Light Portal increases render time

[mckenzie2VER2]

I've got an interior scene with multiple windows lit by a single HDR.

When I add light portals it doubles my render time.

Is that normal?

C4D 2023

C4DtoA 4.4.0

[thiago.ize]

Every light portal is effectively an additional light, so if you replace your one skydome light with say 20 portal lights, you went from 1 light to 20 lights. That will be slower. The benefit is that your render should have less noise and so you can reduce the number of light and camera AA samples and hopefully in the end you end up being faster.

Also, if you can get away with it, changing the skydome_light.portal_mode from interior_exterior to interior_only should give you a speedup.

[mckenzie2VER2]

I didn't remove the Skydome, just added portals to help guide those rays 🙂

I tested it this morning and with the portals there was less shadow noise for sure but it didn't allow me to really reduce the samples etc as still getting GI noise. Trying to find a place where I can use noice and get a decent result.

[thiago.ize]

I didn't mean to imply you removed the skydome, rather that under the hood, if you use the interior_only portal mode, arnold stops sampling the skydome as a light and instead uses the quad_light portals to do so. That's why this results in cleaner renders but takes longer.

If lowering the light samples to 1 still isn't good enough, another thing you can try is combining some of the nearby light portals into a single big portal. It will make it faster at the expense of additional noise. As long as the bigger portal is still mostly windows and not walls, it could help...

[mckenzie2VER2]

I figured that's what you meant 🙂

Light samples with portals still needs to be around 4 or 5 to get clean direct light. The windows are quite large so I did use bigger portals rather than smaller ones for some of them.