OpenGL: Unprojecting a point

I've been searching around the internet for a while now and I'm not finding the answer to my question so I thought I'd give this a whirl...

All I want to do is click on a point in my application's window and have the app draw a dot at that point. My problem comes in when I use the coordinates I get from unprojecting the mouse click to draw the point. The window size is 600 x 600 but my viewing area is set up as glOrtho(-30, 30, -30, 30, -10, 10)

Is there a way to unproject the mouse click to the glOrtho coordinates? If there is, does it lose too much accuracy?

Thanks for any help!!

Take a look at this.

Code:

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// draw point on mouse click
#include <GL/glut.h>

/* to remove Console Window */
#pragma comment(linker, "/subsystem:windows")
#pragma comment(linker, "/entry:mainCRTStartup")

GLint g_iHeight = 500;
GLint g_iWidth = 500;

void display()
{
        glClearColor(1.0, 1.0, 1.0, 1.0);        /* set current color to white */
        glClear(GL_COLOR_BUFFER_BIT);                /* clear the display */
        glFlush();
}

void drawPoint(int x, int y)
{
        glPointSize(3);
        glColor3f(0.0, 0.0, 1.0);
        glBegin(GL_POINTS);
                glVertex2i(x, g_iHeight - y);
        glEnd();
        glFlush();
}

void mouse(int button, int state, int x, int y)
{
        switch (button)
        {
        case GLUT_LEFT_BUTTON:

                if (GLUT_DOWN == state)
                {
                        drawPoint(x, y);
                }
                break;
        }
}

void reshape(GLint width, GLint height)
{
        /* update the value of variable whenever window resize */
        g_iWidth = width;
        g_iHeight = height;
        //gluOrtho2D(0, 0 + g_iWidth, 0, 0 + g_iHeight);
}

int main(int argc, char *argv[])
{
        /* initialize GLUT system */
        glutInit(&argc, argv);

        /* initilise it to use double buffer */
        glutInitDisplayMode(GLUT_RGB);

        /* width=500 pixels height=500 pixels */
        glutInitWindowSize(g_iWidth, g_iHeight);

        /* set window position to 100, 100 */
        glutInitWindowPosition(100, 100);

        /* create window */
        glutCreateWindow("GL Test");

        /* how object is mapped to window */
        gluOrtho2D(0, 0 + g_iWidth, 0, 0 + g_iHeight);

        /* register mouse function */
        glutMouseFunc(mouse);
       
        /* register function when window resize */
        glutReshapeFunc(reshape);

        /* register window display function */
        glutDisplayFunc(display);

        /* start processing events */
        glutMainLoop();
        return 0;
}

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Hope it helps.