Ray-picking in OpenGL ES 2.0

Question

I'm trying to implement ray-picking in OpenGL ES 2.0 to determine if an object has been clicked or not. So far I'm just trying to check if a specific triangle has been pressed. I`m using this site as a motivation http://android-raypick.blogspot.ca/2012/04/first-i-want-to-state-this-is-my-first.html

This is what I have so far:

public void onClick(float x, float y)
{
    float[] temp = new float[4];
    float[] temp2 = new float[4];
    System.out.println("X coordinate: " + x);
    System.out.println("Y coordinate: " + y);
    float[] pos = new float[4];

    y = (float) viewport[3] - y;

    int res = GLU.gluUnProject(x, y, 1.0f, 
            mMVPMatrix, 0,
            mProjectionMatrix, 0,
            viewport, 0, 
            temp, 0);

    Matrix.multiplyMV(temp2, 0, mMVPMatrix, 0, temp, 0);
    float[] nearCoOrds = new float[3];

    if(res == GLES20.GL_TRUE)
    {
        nearCoOrds[0] = temp2[0] / temp2[3];
        nearCoOrds[1] = temp2[1] / temp2[3];
        nearCoOrds[2] = temp2[2] / temp2[3];
        System.out.println("Near0: " + nearCoOrds[0]);
        System.out.println("Near1: " + nearCoOrds[1]);
        System.out.println("Near2: " + nearCoOrds[2]);
    }

    res = GLU.gluUnProject(x, y, 0,
            mMVPMatrix, 0,
            mProjectionMatrix, 0,
            viewport, 0,
            temp, 0);

    Matrix.multiplyMV(temp2,0,mMVPMatrix, 0, temp, 0);
    float[] farCoOrds = new float[3];

    if(res == GLES20.GL_TRUE)
    {
        farCoOrds[0] = temp2[0] / temp2[3];
        farCoOrds[1] = temp2[1] / temp2[3];
        farCoOrds[2] = temp2[2] / temp2[3];
        System.out.println("Far0: " + farCoOrds[0]);
        System.out.println("Far1: " + farCoOrds[1]);
        System.out.println("Far2: " + farCoOrds[2]);
    }

    float[] coords = new float[3];

    coords[0] = farCoOrds[0]-nearCoOrds[0];
    coords[1] = farCoOrds[1]-nearCoOrds[1];
    coords[2] = farCoOrds[2]-nearCoOrds[2];

    System.out.println("REAL COORDS 0: " + coords[0]);
    System.out.println("REAL COORDS 1: " + coords[1]);
    System.out.println("REAL COORDS 2: " + coords[2]);

}

The x and the y float is the x and the y coordinate of where the finger pressed the screen. The function onClick is called from the MainActivity.

In

 GLU.gluUnProject(x, y, 1.0f, 
            mMVPMatrix, 0,
            mProjectionMatrix, 0,
            viewport, 0, 
            temp, 0);

mMVPMatrix is the Modelview Matrix. mProjectionMatrix is the Projection Matrix and the viewport has the values {0,0,screenhwidth,screenheight}.

Example of output I get is (touched around middle of the screen):

REAL COORDS 0: -0.21542415
REAL COORDS 1: 0.31117013
REAL COORDS 2: 9.000003

My question/topic is that I don't know if I'm on the right track here? Have I gotten the right idea or does it seem that I have misunderstood something? Or are there any other ways I can achieve touch detection on triangles?

Thanks for any help or guidance!

By looking only at some sample coordinates I can't tell you whether your results are correct or not. — keaukraine

molnarm molnarm · Accepted Answer · 2013-01-12T19:33:27

There is an excellent OpenGL framework for Android called Rajawali. It supports object picking, the sample code looks very simple, you should try it.

Ray-picking in OpenGL ES 2.0

2 Answers