为什么加上

     <radialGradient id='light' cx='0.5' cy='0.5' r='0.5'>
                <stop offset='0' stop-color='rgba(255,255,255,.8)'></stop>
                <stop offset='1' stop-color='rgba(255,255,255,0)'></stop>
            </radialrGradient>
            <clipPath id='light-clip'>
                 <polygon points='0 0 -400 -15 -400 15' fill='rgba(255,0,0,0.5)'></polygon>
            </clipPath>
        </defs>
        <polygon points='0 0 5 50 -5 50' fill='url(#tower)'></polygon>
        <ellipse cx='0' cy='0' rx='300' ry='100' fill='url(#light)' clip-path='url(#light-clip)'></ellipse>