[Qgis-user] Raster layer opacity expression - how does it work?

[Jonathan Schultz]

Hello,

I'm trying to use opacity on a raster layer to highlight an area of 
interest. So, for example, I would use opacity 100 for an area (defined 
in another layer), and 50 for anywhere outside that area. But I can't 
seem to even find basic documentation on how the Renderer opacity values 
work. If I enable the Assistant I get a bunch of fields: Source (an 
expression), Values from and to, plus a checkbox "Apply transform curve" 
with its own set of fields.

When I google words like "Renderer opacity" all I get are references to 
the source code but no documentation or examples.

This is QGIS version 3.22.0-Białowieża running on Linux/Debian.

Does anyone know how this stuff works, or how I could find out?

Thanks!