Raycasting - Convert touch on screen to point on sphere in OpenGL ES (Rajawali3D)

Question

I have a sphere created using the Rajawali3D OpenGL ES library, with the camera placed inside the sphere at (0, 0, 0). The user can rotate this sphere on swipe.

I want to get the 3D co-ordinates of the spot the user touches on the sphere

Currently I am using the Unproject method to get the near and far planes, calculate vector direction and find the intersection point in the sphere.Here is the code

    mNearPos4 = new double[4];
    mFarPos4 = new double[4];
    mNearPos = new Vector3();
    mFarPos = new Vector3();
    mNewPos = new Vector3();

    // near plane
    GLU.gluUnProject(x, getViewportHeight() - y, 0,
            mViewMatrix.getDoubleValues(), 0,
            mProjectionMatrix.getDoubleValues(), 0, mViewport, 0,
            mNearPos4, 0);

    // far plane

    GLU.gluUnProject(x, getViewportHeight() - y, 1.0f,
            mViewMatrix.getDoubleValues(), 0,
            mProjectionMatrix.getDoubleValues(), 0, mViewport, 0,
            mFarPos4, 0);

    // transform 4D to 3D
    mNearPos.setAll(mNearPos4[0] / mNearPos4[3], mNearPos4[1]
            / mNearPos4[3], mNearPos4[2] / mNearPos4[3]);

    mFarPos.setAll(mFarPos4[0] / mFarPos4[3],
            mFarPos4[1] / mFarPos4[3], mFarPos4[2] / mFarPos4[3]);

    Vector3 dir = new Vector3(mFarPos.x - mNearPos.x, mFarPos.y - mNearPos.y, mFarPos.z - mNearPos.z);
    dir.normalize();

    // compute the intersection with the sphere centered at (0, 0, 0)

    double a = Math.pow(dir.x, 2) + Math.pow(dir.y, 2) + Math.pow(dir.z, 2);
    double b = 2 * (dir.x * (mNearPos.x) + dir.y * (mNearPos.y) + dir.z * (mNearPos.z));
    double c = Math.pow(mNearPos.x, 2) + Math.pow(mNearPos.y, 2) + Math.pow(mNearPos.z, 2) - radSquare;

    double D = Math.pow(b, 2) - 4 * a * c;

    // need only smaller root since the camera is within
    // mNewPos is used as the position of the point
    mNewPos.setAll((mNearPos.x + dir.x * t), (mNearPos.y + dir.y * t), mNearPos.z);

The problem is that i am getting the same range of co-ordinates when i rotate the sphere. For example, If i get the co-ordinates (a, b, c) on one side of the sphere, i get the same on the opposite side of the sphere.

How do i solve this problem and get the correct co-ordinates for all sides?

I am using Rajawali 1.0.232 snapshot

Answer 1

SOLVED: The problem was that i was saving the camera's projection and view matrices in variables.

So when a call was made to unproject() to convert the 2D point to 3D, it was taking the old values and hence the point was not getting plotted correctly.

So a solution would be to get the camera's view and projection matrices on demand without caching them.

mViewport = new int[]{0, 0, getViewportWidth(), getViewportHeight()};

Vector3 position3D = new Vector3();

mapToSphere(event.getX(), event.getY(), position3D, mViewport, 
                                 mCam.getViewMatrix(), mCam.getProjectionMatrix());

where the mapSphere() function does the unproject function as follows

 public static void mapToSphere(float x, float y, Vector3 position, int[] viewport,
                               Matrix4 viewMatrix, Matrix4 projectionMatrix) {

    //please refer for explanation in case of openGL
    //http://myweb.lmu.edu/dondi/share/cg/unproject-explained.pdf

    double[] tempPosition = new double[4];

    GLU.gluUnProject(x, viewport[3] - y, 0.7f,
            viewMatrix.getDoubleValues(), 0,
            projectionMatrix.getDoubleValues(), 0, viewport, 0,
            tempPosition, 0);

    // the co-ordinates are stored in tempPosition as 4d (x, y, z, w)
    // convert to 3D by dividing x, y, z by w

    // the minus (-) for the z co-ordinate worked for me

    position.setAll(tempPosition[0] / tempPosition[3], tempPosition[1]
            / tempPosition[3], -tempPosition[2] / tempPosition[3]);

}