OpenGL maintain object shape on window resize - c++

Question

I have a square and I'm trying to make it stay a square when the window is resized, instead of stretching with the window. I have some code which I thought would work but when I resize the window the square shrinks and disappears. It doesn't come back when I return the window to it's original size. Can someone tell me what I've done wrong and how to fix it?
void display(void)
{
glClear(GL_COLOR_BUFFER_BIT);
glBegin(GL_POLYGON);
glVertex2f(-0.5, -0.5);
glVertex2f(-0.5, 0.5);
glVertex2f(0.5, 0.5);
glVertex2f(0.5, -0.5);
glEnd();
glFlush();
return;
}
void reshape(int w, int h) {
const float aspectRatio = ((float)w) / h;
float xSpan = 1;
float ySpan = 1;
if (aspectRatio > 1) {
xSpan *= aspectRatio;
}
else {
ySpan *= aspectRatio;
}
gluOrtho2D(-1*xSpan, xSpan, -1*ySpan, ySpan);
glViewport(0, 0, w, h);
}
int main(int argc, char **argv)
{
glutInit(&argc, argv);
glutCreateWindow("simple");
glutDisplayFunc(display);
glutReshapeFunc(reshape);
glutMainLoop();
}

Answer 1

The functions gluOrtho2D and glOrtho multiply the current matrix by the new orthographic projection matrix.
This causes that if the reshape is called a 2nd time, the matrix which was set by gluOrtho2D before, is multiplied by the new one and you get consecutive changes.
You have to "reset" the matrix (init the identiy matrix) on the matrix stack by glLoadIdentity. Further you should choose the projection matrix stack by glMatrixMode:
glMatrixMode(GL_PROJECTION);
glLoadIdentity();
gluOrtho2D(-1*xSpan, xSpan, -1*ySpan, ySpan);
The setup of the viewport glViewport is correct and you also consider the aspect ratio correctly (in gluOrtho2D). But there is an issue if the aspect ratio is less than 1.0. It should be ySpan /= aspectRatio;
I recommend to do the setting of the viewport and the projection matrix in the display function and just to set a notification flag in the reshape function. Note, the viewport and the projection matrix should be changed as rare as possible.
bool vp_valid = true;
int width, height;
void reshape(int w, int h) {
vp_valid = false;
width = w;
height = h;
}
void display(void)
{
if (!vp_valid)
{
const float aspectRatio = (float)width / height;
float sx = aspectRatio > 1.0f ? aspectRatio : 1.0f;
float sy = aspectRatio > 1.0f ? 1.0f : 1.0f/aspectRatio;
glMatrixMode(GL_PROJECTION);
glLoadIdentity();
gluOrtho2D(-sx, sx, -sy, sy);
glViewport(0, 0, width, height);
}
.....
}