex2f(x, y + height);
glEnd();
It all loads fine, it draws the square and everything, but it just doesn't move the square, I'm using SDL to draw the window incase you want to know.

Answer 1

OpenGL expects you to send relative coordinates between 0 and 1. Moreover, you create new variables every frame, so they can't really be incremented along all frames.
// box parameters in pixels
int boxleft = 100,
boxbottom = 100;
int boxwidth = 50,
boxheight = 50;
// window dimensions
int screenwidth = 1920,
screenheight = 1080;
for(;;)
{
// clear last frame
glClear(GL_COLOR_BUFFER_BIT);
// calculate screen space coordinates
float left = (float)boxleft / screenwidth,
right = left + (float)boxwidth / screenwidth,
bottom = (float)boxbottom / screenheight,
top = bottom + (float)boxheight / screenheight;
// draw the box
glBegin(GL_QUADS);
glColor3f(r, g, b);
glVertex2f(left, top);
glVertex2f(right, top);
glVertex2f(right, bottom);
glVertex2f(left, bottom);
glEnd();
// shift box for next frame
boxleft++;
}
Update: Okay, you say the square draws fine with your coordinates, so you might not change that. But defining the variables outside your draw loop is essential. Tell me if this works for you.

Answer 2

Assuming that's all one function, the issue is that the begining of the function is constantly resetting your value for x to 100. Move your variable definitions out of the function. As an example:
int x = 100;
int y = 100;
int width = 50;
int height = 50;
function drawSquare()
{
x += 1;
glBegin(GL_QUADS);
glColor3f(r, g, b);
glVertex2f(x, y);
glVertex2f(x + width, y);
glVertex2f(x + width, y + height);
glVertex2f(x, y + height);
glEnd();
}
Each time you call that function, the square will have x incremented by one and so will move progressively over.