Question

I am making a rollercoaster inside of a skybox in OpenGL, and without much background on it’s functions or computer graphics it is proving to be very difficult. I drew a rollercoaster using Catmull-Rom spline interpolation, and drew each point with glVertex3f. Now I want to call an update() function every 50ms to move the camera around the track. gluLookAt() is producing weird results, either removing the track from the screen, producing a black screen, etc. I think I need to move some of the matrix functions around but I am not sure where to put each one. Here is my code so far:

int main(int argc, char** argc)
{
    // ... load track, etc ...

    // Init currpos, nextpos, iter, up
    currpos = Vec3f(0, 0, 0);
    nextpos = currpos;
    iter = 0;
    up = Vec3f(0, 1, 0);

    deque<Vec3f> points;
    Vec3f newpt;

    // Loop through the points and interpolate 
    for (pointVectorIter pv = g_Track.points().begin(); pv != g_Track.points().end(); pv++)
    {
        Vec3f curr(*pv);            // Initialize the current point and a new point (to be drawn)
        points.push_back(curr);     // Push the current point onto the stack
        allpoints.push_back(curr);  // Add current point to the total stack

        if (points.size() == 4) // Check if there are 4 points in the stack, if so interpolate
        {
            for (float u = 0.0f; u < 1.0f; u += 0.01f)
            {
                newpt = interpolate(points[0], points[1], points[2], points[3], u);
                glColor3f(1, 1, 1);
                glVertex3f(newpt.x(), newpt.y(), newpt.z());

                allpoints.push_back(newpt);
            }

            points.pop_front();
        }
    }

    // glutInit, InitGL(), etc...
}

void InitGL(GLvoid)
{
    glEnable(GL_DEPTH_TEST);
    glDepthFunc(GL_LEQUAL);
    glMatrixMode(GL_PROJECTION);
    glLoadIdentity();
    gluPerspective(100.0, (GLfloat)WINDOW_WIDTH / (GLfloat)WINDOW_HEIGHT, .0001, 999999);
    glMatrixMode(GL_MODELVIEW);
    glClearColor(0.0f, 0.0f, 0.0f, 0.5f);
}

void display (void)
{
    glClear(GL_COLOR_BUFFER_BIT | GL_DEPTH_BUFFER_BIT);
    glMatrixMode(GL_MODELVIEW);
    glLoadIdentity();
    gluLookAt(currpos.x(), currpos.y(), currpos.z(), nextpos.x(), nextpos.y(), nextpos.z(), up.x(), up.y(), up.z());

    glPushMatrix();
    glEnable(GL_TEXTURE_2D); // Enable texturing from now on

    /* draw skybox, this was from previous assignment and renders correctly */

    glPopMatrix();

    // now draw rollercoaster ...

    glPushMatrix();
    glBegin(GL_LINE_STRIP);

    deque<Vec3f> points;
    Vec3f newpt;

    for each (Vec3f pt in allpoints)
    {
        glColor3f(1, 1, 1);
        glVertex3f(pt.x(), pt.y(), pt.z());
    }

    glutTimerFunc(50, update, 1);
    glEnd();
    glPopMatrix();

    // Swap buffers, so one we just drew is displayed
    glutSwapBuffers();
}

void update(int a)
{
    if (iter < allpoints.size())
    {
        currpos = allpoints[iter];
        nextpos = allpoints[iter + 1];

        gaze = nextpos - currpos;
        gaze.Normalize();

        Vec3f::Cross3(binorm, gaze, up);
        binorm.Normalize();

        Vec3f::Cross3(up, binorm, gaze);
        up.Normalize();

        glutPostRedisplay();
    }

    iter++;
}

The idea is that I am keeping a global deque allpoints that includes the control points of the spline and the interpolated points. Once that is complete, I call update() every 50ms, and move the camera along each point in allpoints. In a previous version of the project, I could see that the rollercoaster was being drawn correctly. It is gluLookAt() that doesn’t seem to work how I want it to. With the code above, the program starts with the camera looking at one side of the skybox with a part of the rollercoaster, and then when update() is called, the rollercoaster disappears but the camera does not move. I have been messing around with where I am putting the OpenGL matrix functions, and depending on where they are sometimes update() will cause a blank screen as well.

Answer 1

Besides the absence of glPopMatrix (which user971377 already spotted), you call glLoadIdentity in your drawing routine, which of course overwrites any changes you did on the modelview matrix in the update method (using gluLookAt).

Always keep in mind: gluLookAt, glOrtho, gluPerspective, glTranslate, glRotate, and all other matrix and transformation functions always work on the top element (changed by glPush/PopMatrix) of the currently selected matrix stack (changed by glMatrixMode). And they always multiply the current matrix, istead of replacing it. So like for gluPerspective, you should call glLoadIdentity before calling gluLookAt. And the whole camera change should be done in the rendering routine, istead of the update routine.

Instead of doing any GL transformations in update you should rather change the variables on which the camera depends and set the camera (gluLookAt on the modelview matrix) in the display method. To demonstrate the standard use of these functions, your code should be something like:

void display()
{
    <general state setup (glClear, ...)>

    glMatrixMode(GL_MODELVIEW);
    glLoadIdentity();
    glLookAt(camera);     //view transformation (camera)

    //object 1
    glPushMatrix();       //save modelview
    glTranslate/glRotate/glScale;        //local model transformations
    <draw object 1>
    glPopMatrix();
    ...
    //object n
    glPushMatrix();       //save modelview
    glTranslate/glRotate/glScale;        //local model transformations
    <draw object n>
    glPopMatrix();

    gluSwapBuffers();
}

void update()
{
    camera = ...;
}

}