Hey James,
I’m not sure if you’re dealing with the same thing as I am (wanting a triangle “above” the background image as a section divider") but, if so, this may work.
Add a row at the top and bottom of your section, ensuring that there’s no padding/margin, then for the triangles I’m using an SVG mask. Top row:
<svg xmlns="http://www.w3.org/2000/svg" version="1.1" width="100%" height="100" viewBox="0 0 100 100" preserveAspectRatio="none">
<path d="M0 0 L50 100 L100 0 Z" fill="white"/>
</svg>
Then in the bottom row:
<svg xmlns="http://www.w3.org/2000/svg" version="1.1" width="100%" height="100" viewBox="0 0 100 100" preserveAspectRatio="none">
<path d="M100 100 L0 100 L50 0 Z" fill="white"/>
</svg>
It’s not the most elegant, but it works.