OpenGl transformation question newbie

Newbie & Debugging Questions
#1

Hi all,

i am a complete newbie to opengl taking a computer graphics course this semester. we just did transformations so i am playing around with some code to make sure i understand them. however there seems to be some basic flaw in my understanding as i always get output different that what i predicted.

I have posted a sample code below and hoping someone would be kind enough to walk me through it to point what i am not getting right.

the code below is supposed to draw x,y and z axis with the viewpoint situated at (1,1,1) so that i can see all the axis.

what i want to see is all 3 axes in something like

          *         *
          *       *
          *     *
          *   *

        **
     *   *
   *     *
*        *

gl.glClear(GL.GL_COLOR_BUFFER_BIT|gl.GL_DEPTH_BUFFER_BIT);   //clear gl buffers
gl.glMatrixMode(GL.GL_MODELVIEW); //set the current matrix as modelview
gl.glLoadIdentity(); //load identity matrix => locate viewpoint at (0,0,0)

gl.glClearColor(1.0f,1.0f,1.0f,0); //clear window with white
gl.glClear(GL.GL_COLOR_BUFFER_BIT);
gl.glColor3f(0.0f, 0.0f, 0.0f); //set drawing color as black
        
gl.glBegin(GL.GL_LINES);  //draw all three axis within -0.5 to +0.5 with viewer at (0,0,0). this when rendered will show a
     gl.glVertex3f( -0.5f, 0.0f, 0.0f ); // a plus sign since z axis would be 'hidden'
     gl.glVertex3f( 0.5f, 0.0f, 0.0f );
         
     gl.glVertex3f( 0.0f, -0.5f, 0.0f );
     gl.glVertex3f( 0.0f, 0.5f, 0.0f );
            
     gl.glVertex3f( 0.0f, 0.0f, -0.5f );
     gl.glVertex3f( 0.0f, 0.0f, 0.5f );
            
gl.glEnd();
gl.glTranslatef(1.0f, 1.0f, 1.0f); //finally move the viewer to (1,1,1) to get the desired view

when i run this all i see is half a part of of the plus sign drawn at the top right corner of the gl canvas.

thank you for all the help
-ankit

#2

First thing with opengl:
it is a state machine, so you need to set up all colors, lights and transforms before rendering anything with that state (and then make any changes before rendering something else, rinse and repeat).

So to get a transform affecting the axis you’re drawing, you need to call glTranslate() before glBegin().
Also there is no need to call glClear() twice, just move glClearColor(white) to be above 1st clear and remove second.

Finally, translations and transforms in opengl are all matrices that get multiplied together, so the last translation used is the first actually applied. Also, the translations move what is being drawn (effectively), so to simulate the viewing position (which is always at the origin, looking down neg. z axis), you want to do the opposite transform for the viewer.

#3

ok …makes sense … so i modified my code and now i can see the axis with the desired view however there is a small problem in the code

 final GL gl = drawable.getGL();
        final GLU glu = new GLU();
        gl.glClear(GL.GL_COLOR_BUFFER_BIT|GL.GL_DEPTH_BUFFER_BIT);
        gl.glMatrixMode(GL.GL_MODELVIEW);
        gl.glLoadIdentity();
        gl.glTranslatef(0.0f, 0.0f, -1.0f);
        gl.glRotatef(45.0f, 1.0f, 0.0f, 0.0f);
        gl.glRotatef(45.0f, 0.0f, 1.0f, 0.0f);
        //gl.glRotatef(xy_angle0 , xz_angle, xy_angle, xz_angle);
        
 

        gl.glColor3f(0.0f, 0.0f, 0.0f);
        
        //gl.glRotatef(xz_angle, 0.0f, 0.0f, 1.0f);
        gl.glBegin(GL.GL_LINES);
        
            
            //x axis = green
            gl.glColor3f(0.0f, 1.0f, 0.0f);
            gl.glVertex3f( -1.5f, 0.0f, 0.0f );
            gl.glVertex3f( 1.5f, 0.0f, 0.0f );
            
            //y axis = red
            gl.glColor3f(1.0f, 0.0f, 0.0f);
            gl.glVertex3f( 0.0f, -1.5f, 0.0f );
            gl.glVertex3f( 0.0f, 1.5f, 0.0f );
            
            //z axis = blue
            gl.glColor3f(0.0f, 0.0f, 1.0f);
            gl.glVertex3f( 0.0f, 0.0f, -1.5f );
            gl.glVertex3f( 0.0f, 0.0f, 1.5f );
            
                     
            
        gl.glEnd();
        gl.glFlush();

the output that i get is :


http://img49.imageshack.us/img49/1240/outputyh2.th.jpg

http://img49.imageshack.us/images/thpix.gif

my question … why cant i see the -ve part of the axis ?

thanks again
-ankit

#4

Not exactly sure what you mean by -ve, can you elaborate?

#5

the code is supposed to draw all the axes between the limits [-0.5,0.5] however if you look at the attached picture
of the output all i see is the positive half of the axis lines.

-ankit

#6

Gotcha, here are a couple of suggestions:

  1. Move the camera farther back. You’re drawing axis each with a length of 3 (-1.5 to 1.5), but you’ve only moved back 1 unit.
  2. Use a longer draw range when you’re setting the projection matrix (last argument in gluPerspective if that’s what you’re using).

If none of that works, can you post the entire code of your setup and GLEventListener and I can toy around with it.

#7

lhkbob,

thank you so much ! fixing the camera project did the trick ;D

have another noob question.

i added code so that every time the user presses the right key i change the camera rotate angle to rotate the final projection. it works fine except that right now what i do is (pseudocode)

void keypressed(....)
{
       if key(==right key)
       {
               angle+=2.0f;
               init();
               display();
        }
}

so i am calling the display() and init() every time => redrawing the whole scene. i am wondering since opengl is a state machine there should be a way to just recompute the projection according to the new angle instead of drawing the whole thing again … any ideas how would i do that ?

thanks
-ankit

#8

There are a couple of ways to do this, but first make sure that you never actually call display() from an event listener for AWT events (key, mouse, etc) because they are supposed to be fast executing. Also, to actually make opengl calls with JOGL, you need to have a current context (which is guaranteed when you use the GLEventListener pattern and let JOGL call those methods when you’re updating the frame).

Now, it is best to only have to call init() once per startup of the application, and include only 1-time things in there. If you need to change the projection matrix possibly each frame, then the key listener could set a boolean saying to re-compute the projection matrix, and actually do it in the display() method. Unfortunately you have to re-draw the whole thing each frame. opengl doesn’t actually remember what was rendered, the state only remembers how to draw something. So when changed, you need to clear the frame and re-render the geometry.

#9

hmm … .i am not sure if i follow ur advice completely.

so i use init() for 1-time stuff. now you said that in the keylistener i should set a boolean to recompute the projection matrix … assuming i do that , how would the display() function of opengl get called ? wont i need to explicitly call it from my keylistener ?

thanks
-ankit

#10

Here is the best way to render simple things with JOGL (read best as safest):

  1. Create a GLCanvas and add it to a JFrame or other window type
  2. Create your GLEventListener (that has display(GLDrawable), init(GLDrawable), etc. methods) and add to the GLCanvas
  3. Make the frame visible, and use an Animator or FPSAnimator that automatically calls the GLCanvas’s display() method
    repeatedly.
    For more details, I’d check out the jogl-demos source code.

Too recompute the projection matrix, you’d do something like:

public void display(GLAutoDrawable glad) {
  GL gl = glad.getGL();

  gl.glClear(GL.GL_COLOR_BUFFER_BIT | GL.GL_DEPTH_BUFFER_BIT);

  if (this.redoProjection) {
    gl.glMatrixMode(GL.GL_PROJECTION);
    gl.glLoadIdentity();
    new GLU().gluPerspective(...); // don't actually create a new GLU each time.
    gl.glMatrixMode(GL.GL_MODELVIEW);
    this.redoProjection = false;
  }
  gl.glLoadIdentity();
  // set up view transform
  // set up model transform and others
  // render stuff
}

Also, there is a forum category specifically for JOGL at java-gaming.org, so more specific questions could be asked (and then you’d get more advice than just mine).

#11

lhkbob,

thank you for the advice … i think i understand it now :smiley:

-ankit