objective c - iOS Using GLKMathUnproject to find screen point in world space -

how use glkmathunproject determine location in world space? user needs select 1 of several objects in world space.

in doc says

glkvector3 glkmathunproject (    glkvector3 window,    glkmatrix4 model,    glkmatrix4 projection,    int *viewport,    bool *success ); 

but modelview matrix? have bunch of models in world. model matrix should use.

my world 2d, on x,y plane. z used camera movement.

if understood correctly, glkmathunproject used find point in model space. how know model? need determine first model below fingers or what?

- (ibaction)handletapunproject:(id)recognizer {         bool success = no;     glfloat realy;      glint viewport[4] = {};     glgetintegerv(gl_viewport, viewport);     nslog(@"%d, %d, %d, %d", viewport[0], viewport[1], viewport[2], viewport[3]);      cgpoint touchorigin = [recognizer locationinview:self.view];     nslog(@"tap coordinates: %8.2f, %8.2f", touchorigin.x, touchorigin.y);      realy = viewport[3] - touchorigin.y;      glkmatrix4 modelview = lookat;      // near      glkvector3 origininwindownear = glkvector3make(touchorigin.x, realy, 0.0f);      glkvector3 result1 = glkmathunproject(origininwindownear, modelview, projectionmatrix, viewport, &success);     nsassert(success == yes, @"unproject failure");      glkmatrix4 matrix4_1 = glkmatrix4translate(glkmatrix4identity, result1.x, result1.y, 0.0f);     _squareunprojectnear.modelmatrixusage = glkmatrix4multiply(matrix4_1, _squareunprojectnear.modelmatrixbase);      glkvector3 rayorigin = glkvector3make(result1.x, result1.y, result1.z);      // far      glkvector3 origininwindowfar = glkvector3make(touchorigin.x, realy, 1.0f);      glkvector3 result2 = glkmathunproject(origininwindowfar, modelview, projectionmatrix, viewport, &success);     nsassert(success == yes, @"unproject failure");      glkmatrix4 matrix4_2 = glkmatrix4translate(glkmatrix4identity, result2.x, result2.y, 0.0f);      glkvector3 raydirection = glkvector3make(result2.x - rayorigin.x, result2.y - rayorigin.y, result2.z - rayorigin.z); } 

so if want unproject world space, use lookat matrix. , if want unproject model space of specific object, use model * view matrix.