protected function CalcSlice($angle_start, $angle_end, &$x_start, &$y_start, &$x_end, &$y_end)
{
$x_start = $this->radius_x * cos($angle_start);
$y_start = ($this->reverse ? -1 : 1) * ($this->radius_y * sin($angle_start));
$x_end = $this->radius_x * cos($angle_end);
$y_end = ($this->reverse ? -1 : 1) * ($this->radius_y * sin($angle_end));
$x_start += $this->x_centre;
$y_start += $this->y_centre;
$x_end += $this->x_centre;
$y_end += $this->y_centre;
}