如何获取场景中每个对象的当前位置？

Question

I am rendering several square objects in sphere (I am in the center of the sphere and the objects are around me). I am using the phone's Rotation sensor to view the the objects in the sphere.

all the objects start from position around (0,0,0) , but at rendering time I'm rotating each object to a different angle in the sphere .

this is how I handle the matrices until from the moment I get the rotation matrix :

public void position(float[] rotationMatrix , float[] projectionMatrix , float rotationAngle , float xRotation , float yRotation , float zRotation , float xTranslation , float yTranslation , float zTranslation){


        float[] MVP = new float[16];

        float[] modelMatrix = new float[16];

        System.arraycopy(rotationMatrix, 0, modelMatrix, 0, rotationMatrix.length);

        Matrix.rotateM(modelMatrix, 0, -90f, 1f, 0f, 0f); //correct the axis so that the direction of Y axis is to the sky and Z is to the front

        Matrix.rotateM(modelMatrix, 0, rotationAngle, xRotation, yRotation, zRotation); //rotate the object around axis

        // used to control the distance of viewing the object (currently only z translation is used)
        Matrix.translateM(modelMatrix, 0, xTranslation, yTranslation, zTranslation);



        Matrix.multiplyMM(MVP , 0 , projectionMatrix , 0 , modelMatrix , 0);

        textureShaderProgram.setUniforms(MVP , texture);

        return;
    }

then in the shader I multiply each object location(which is the same location basically) with this MVP matrix , and they are rendered around in a sphere like world.

this works good . now what I like to do is identify when an object is right in front of me . get each object's location at all times , and when I view a certain object make it selectable or lighted.

but since every object is multiplied several times , how can I know it's location and when I am actually viewing it now ?

Answer 1

The translation is always stored in the last column of a transformation matrix (or in the last row depending whether the matrix is stored column major or row major).

Thus the position of an object in worldspace is the last column of the model matrix.