Ray object picking on Android OpenGL ES

Question

I'm drawing a globe using OpenGL on Android platform and trying to find a point on the sphere where user tapped. I am new to OpenGL at all and I'm using opensource lib Rajawali as it is show in example Touch & Drag but if someone know better way to do this please suggest me as the code of Rajawali is completely without comments and huge part of docs are outdated :(

double[] np4 = new double[4]; //near plane
double[] fp4 = new double[4]; // far plane

GLU.gluUnProject(
        x, mViewportHeight - y, 0.0f, 
        mViewMatrix.getDoubleValues(), 0, 
        mProjectionMatrix.getDoubleValues(), 0, 
        mViewport, 0, 
        np4, 0
);

GLU.gluUnProject(
        x, mViewportHeight - y, 1.0f, 
        mViewMatrix.getDoubleValues(), 0, 
        mProjectionMatrix.getDoubleValues(), 0, 
        mViewport, 0, 
        fp4, 0
);

// transform 4D coordinates (x, y, z, w) to 3D (x, y, z) by dividing each coordinate (x, y, z) by w.
return new Ray(
        new Vector3(np4[0] / np4[3], np4[1] / np4[3], np4[2] / np4[3]),
        new Vector3(fp4[0] / fp4[3], fp4[1] / fp4[3], fp4[2] / fp4[3])
    );

and then find an intersection point with sphere (radius = 1, center (0,0,0)) as described here

    /**
     * Calculate point of intersection of line and sphere.
     * 
     * @param p1 Point the line goes throw
     * @param p2 Point the line goes throw
     * @param center Center of the sphere
     * @param r Radius of the sphere
     * @return <code>Vector3</code> Coordinates of intersection point
     */
    private Vector3 getIntersectionPoint(Vector3 p1, Vector3 p2, Vector3 center, double r){
        if (p1 == null || p2 == null || center == null)
            return null;

        double a = in2(p2.x - p1.x) + in2(p2.y - p1.y) + in2(p2.z - p1.z);
        double b = 2*( (p2.x - p1.x)*(p1.x - center.x) + (p2.y - p1.y)*(p1.y - center.y) + (p2.z - p1.z)*(p1.z - center.z) );
        double c = in2(center.x) + in2(center.y) + in2(center.z) + in2(p1.x) + in2(p1.y) + in2(p1.z) - 2*(center.x*p1.x + center.y*p1.y + center.z*p1.z) - in2(r);

        //simplified equations for Sphere coordinates are (0, 0, 0) r=1 and cam.x = cam.y = 0
//      double a = in2(p2.x) + in2(p2.y) + in2(p2.z - p1.z);
//      double b = 2*( (p2.z - p1.z)*p1.z );
//      double c = in2(p1.z) - 1;

        for (Vector3 poi : solveQuadraticEquation(a, b, c, p1, p2)){
            if (poi.z > 0)
                return poi;
        }

        return null;
    }


    /**
     * Solve quadratic equation.
     * 
     * @param a Parameter A in equation Ax^2 + Bx + C = 0
     * @param b Parameter B in equation Ax^2 + Bx + C = 0
     * @param c Parameter C in equation Ax^2 + Bx + C = 0
     * @param p1 <code>Vector3</code> Start point for the line
     * @param p2 <code>Vector3</code> End point for the line
     * 
     * @return <code>List<Vector3></code> of results. Empty if no results found.
     */
    private List<Vector3> solveQuadraticEquation(double a, double b, double c, Vector3 p1, Vector3 p2){
        double D = in2(b) - 4*a*c;

        List<Vector3> rez = new ArrayList<Vector3>();

        if (D < 0) {

            return rez;

        } else  if (D == 0) {

            double t = (-b) / (2*a);
            rez.add(new Vector3(p1.x + (p2.x - p1.x)*t, p1.y + (p2.y - p1.y)*t, p1.z + (p2.z - p1.z)*t));

            return rez;

        } else {


            double t = (Math.sqrt(D)-b) / (2*a);
            rez.add(new Vector3(p1.x + (p2.x - p1.x)*t, p1.y + (p2.y - p1.y)*t, p1.z + (p2.z - p1.z)*t));

            t = (-Math.sqrt(D)-b) / (2*a);
            rez.add(new Vector3(p1.x + (p2.x - p1.x)*t, p1.y + (p2.y - p1.y)*t, p1.z + (p2.z - p1.z)*t));

            return rez;

        }
    }

and here is a result:

Here: BLUE dots are (-1, -1, -1), (1, -1, -1), (1, 1, -1) and (-1, 1, -1) GREEN dots are (-1, -1, 1), (1, -1, 1), (1, 1, 1) and (-1, 1, 1) YELLOW dots are (1, 0, 0), (0, 1, 0), (-1, 0, 0) and (0, -1, 0)

small red dot is a point of intersection when I tapped on top yellow circle: x: 405.10202 y: 430.5059 (event.getX(), evet.getY() in touch listener) here is a unprojected points: near: Vector3 : <0.0038529188490706075, 0.08910624125329661, 4.999999999999999> far: Vector3 : <0.46235026188847467, 10.692748950395632, -114.00000000000048> intersection: Vector3 : <0.019687997272589165, 0.45532322467387265, 0.8901085011592542>

camera position is (0,0,-6)

Could anybody help me put this small red dot under the finger, please ?

P.S. I need a point of intersection not just an object picking beckause I'm going to get country code by this point later...

Answer 1

The problem is that ViewPort correspond to the view dimensions (GLSurfaceView in this case) but point hase absolute coordinates, so the should be converted to relative, for example like this:

final float x = e.getX() - getLeft();
final float y = e.getY() - getTop();