OK, I’ve watched a hundred videos (OK, a few) to get convincing volumetric lights localised to the visible lights. The image is where I managed to get myself. I did this by parenting a cube (suitably modified to be an elongated cube with the same profile as the window) to sun lamp. That way, if I adjust the sun, the volumetrics are adjusted automatically. Density is a tiny .01, and whilst it works:
Sun intensity is high - at least 10 and up to 100.
Samples (even with denoising) is 10,000. Render time for this very simple test was over 50 minutes, on GPU.
What is the most efficient way to get volumetric lights that only appear in direct light?
OK, made some progress using my technique. Got it down to 1000 samples (could probably do with 2000), and added dust. I was especially worried that the denoiser would remove the disu, but it seems to handle it OK.
Unfortunately, I didn’t save my original scene. This one is slightly more complicated, though uses the same materials. I’ve been able to drop the sun brightness back to 8 by adding an area light behind the camera to perfom the bounce light more efficiently, and reduced sun bounces to just 16.
Also improved the volumetric falloff, not with any fancy tricks, just learning better how the shader works.
Render time down to 17 minutes.
MOD: If you think that this would be better in experiments, please move it.