feFlood

想象成油漆桶工具画一个矩形

<svg xmlns="http://www.w3.org/2000/svg" width="200" height="200">
  <defs>
    <filter id="floodFilter" filterUnits="userSpaceOnUse">
      <feFlood x="50" y="50" width="100" height="100"
          flood-color="green" flood-opacity="0.5"/>
    </filter>
  </defs>

  <use style="filter: url(#floodFilter);"/>
</svg>