glUnProject problem in Android

Question

In my game, I need to find out where the player is touching. MotionEvent.getX() and MotionEvent.getY() return window coordinates. So I made this function to test converting window coordinates into OpenGL coordinates:

public void ConvertCoordinates(GL10 gl) {
        float location[] = new float[4];

        final MatrixGrabber mg = new MatrixGrabber(); //From the SpriteText demo in the samples.
        mg.getCurrentProjection(gl);
        mg.getCurrentModelView(gl);

        int viewport[] = {0,0,WinWidth,WinHeight};

        GLU.gluUnProject((float) WinWidth/2, (float) WinHeight/2, (float) 0, mg.mModelView, 0, mg.mProjection, 0, viewport, 0, location,0);
        Log.d("Location",location[1]+", "+location[2]+", "+location[3]+"");
    }

X and y oscillated from almost -33 to almost +33. Z is usually 10. Did I use MatrixGrabber wrong or something?

Answer 1

Well the easiest way for me to get into this was imagining the onscreen click as a ray that starts in camera position and goes into infinity

To get that ray i needed to ask for it's world coords in at least 2 Z positions (in view coords).

Here's my method for finding the ray(taken from the same android demo app i guess). It works fine in my app:

public void Select(float x, float y) {
    MatrixGrabber mg = new MatrixGrabber();
    int viewport[] = { 0, 0, _renderer.width, _renderer.height };

    _renderer.gl.glMatrixMode(GL10.GL_MODELVIEW);
    _renderer.gl.glLoadIdentity();
    // We need to apply our camera transformations before getting the ray coords 
    // (ModelView matrix should only contain camera transformations)
    mEngine.mCamera.SetView(_renderer.gl);

    mg.getCurrentProjection(_renderer.gl);
    mg.getCurrentModelView(_renderer.gl);

    float realY = ((float) (_renderer.height) - y);
    float nearCoords[] = { 0.0f, 0.0f, 0.0f };
    float farCoords[] = { 0.0f, 0.0f, 0.0f };
    gluUnProject(x, realY, 0.0f, mg.mModelView, 0, mg.mProjection, 0,
            viewport, 0, nearCoords, 0);
    gluUnProject(x, realY, 1.0f, mg.mModelView, 0, mg.mProjection, 0,
            viewport, 0, farCoords, 0);

    mEngine.RespondToClickRay(nearCoords, farCoords);
}

In OGL10 AFAIK you don't have access to Z-Buffer so you can't find the Z of the closest object at the screen x,y coords. You need to calculate the first object hit by your ray on your own.