Question about kinect fuction

Hello dude. I have a quick question about kinect addon’s function which is getWorldCoordiateAt().
I already knew this function returns ofVec3f value but when I tried to get those return values in loop, there was nothing coming in. It was just 0, 0, 0.
However if I add some function in that which is getdistanceat() in if statement. like if(kinect.getDistanceAt( )>0) then I could got proper values. Do you guys know how this is working? why I need getdistanceat( )> 0 ?
The funtion getdistanceat( ) returns ofshortpixels.

My code is:

void ofApp::draw(){

int w = 640;
int h = 480;
int step = 2;
for(int y = 0; y < h; y += step) {
    for(int x = 0; x < w; x += step) {
       if(kinect.getDistanceAt(x, y) > 0) {
           cout<<kinect.getWorldCoordinateAt(x, y)<<endl;
            
       }
    }
}