How to transform vertex coordinates into screen pixel coordinates?

Question

Im trying to show a square polygon with the 100% of the width of the screen, then, i supose that i must zoom it (with Z axis) until the polygon borders are tounching the screen borders.

I'm trying to achieve this using gluProject to project a coordinate in 3D into a 2D screen coordinate. If the screen coordinate is either 0 or matches the width or height, then it is touching a screen border.

The problem is that something is going wrong, the outputCoords array returned with gluProject is giving me fake values, then i'm doing something wrong. For example, if i put a Z value of -1,15, i can see how my polygon haves exactly the same width of the screen... but the outputCootrds array is telling me these rare values: 175,16,0.9 !!! 175,16!!! it is wrong, because the polygon haves exactly the same width of the screen and it is starting on the 0 pixel of the screen :S

If someone can help me explaining the correct way to achieve this with code examples, please i really need help with this. I readed thousands of tutorials and stackoverflow questions about transforming vertices into pixel coordinates, but all those tutorials, guides and questions doesn't help me to achieve this with my code.

This is myGlSurfaceView class, the code to calcule the screen pixel coordinates is on the method onDrawFrame.

public class MySurfaceView extends GLSurfaceView implements Renderer {  
private Context context;
private Square square;
private float xrot;                 //X Rotation
private float yrot;                 //Y Rotation
private float zrot;                 //Z Rotation
private float xspeed;               //X Rotation Speed
private float yspeed;               //Y Rotation Speed
private float z = -1.15f;           //Profundidad en el eje Z
private float oldX; //valor anterior de X, para rotación
private float oldY; //valor anterior de Y, para rotación
private final float TOUCH_SCALE = 0.2f;     //necesario para la rotación

//create the matrix grabber object in your initialization code  
private MatrixGrabber mg = new MatrixGrabber();           

private boolean firstTimeDone=false; //true si la aplicación ya ha sido inicializada.

public MySurfaceView(Context context, Bitmap image) {
    super(context);
    this.context = context;
    setEGLConfigChooser(8, 8, 8, 8, 16, 0); //fondo transparente
    getHolder().setFormat(PixelFormat.TRANSLUCENT); //fondo transparente
    //Transformamos esta clase en renderizadora
    this.setRenderer(this);
    //Request focus, para que los botones reaccionen
    this.requestFocus();
    this.setFocusableInTouchMode(true);
    square = new Square(image);                                 
}

public void onSurfaceCreated(GL10 gl, EGLConfig config) {       
    gl.glDisable(GL10.GL_DITHER);               //dithering OFF
    gl.glEnable(GL10.GL_TEXTURE_2D);            //Texture Mapping ON
    gl.glShadeModel(GL10.GL_SMOOTH);            //Smooth Shading 
    gl.glClearDepthf(1.0f);                     //Depth Buffer Setup
    gl.glEnable(GL10.GL_DEPTH_TEST);            //Depth Testing ON
    gl.glDepthFunc(GL10.GL_LEQUAL);
    gl.glClearColor(0,0,0,0); //fondo transparente
    gl.glHint(GL10.GL_PERSPECTIVE_CORRECTION_HINT, GL10.GL_NICEST);         
    //Cargamos la textura del cubo.
    square.loadGLTexture(gl, this.context);
}

public void onDrawFrame(GL10 gl) {
    //Limpiamos pantalla y Depth Buffer
    gl.glClear(GL10.GL_COLOR_BUFFER_BIT | GL10.GL_DEPTH_BUFFER_BIT);
    gl.glLoadIdentity();
    //Dibujado
    gl.glTranslatef(0.0f, 0.0f, z);         //Move z units into the screen
    gl.glScalef(0.8f, 0.8f, 0.8f);          //Escalamos para que quepa en la pantalla
    //Rotamos sobre los ejes.
    gl.glRotatef(xrot, 1.0f, 0.0f, 0.0f);   //X
    gl.glRotatef(yrot, 0.0f, 1.0f, 0.0f);   //Y
    gl.glRotatef(zrot, 0.0f, 0.0f, 1.0f);   //Z
    //Dibujamos el cuadrado
    square.draw(gl);    
    //Factores de rotación.
    xrot += xspeed;
    yrot += yspeed;         


    if (!firstTimeDone)
    {       
        /////////////// NEW CODE FOR SCALING THE AR IMAGE TO THE DESIRED WIDTH /////////////////            
        mg.getCurrentProjection(gl); 
        mg.getCurrentModelView(gl);                     
        float [] modelMatrix = new float[16];
        float [] projMatrix = new float[16];
        modelMatrix=mg.mModelView;
        projMatrix=mg.mProjection;          
        int [] mView = new int[4];
        mView[0] = 0;
        mView[1] = 0;
        mView[2] = 800; //width
        mView[3] = 480; //height
        float [] outputCoords = new float[3];
        GLU.gluProject(-1.0f, -1.0f, z, modelMatrix, 0, projMatrix, 0, mView, 0, outputCoords, 0);

        int i=0;
        System.out.print(i);
       // firstTimeDone=true;
    }
}

//si el surface cambia, resetea la vista, imagino que esto pasa cuando cambias de modo portrait/landscape o sacas el teclado físico en móviles tipo Droid.
public void onSurfaceChanged(GL10 gl, int width, int height) {
    if(height == 0) {                       
        height = 1;                         
    }
    gl.glViewport(0, 0, width, height);     //Reset Viewport
    gl.glMatrixMode(GL10.GL_PROJECTION);    //Select Projection Matrix
    gl.glLoadIdentity();                    //Reset Projection Matrix
    //Aspect Ratio de la ventana
    GLU.gluPerspective(gl, 45.0f, (float)width / (float)height, 0.1f, 100.0f);
    gl.glMatrixMode(GL10.GL_MODELVIEW);     //Select Modelview Matrix
    gl.glLoadIdentity();                    //Reset Modelview Matrix        

}

public boolean onTouchEvent(MotionEvent event) {
    float x = event.getX();
    float y = event.getY();
    switch (event.getAction()) 
    {
        case MotionEvent.ACTION_MOVE:
            //Calculamos el cambio
            float dx = x - oldX;
            float dy = y - oldY;
            xrot += dy * TOUCH_SCALE;
            yrot += dx * TOUCH_SCALE;
            //Log.w("XXXXXX", "ACTION_MOVE_NO_ZOOM");
            break;
    }
    oldX = x;
    oldY = y;
    return true; //El evento ha sido manejado
}

public void zoomIn(){ 
    z=z+0.2f;   
    if (z>-1.0f)
        z=-1.0f;
}
public void zoomOut(){ 
    z=z-0.2f; 
    if (z<-20.0f)
       z=-20.0f;
}
public void rotateL(){ 
    zrot=zrot+3.0f; 
}
public void rotateR(){ 
    zrot=zrot-3.0f; 
}   
public void reset()
{
    xrot=0;
    yrot=0;
    zrot=0;
    xspeed=0;
    yspeed=0;
    z = -5.0f;
}
}

This is my square class:

public class Square {
//Buffer de vertices
private FloatBuffer vertexBuffer;
//Buffer de coordenadas de texturas
private FloatBuffer textureBuffer;
//Puntero de texturas
private int[] textures = new int[3];
//El item a representar
private Bitmap image;
//Definición de vertices

private float vertices[] = 
{ 
    -1.0f, -1.0f, 0.0f,     //Bottom Left
    1.0f, -1.0f, 0.0f,      //Bottom Right
    -1.0f, 1.0f, 0.0f,      //Top Left
    1.0f, 1.0f, 0.0f        //Top Right
};
/*  
private float vertices[] = 
{ 
-0.8f, -0.8f, 0.0f,     //Bottom Left
0.8f, -0.8f, 0.0f,      //Bottom Right
-0.8f, 0.8f, 0.0f,      //Top Left
0.8f, 0.8f, 0.0f 
};
*/
//Coordenadas (u, v) de las texturas    
/*
private float texture[] = 
{           
    //Mapping coordinates for the vertices
    0.0f, 0.0f,
    0.0f, 1.0f,
    1.0f, 0.0f,
    1.0f, 1.0f
};
*/
private float texture[] =
{
    //Mapping coordinates for the vertices
    0.0f, 1.0f,
    1.0f, 1.0f,
    0.0f, 0.0f,
    1.0f, 0.0f
};
//Inicializamos los buffers
public Square(Bitmap image) {
    ByteBuffer byteBuf = ByteBuffer.allocateDirect(vertices.length * 4);
    byteBuf.order(ByteOrder.nativeOrder());
    vertexBuffer = byteBuf.asFloatBuffer();
    vertexBuffer.put(vertices);
    vertexBuffer.position(0);

    byteBuf = ByteBuffer.allocateDirect(texture.length * 4);
    byteBuf.order(ByteOrder.nativeOrder());
    textureBuffer = byteBuf.asFloatBuffer();
    textureBuffer.put(texture);
    textureBuffer.position(0);

    this.image=image;
} 
//Funcion de dibujado
public void draw(GL10 gl) {
    gl.glFrontFace(GL10.GL_CCW);
    //gl.glEnable(GL10.GL_BLEND);
    //Bind our only previously generated texture in this case
    gl.glBindTexture(GL10.GL_TEXTURE_2D, textures[0]);
    //Point to our vertex buffer
    gl.glVertexPointer(3, GL10.GL_FLOAT, 0, vertexBuffer);
    gl.glTexCoordPointer(2, GL10.GL_FLOAT, 0, textureBuffer);
    //Enable vertex buffer
    gl.glEnableClientState(GL10.GL_VERTEX_ARRAY);
    gl.glEnableClientState(GL10.GL_TEXTURE_COORD_ARRAY);
    //Draw the vertices as triangle strip
    gl.glDrawArrays(GL10.GL_TRIANGLE_STRIP, 0, vertices.length / 3);
    //Disable the client state before leaving
    gl.glDisableClientState(GL10.GL_VERTEX_ARRAY);
    gl.glDisableClientState(GL10.GL_TEXTURE_COORD_ARRAY);
    //gl.glDisable(GL10.GL_BLEND);      
}
//Carga de texturas
public void loadGLTexture(GL10 gl, Context context) {
    //Generamos un puntero de texturas
    gl.glGenTextures(1, textures, 0);       
    //y se lo asignamos a nuestro array
    gl.glBindTexture(GL10.GL_TEXTURE_2D, textures[0]);
    //Creamos filtros de texturas
    gl.glTexParameterf(GL10.GL_TEXTURE_2D, GL10.GL_TEXTURE_MIN_FILTER, GL10.GL_NEAREST);
    gl.glTexParameterf(GL10.GL_TEXTURE_2D, GL10.GL_TEXTURE_MAG_FILTER, GL10.GL_LINEAR);
    //Diferentes parametros de textura posibles GL10.GL_CLAMP_TO_EDGE
    gl.glTexParameterf(GL10.GL_TEXTURE_2D, GL10.GL_TEXTURE_WRAP_S, GL10.GL_REPEAT);
    gl.glTexParameterf(GL10.GL_TEXTURE_2D, GL10.GL_TEXTURE_WRAP_T, GL10.GL_REPEAT);     
    /*
    String imagePath = "radiocd5.png";
    AssetManager mngr = context.getAssets();
    InputStream is=null;
    try {
        is = mngr.open(imagePath);
    } catch (IOException e1) {  e1.printStackTrace();   }
    */
    //Get the texture from the Android resource directory
    InputStream is=null;
    /*
    if (item.equals("rim"))
        is = context.getResources().openRawResource(R.drawable.rueda);
    else if (item.equals("selector"))
        is = context.getResources().openRawResource(R.drawable.selector);
    */      
    /*
    is = context.getResources().openRawResource(resourceId);
    Bitmap bitmap = null;
    try {
        bitmap = BitmapFactory.decodeStream(is);
    } finally {
        try {
            is.close();
            is = null;
        } catch (IOException e) {
        }
    }
    */
    Bitmap bitmap =image;       
    //con el siguiente código redimensionamos las imágenes que sean mas grandes de 256x256.
    int newW=bitmap.getWidth();
    int newH=bitmap.getHeight();
    float fact;
    if (newH>256 || newW>256)
    {
        if (newH>256)
        {
            fact=(float)255/(float)newH; //porcentaje por el que multiplicar para ser tamaño 256
            newH=(int)(newH*fact); //altura reducida al porcentaje necesario
            newW=(int)(newW*fact); //anchura reducida al porcentaje necesario   
        }
        if (newW>256)
        {
            fact=(float)255/(float)newW; //porcentaje por el que multiplicar para ser tamaño 256
            newH=(int)(newH*fact); //altura reducida al porcentaje necesario
            newW=(int)(newW*fact); //anchura reducida al porcentaje necesario
        }
        bitmap=Bitmap.createScaledBitmap(bitmap, newW, newH, true);
    }       
    //con el siguiente código transformamos imágenes no potencia de 2 en imágenes potencia de 2 (pot)
    //meto el bitmap NOPOT en un bitmap POT para que no aparezcan texturas blancas.
    int nextPot=256;
    int h = bitmap.getHeight();
    int w = bitmap.getWidth();
    int offx=(nextPot-w)/2; //distancia respecto a la izquierda, para que la imagen quede centrada en la nueva imagen POT
    int offy=(nextPot-h)/2; //distancia respecto a arriba, para que la imagen quede centrada en la nueva imagen POT
    Bitmap bitmap2 = Bitmap.createBitmap(nextPot, nextPot, Bitmap.Config.ARGB_8888); //crea un bitmap transparente gracias al ARGB_8888
    Canvas comboImage = new Canvas(bitmap2);
    comboImage.drawBitmap(bitmap, offx, offy, null);
    comboImage.save();

    //Usamos Android GLUtils para espcificar una textura de 2 dimensiones para nuestro bitmap
    GLUtils.texImage2D(GL10.GL_TEXTURE_2D, 0, bitmap2, 0);

    //Checkeamos si el GL context es versión 1.1 y generamos los Mipmaps por Flag. Si no, llamamos a nuestra propia implementación
    if(gl instanceof GL11) {
        gl.glTexParameterf(GL11.GL_TEXTURE_2D, GL11.GL_GENERATE_MIPMAP, GL11.GL_TRUE);
        GLUtils.texImage2D(GL10.GL_TEXTURE_2D, 0, bitmap2, 0);
    } else {
        buildMipmap(gl, bitmap2);
    }   
    //Limpiamos los bitmaps
    bitmap.recycle();
    bitmap2.recycle();
}
//Nuestra implementación de MipMap. Escalamos el bitmap original hacia abajo por factor de 2 y lo asignamos como nuevo nivel de mipmap
private void buildMipmap(GL10 gl, Bitmap bitmap) {
    int level = 0;
    int height = bitmap.getHeight();
    int width = bitmap.getWidth();
    while(height >= 1 || width >= 1) {
        GLUtils.texImage2D(GL10.GL_TEXTURE_2D, level, bitmap, 0);
        if(height == 1 || width == 1) {
            break;
        }
        level++;
        height /= 2;
        width /= 2;
        Bitmap bitmap2 = Bitmap.createScaledBitmap(bitmap, width, height, true);
        bitmap.recycle();
        bitmap = bitmap2;
    }
}
}

Answer 1

If I see correctly, the z-value you put into gluProject is the same value you put into glTranslate. But you still draw the polygon using the vertex (-1, -1, 0), the z translation comes from the glTranslate call (which in turn modifies the modelview matrix). But this matrix is also used in gluProject, so what actually happens is that you translate by z two times (not exactly, as the first translation is further distorted by the rotation). So put in the same vertex you also draw the polygon with, which would be (-1, -1, 0) and not (-1, -1, z).

Keep in mind that gluProject does the same thing as OpenGL's transformation pipeline (like explained in my answer to your other nearly exact same question), so you have to feed it with the same values you feed the OpenGL pipeline with (your polygon's vertices) if you want the same results (the polygon covering the screen).