模型矩阵转换后的3D选择OpenGL ES 2.0

Question

Hey all I'm trying to implement 3D picking into my program, and it works perfectly if I don't move from the origin. It is perfectly accurate. But if I move the model matrix away from the origin (the viewmatrix eye is still at 0,0,0) the picking vectors are still drawn from the original location. It should still be drawing from the view matrix eye (0,0,0) but it isn't. Here's some of my code to see if you can find out why..

        Vector3d near = unProject(x, y, 0, mMVPMatrix, this.width, this.height);
        Vector3d far = unProject(x, y, 1, mMVPMatrix, this.width, this.height);
        Vector3d pickingRay = far.subtract(near);
        //pickingRay.z *= -1;
        Vector3d normal = new Vector3d(0,0,1);
        if (normal.dot(pickingRay) != 0 && pickingRay.z < 0)
        {
            float t = (-5f-normal.dot(mCamera.eye))/(normal.dot(pickingRay));
            pickingRay = mCamera.eye.add(pickingRay.scale(t));
            addObject(pickingRay.x, pickingRay.y, pickingRay.z+.5f, Shape.BOX);


        //a line for the picking vector for debugging
        PrimProperties a = new PrimProperties(); //new prim properties for size and center
        Prim result = null;
        result = new Line(a, mCamera.eye, far);//new line object for seeing look at vector
        result.createVertices();
        objects.add(result);
        }

public static Vector3d unProject(
        float winx, float winy, float winz,
        float[] resultantMatrix,
        float width, float height)
{
    winy = height-winy;
    float[] m = new float[16],
    in = new float[4],
    out = new float[4];
    Matrix.invertM(m, 0, resultantMatrix, 0);
    in[0] = (winx / width) * 2 - 1;
    in[1] = (winy / height) * 2 - 1;
    in[2] = 2 * winz - 1;
    in[3] = 1;
    Matrix.multiplyMV(out, 0, m, 0, in, 0);

    if (out[3]==0)
        return null;

    out[3] = 1/out[3];
    return new Vector3d(out[0] * out[3], out[1] * out[3], out[2] * out[3]);
}

Matrix.translateM(mModelMatrix, 0, this.diffX, this.diffY, 0); //i use this to move the model matrix based on pinch zooming stuff.

Any help would be greatly appreciated! Thanks.

Answer 1

I wonder which algorithm you have implemented. Is it a ray casting approach to the problem?

I didn't focus much on the code itself but this looks a way too simple implementation to be a fully operational ray casting solution.

In my humble experience, i would like to suggest you, depending on the complexity of your final project (which I don't know), to adopt a color picking solution.

This solution is usually the most flexible and the easiest to be implemented.

It consist in the rendering of the objects in your scene with unique flat colors (usually you disable lighting as well in your shaders) to a backbuffer...a texture, then you acquire the coordinates of the click (touch) and you read the color of the pixel in that specific coordinates.

Having the color of the pixel and the tables of the colors of the different objects you rendered, makes possible for you to understand what the user clicked from a logical perspective.

There are other approaches to the object picking problem, this is probably universally recognized as the fastest one.

Cheers Maurizio