Scaling 3D shapes (OpenGL and C++)

Question

I've created a program in OpenGL that draws some shapes. I want the user to be able to zoom in on the shapes if they want to. This is the code that draws the shapes:

/*Initialise the required OpenGL functions*/
glClear(GL_COLOR_BUFFER_BIT | GL_DEPTH_BUFFER_BIT);
glMatrixMode(GL_PROJECTION);
glPushMatrix();
glLoadIdentity();
glOrtho(0.0, screenWidth, screenHeight, 0.0, -1.0, 10.0);
glMatrixMode(GL_MODELVIEW);
glPushMatrix();
glLoadIdentity();
glDisable(GL_CULL_FACE);
glClear(GL_DEPTH_BUFFER_BIT);

/*Draw a square*/
glColor3f(1, 0, 0);
glBegin(GL_QUADS);
glVertex2f(screenWidth * 0.75, screenHeight * 0.08333);
glVertex2f(screenWidth * 0.75, screenHeight * 0.16666);
glVertex2f(screenWidth * 0.86666, screenHeight * 0.16666);
glVertex2f(screenWidth * 0.86666, screenHeight * 0.08333);
glEnd();
glColor3f(0, 0, 0);

/*Let the user zoom*/
if (GetAsyncKeyState(VK_UP))
{
    /*"zoom" is a global variable*/
    zoom += 0.005;
}

glScaled(1 + zoom, 1 + zoom, 1);

/*Everything that is drawn from this point on (A sphere and a cube) should be scaled*/
glMatrixMode(GL_PROJECTION);
glLoadIdentity();
glMatrixMode(GL_MODELVIEW);
glLoadIdentity();
glTranslatef(-0.3, 0, 0);
glutSolidSphere(3, 20, 20);

glMatrixMode(GL_MODELVIEW);
glLoadIdentity();
glTranslatef(0.55, 0.36, 0);
glutSolidCube(0.05);

glMatrixMode(GL_PROJECTION);
glPopMatrix();
glMatrixMode(GL_MODELVIEW);
glPopMatrix();

glutSwapBuffers();

The code draws the shapes properly, but the shapes can't be scaled. I've used similar code in some other functions, so I believe that it may be because I am using 3D shapes or it may have something to do with me calling "glMatrixMode" multiple times. Either way, how should I change my code so that the cube and sphere are scaled based on user input, but the first square is not affected?

Answer 1

glScaled() changes the current matrix. So as soon as you call glLoadIdentity() you are undoing your scaling. You are doing lots of unnecessary calls to glMatrixMode() and glLoadIdentity() that should be eliminated. So try something more like this:

// You probably don't really need to do these, but if you do, do it once up top.
glMatrixMode(GL_PROJECTION);
glLoadIdentity();
glMatrixMode(GL_MODELVIEW);
glLoadIdentity();

glPushMatrix(); // Save the current matrix
glScaled(1 + zoom, 1 + zoom, 1); // Scale it

/*Everything that is drawn from this point on (A sphere and a cube) should be scaled*/
glTranslatef(-0.3, 0, 0);
glutSolidSphere(3, 20, 20);

glTranslatef(0.55, 0.36, 0);
glutSolidCube(0.05);

glPopMatrix(); // Undo the glScaled() call above

glutSwapBuffers();