Mouse click changing the screen face in OpenGL - c

Question

I'm programming a code that when the user click on the screen It marks a point and when he continues clicking on other points on the screen It'll be linking the points with lines.
I don't know why but I'm working with two different screen plan. When I click with the mouse It marks a point in the screen that I only see when I'm with the left mouse button clicked and when the button isn't clicked I see a clean screen.
My code that marks the points and link them with lines:
void exerciseThree(int x, int y){
write("Exercise Three", -5, 18);
float cx = 0, cy = 0;
if(x != 0 && y != 0 && isFinished == false){
glPushMatrix();
glPointSize(6.0f);
//Draw the points
glBegin(GL_POINTS);
cx = ((x * (maxx - minx)) / width) + minx;
cy = (((height - y) * (maxy - miny)) / height) + miny;
glVertex2f(cx , cy);
glEnd();
if(lastCx != 0 && lastCy != 0){
glBegin(GL_LINE_STRIP);
glVertex2f(lastCx , lastCy);
glVertex2f(cx , cy);
glEnd();
}
lastCx = cx;
lastCy = cy;
glPopMatrix();
}
write("Press 0 (zero) to come back.", -10, -18);
}
The mouse function:
void mouse(int button, int state, int x, int y){
switch(button){
case GLUT_LEFT_BUTTON:
if( option == 3){
exerciseThree(x, y);
projecao();
glutSwapBuffers();
}
break;
}
}
I know that I should handle the GLUT_DOWN and GLUT_UP of the mouse, but does exist a way of work with only one screen face?

Answer 1

You're only seeing something on the screen when you click because that's the only time you are drawing to and updating the buffer. You should just update the list of x/y coordinates when the mouse is clicked. However, you should draw the points and call glutSwapBuffers() every time in your main loop so that they are always on screen regardless of a button press.
The flow should be something like the following pseudo code:
ArrayOfPoints[];
while(Running)
{
CheckForMouseInput(); // Update array of points to draw if pressed
DrawPoints(); // Use same code draw code from exerciseThree()
glutSwapBuffers(); // Update video buffer
}