Drawing polygons with box2d

[tuxier]

Hello,
box2d and qml works together nice as i see in demo projects
but
i write this in main.qml

import QtQuick 1.0
import Box2D 1.0

Polygon {
    vertices: [
        Qt.point(20,40),
        Qt.point(40,20),
        Qt.point(20,20)
    ]
/*Rectangle{
    width:50
    height:50
*/
    
   

}

i can see only a black window

you might say there is no color,

color or gradient tag is not allowed in polygon object.
i am going slightly mad, because i could not find any doc on box2d+qml and asked several forums, nobody use box2d+qml before.
i dig into demo projects but can not applicate them exactly
thanks