Example
a green rect (again...) with 4 holes created using 4 greyscale values resulting in 4 different opacities.
<svg viewBox="0 0 100 100" xmlns="http://www.w3.org/2000/svg" xmlns:xlink="http://www.w3.org/1999/xlink">
<mask id="myMask">
<rect x="0" y="0" width="100" height="100" fill="white"/>
<circle cx="25" cy="25" r="20" fill="black"/>
<circle cx="75" cy="25" r="20" fill="#333"/>
<circle cx="25" cy="75" r="20" fill="#666"/>
<circle cx="75" cy="75" r="20" fill="#999"/>
</mask>
<image xlink:href="https://cdn.pixabay.com/photo/2013/04/06/05/06/ship-100978_960_720.jpg" width="100" height="100"/>
<rect x="0" y="0" width="100" height="100" fill="green" mask="url(#myMask)"/>
</svg>
