OpenGL when switch fron 2D to 3D - opengl

Question

Here's what I want to achieve, I have a flag called switch_2D_3D in the code below, and when it's true I switch to 2D mode, otherwise 3D.
void reshape(GLsizei width, GLsizei height)
{
if (switch_2D_3D)
{
// GLsizei for non-negative integer
// Compute aspect ratio of the new window
if (height == 0)
height = 1; // To prevent divide by 0
GLfloat aspect = (GLfloat)width / (GLfloat)height;
// Reset transformations
glLoadIdentity();
// Set the aspect ratio of the clipping area to match the viewport
glMatrixMode(GL_PROJECTION); // To operate on the Projection matrix
// Set the viewport to cover the new window
glViewport(0, 0, width, height);
if (width >= height)
{
// aspect >= 1, set the height from -1 to 1, with larger width
gluOrtho2D(-1.0 * aspect, 1.0 * aspect, -1.0, 1.0);
}
else
{
// aspect < 1, set the width to -1 to 1, with larger height
gluOrtho2D(-1.0, 1.0, -1.0 / aspect, 1.0 / aspect);
}
winWidth = width;
winHeight = height;
} // 2D mode
else
{
// Prevent a divide by zero, when window is too short
// (you cant make a window of zero width).
if (height == 0)
height = 1;
float ratio = width * 1.0 / height;
// Use the Projection Matrix
glMatrixMode(GL_PROJECTION);
// Reset Matrix
glLoadIdentity();
// Set the viewport to be the entire window
glViewport(0, 0, width, height);
// Set the correct perspective.
gluPerspective(45.0f, ratio, 0.1f, 100.0f);
// Get Back to the Modelview
glMatrixMode(GL_MODELVIEW);
winWidth = width;
winHeight = height;
}// 3D mode
}
Everything works perfectly when drawing only in 2d mode, but when I change the flag to switch to the 3d mode, here comes the problem
Every time I resize the window, the things I draw in the 3d scene(for example a cube) would be come smallerand smaller, eventually disappeared, why is this happening
And if I switch back to 2D mode, everything in 2d mode still works fine, the problem is with the 3d mode
Also, if I start the program with the flag set to false, I would see a cube and it still gets smaller as I resize the window each time
Why is this happening?

Answer 1

You should look at your glLoadIdentity() / glMatrixMode() interactions.
Right now, you have two different behaviors:
In 2D: you're resetting your matrix for whatever is active when you enter the function, presumably GL_MODELVIEW, which causes the gluOrtho2D calls to "stack up".
In 3D: you're always resetting the projection matrix, which seems more correct.
Try swapping the order of the glLoadIdentity and glMatrixMode calls in your first path (2D) only.
It's a wise idea to always explicitly set the matrix you want to modify before actually modifying it.