Question

I have the following code

int i = 0;

void display(void)
{
    glLoadIdentity();
    glColor3f(1.0f, 0.0f, 0.0f);
    glPushMatrix();
    glTranslatef(20.0f, 20.0f, -40.f);
    glRotatef(180.f, 0.0, 0.0, 1.0);

    glBegin(GL_TRIANGLES);
    glVertex3f(pts[tri[i]], (pts[ptsLength/2 + tri[i]] - maximum) * -1, 0);
    glVertex3f(pts[tri[i+1]], (pts[ptsLength/2 + tri[i+1]] - maximum) * -1, 0);
    glVertex3f(pts[tri[i+2]], (pts[ptsLength/2 + tri[i+2]] - maximum) * -1, 0);
    glEnd();

    i+=3;
    glPopMatrix();
    glutSwapBuffers();
}

How can I make this method pause and wait for a keystroke before rendering the next triangle?

Answer 1

Is this the glutIdleFunc, or the one called when you glutPostRedisplay?

In either case, you need to first setup a glutKeyboardFunc, in which you detect whether a certain key is pressed or not.

In the case you update the screen only on certain events:

... inside the keyboard function
if (key_pressed == THE_KEY_YOU_WANT)
    glutPostRedisplay();

In the case you are using display as the idle function, you can do:

global:
bool go_next = false;

... inside the keyboard function
if (key_pressed == THE_KEY_YOU_WANT)
    go_next = true;

... inside display:
void display(void)
{
    if (go_next)
    {
        go_next = false;
        glLoadIdentity();
        glColor3f(1.0f, 0.0f, 0.0f);
        glPushMatrix();
        ... etc
        glPopMatrix();
    }
    glutSwapBuffers();
}

---

Side note: It is best to keep the logic and rendering separate. So perhaps a code like this is best:

int triangles_to_draw = 0;

... inside the keyboard function
if (key_pressed == THE_KEY_YOU_WANT)
    process_event_next_triangle();

void process_event_next_triangle()
{
    if (triangles_to_draw < maximum)
        ++triangles_to_draw;
}

void display(void)
{
    go_next = false;
    glLoadIdentity();
    glColor3f(1.0f, 0.0f, 0.0f);
    glPushMatrix();
    glTranslatef(20.0f, 20.0f, -40.f);
    glRotatef(180.f, 0.0, 0.0, 1.0);

    glBegin(GL_TRIANGLES);
    for (int i = 0; i < 3*triangles_to_draw; i += 3)
    {
        glVertex3f(pts[tri[i]], (pts[ptsLength/2 + tri[i]] - maximum) * -1, 0);
        glVertex3f(pts[tri[i+1]], (pts[ptsLength/2 + tri[i+1]] - maximum) * -1, 0);
        glVertex3f(pts[tri[i+2]], (pts[ptsLength/2 + tri[i+2]] - maximum) * -1, 0);
    }
    glEnd();

    glPopMatrix();
    glutSwapBuffers();
}

Note that this way, you can make the process_event_next_triangle as complicated as you want without affecting the display function. This makes life much easier when you have many keys doing different stuff.