$image = new Imagick();
    $image->newImage(800, 600, new ImagickPixel("transparent"));
    //or
    $image->newImage(800, 600, "none");

    $draw = new ImagickDraw();
    $draw->setStrokeWidth(4);
    $draw->setStrokeColor( new ImagickPixel('red') );
    $draw->setFillColor( new ImagickPixel('transparent') );
    $draw->polygon(
        array(
            array( 'x' => 10,'y'=>150),
            array( 'x' => 30,'y'=>140),
            array( 'x' => 50,'y'=>170 ),
    ));
    $image->drawimage($draw);

    $image->setImageFormat( "png" );
    $image->writeimage('xxx.png');
    $image->destroy();
    $draw->destroy();