Draw oval with sphere in Opengl - c++

Question

I want to draw an oval by projection the sphere on the screen (like rasterize). Here is my code but it doesn't show anything on the screen. Should I use more functions to initialize the projection? Is this way possible to draw oval on screen by using sphere?
GLfloat xRotated, yRotated, zRotated;
GLdouble radius=1;
void display(void);
void reshape(int x, int y);
int main (int argc, char **argv)
{
glutInit(&argc, argv);
glutInitWindowSize(800,800);
glutCreateWindow("OVAL");
zRotated = 30.0;
xRotated=43;
yRotated=50;
glutDisplayFunc(display);
glutReshapeFunc(reshape);
glutMainLoop();
return 0;
}
void display(void)
{
glMatrixMode(GL_PROJECTION);
glOrtho(0.1, 1.0, 0.1, 1.0, -1.0, 1.0);
glClear(GL_COLOR_BUFFER_BIT);
glLoadIdentity();
glTranslatef(0.0,0.0,-5.0);
glColor3f(0.9, 0.3, 0.2);
glRotatef(xRotated,1.0,0.0,0.0);
glRotatef(yRotated,0.0,1.0,0.0);
glRotatef(zRotated,0.0,0.0,1.0);
glScalef(1.0,1.0,1.0);glutSolidSphere(radius,20,20);
glFlush();
}
void reshape(int x, int y)
{
if (y == 0 || x == 0) return;
glMatrixMode(GL_PROJECTION);
glLoadIdentity();
gluPerspective(39.0,(GLdouble)x/(GLdouble)y,0.6,21.0);
glMatrixMode(GL_MODELVIEW);
glViewport(0,0,x,y);
}

Answer 1

You are drawing a sphere compltely outside of the viewing volume, so it should be no surprise that it can't be seen.
There are a couple of issues with your code:
All OpenGL matrix functions besides glLoadIndentity and glLoadMatrix always post-multiply a matrix to the current top element of the current matrix stack. In your display function, you call glOrtho without resetting the projection matrix to identity before. This will result in totally weird - and different - results if the display callback is called more than once.
You should add a call to glLoadIdentity() right before calling glOrtho.
You set up the model view transformations so that the sphere's center will always end up at (0,0,-5) in eye space. However, you set a projectiom matrix which defines a viewing volume which goes from z=1 (near plane) to z=-1 (far plane) in eye space, so your spehre is actually behind the far plane.
There are several ways this could be fixed. Changing the viewing frustum by modifying the parameters of glOrtho might be the easisest. You could for example try (-2, 2, -2, 2, 1, 10) to be able to see the sphere.
It is not really clear what
I want to draw an oval by projection the sphere on the screen (like rasterize).
exactly means. If you just want the sphere to be distorted to an ellipsoid, you could just apply some non-uniform scaling. This in principle could be done in the projection matrix (if no other objects are to be shown), but this would make much more sense to apply it to the model matrix of the sphere - you already have the glScale call there, you could try something like glScalef(1.0f, 0.5f, 1.0f);.
Also note that the ortho parameters I suggested previously will result in some distortion if your viewport is not exactly square. In a real world, one wants to incorporate the aspect ratio of the viewport into the projection matrix.
If you want to see the sphere deformed as by a perspective projection, you would have to skip the glOrtho altogheter and switch to a perspective projection matrix.
The code you are using is totally outdated. The OpenGL matrix stack has been deprecated in OpenGL 3.0 (2008) and is not available in core profiles of modern OpenGL. The same applies for builtin vertex attributes like glColor or immediate mode drawing and client-side vertex arrays. As a result, GLUT's drawing functions can also not be used with modern GL any more.
If you really intend learning OpenGL nowadays, I stronly advise you to ignore this old cruft and star learning the modern way.