Drawing multiple cubes at random positions in opengl es android

Question

Using OpenGL ES I am rendering a simple cube class that draws it at the centre of the screen. However, I want to be able to draw multiple such cubes on the screen at random positions but don't know how to. Here is the my custom surface view that renders the cube as a private class. I haven't included my main ActivityManager since its not the concern.

 public class TouchSurfaceView extends GLSurfaceView  {

private final float TRACKBALL_SCALE_FACTOR=52.0f;
private final float TOUCH_SCALE_FACTOR=100.0f/320;
private MyGLRenderer mRenderer;
private float mPreviousX;
private float mPreviousY;


public TouchSurfaceView(Context context) {
    super(context);
    // TODO Auto-generated constructor stub

        mRenderer=new MyGLRenderer();
        setRenderer(mRenderer);
        setRenderMode(GLSurfaceView.RENDERMODE_WHEN_DIRTY);

}

    private class MyGLRenderer implements GLSurfaceView.Renderer{

    private MyGLCube mCube;
    public float mAngleX;
    public float mAngleY; 


    public MyGLRenderer(){
        mCube=new MyGLCube();
    }



    @Override
    public void onDrawFrame(GL10 gl) {
        // TODO Auto-generated method stub


        gl.glDisable(GL10.GL_DITHER);
         gl.glClear(GL10.GL_COLOR_BUFFER_BIT | GL10.GL_DEPTH_BUFFER_BIT);   

         gl.glMatrixMode(GL10.GL_MODELVIEW);
         gl.glLoadIdentity();
         gl.glClientActiveTexture(DRAWING_CACHE_QUALITY_HIGH);
         // gl.glTranslatef(0, 0, -3.0f);
         gl.glTranslatef(0, 0, -3.0f);
         gl.glRotatef(mAngleX, 0, 1, 0);
         gl.glRotatef(mAngleY, 1, 0, 0);
         mCube.draw(gl);

    }

    @Override
    public void onSurfaceChanged(GL10 gl, int width, int height) {
        // TODO Auto-generated method stub
        gl.glViewport(0, 0, width, height);
        float ratio=(float) width/height;
        gl.glMatrixMode(GL10.GL_PROJECTION);
        gl.glLoadIdentity();
        gl.glFrustumf(-ratio, ratio, -1, 1, 1, 10);
    }

    @Override
    public void onSurfaceCreated(GL10 gl, EGLConfig config) {
        // TODO Auto-generated method stub
        gl.glDisable(GL10.GL_DITHER);
        gl.glHint(GL10.GL_PERSPECTIVE_CORRECTION_HINT, GL10.GL_FASTEST);
        gl.glClearColor(0.5f, 2, 0.63f, 0.2f);

    }


}
 }

Here is MYGLCube that contains vertices, indices and other stuff for constructing the cube

    public class MyGLCube {


private float vertices[]={
        0.3f,0.3f,-0.3f, // topFront Right
        0.3f,-0.3f,-0.3f, //BottomFront Right
       -0.3f,-0.3f,-0.3f, //BottomFront Left
       -0.3f,0.3f,-0.3f,  //topFront Left
       0.3f,0.3f,0.3f, // topBack Right
       0.3f,-0.3f,0.3f, //BottomBack Right
       -0.3f,-0.3f,0.3f, //BottomBack Left
       -0.3f,0.3f,0.3f,  //topBack Left
 }; 

private float rgbaVals[]={
        1,1,0,.5f,   // topFront Right color
        0.25f,0, 0.8f,1, //BottomFront Right color
        0,1,1,1, //BottomFront Left color
        0.35f,0.26f,1,0.5f,  //topFront Left color
        0.23f,0.62f,3,0.2f,  // topBack Right color
        0.3f,0.43f,1,0.2f,  //BottomBack Right color
        0.2f, 0.3f, 0.73f,0.6f,  //BottomBack Left color
        0.6f, 0.9f, 0.65f, 0.2f //topBack Left color            
};

private short pIndex[]={
          3,4,0,  0,4,1,   3,0,1, 
          3,7,4,   7,6,4,  7,3,6,
          3,1,2,   1,6,2,  6,3,2,
          1,4,5,   5,6,1,  6,5,4            
};
private FloatBuffer vertBuff, cBuff;
private ShortBuffer pBuff;


public MyGLCube(){
  ByteBuffer bBuff=ByteBuffer.allocateDirect(vertices.length*4);    
  bBuff.order(ByteOrder.nativeOrder());
  vertBuff=bBuff.asFloatBuffer();
  vertBuff.put(vertices);
  vertBuff.position(0);

  ByteBuffer pbBuff=ByteBuffer.allocateDirect(pIndex.length*2);
  pbBuff.order(ByteOrder.nativeOrder());
  pBuff=pbBuff.asShortBuffer();
  pBuff.put(pIndex);
  pBuff.position(0);

  ByteBuffer colorBuff=ByteBuffer.allocateDirect(rgbaVals.length*4);
  colorBuff.order(ByteOrder.nativeOrder());
  cBuff=colorBuff.asFloatBuffer();
  cBuff.put(rgbaVals);
  cBuff.position(0);


}

public void draw(GL10 gl){
    gl.glFrontFace(GL10.GL_CW);
    gl.glEnable(GL10.GL_CULL_FACE);
    gl.glEnable(GL10.GL_SMOOTH);
    gl.glEnable(GL10.GL_DEPTH_TEST);
    gl.glCullFace(GL10.GL_BACK);
    gl.glEnableClientState(GL10.GL_VERTEX_ARRAY);
    gl.glEnableClientState(GL10.GL_COLOR_ARRAY);
    gl.glVertexPointer(3, GL10.GL_FLOAT, 0, vertBuff);
    gl.glColorPointer(4, GL10.GL_FLOAT, 0, cBuff);
    gl.glDrawElements(GL10.GL_TRIANGLES, pIndex.length, GL10.GL_UNSIGNED_SHORT, pBuff);
    gl.glDisableClientState(GL10.GL_COLOR_ARRAY);
    gl.glDisableClientState(GL10.GL_VERTEX_ARRAY);
    gl.glDisable(GL10.GL_CULL_FACE);
 }  



}

Answer 1

You must use glPushMatrix and glPopMatrix to return to previus possition after translate,rotate...

@Override
public void onDrawFrame(GL10 gl) {
    // TODO Auto-generated method stub


    gl.glDisable(GL10.GL_DITHER);
     gl.glClear(GL10.GL_COLOR_BUFFER_BIT | GL10.GL_DEPTH_BUFFER_BIT);   

     gl.glMatrixMode(GL10.GL_MODELVIEW);
     gl.glLoadIdentity();
     gl.glClientActiveTexture(DRAWING_CACHE_QUALITY_HIGH);

     //Draw cube 1
     gl.glPushMatrix();
     gl.glTranslatef(-0.5f, 0, -3.0f);
     gl.glRotatef(mAngleX, 0, 1, 0);
     gl.glRotatef(mAngleY, 1, 0, 0);
     mCube.draw(gl);
     gl.glPopMatrix();

     //Draw cube 2
     gl.glPushMatrix();
     gl.glTranslatef(0, 0, -3.0f); 
     gl.glRotatef(mAngleX, 0, 1, 0);
     gl.glRotatef(mAngleY, 1, 0, 0);
     mCube.draw(gl);
     gl.glPopMatrix();

     //Draw cube 3
     gl.glPushMatrix();
     gl.glTranslatef(0.5f, 0, -3.0f); 
     gl.glRotatef(mAngleX, 0, 1, 0);
     gl.glRotatef(mAngleY, 1, 0, 0);
     mCube.draw(gl);
     gl.glPopMatrix();

}

Answer 2

The code you have that actually draws the cube are these three lines:

gl.glTranslatef(0, 0, -3.0f);
gl.glRotatef(mAngleX, 0, 1, 0);
gl.glRotatef(mAngleY, 1, 0, 0);
mCube.draw(gl);

It moved the coordinate system, then rotates it and then draws the cube. What you want to do it move it somewhere else and draw the cube there. Like this:

gl.glTranslatef(1.0, 0, 0f);
mCube.draw(gl);