Ok, I think I understand what you are saying. And obviously there are some problems with lightmass. Here is an example. I did try to replicate your scene to see if I get a better result. Well - it didn’t work. I think because it is an extreme case, at least for light mass. You have no light other then the sun light and you have only a very thin wall between a totally dark and a pretty good lit spot. So there is some pretty bad light bleeding.
I used standard settings with a stationary sun light with 3 bounces and a skylight . It doesn’t look a lot worse than your version other than the bad light bleeding but it took less than a minute to render. Light map sizes are 256 for the floor and 512 for the wall/ceiling mesh. Pumping up the settings doesn’t seem to make a difference so I usually leave it close to the default settings. You only end up with ridiculous render times if you change some of the default settings too much without any real benefit from it.
The light bleeding is a huge problem in this image. But only a theoretical one because we would definitely put some lights in this nook. Then the light bleeding disappears. My theory is that lightmass tries to blend the totally dark and the bright spots so the transition isn’t harsh. Because it doesn’t change when I change the UV layout of the light map or the resolution.
So I put some lights in the nook and applied a few simple materials. It still only takes a minute to render on my 4 year old i7 PC.
This isn’t perfect but I think it isn’t too bad either as long as you don’t compare it to traditional vray renders. Part of the problem is that traditional vray renders are often unrealistically clean. If I look at my ceiling in my flat at night, I can seeall kind of weird shades of grey.
It is like a photoshoped woman in a fashion magazine, no one looks like that. I wish we could train our clients to accept little imperfections for the greater picture.
I am not saying that light mass shouldn’t be better. I think they are trying but they have to balance a lot of things and currently still image isn’t on the top of their list. I personally try to work around the problems. I learned that from games where the process seems to be a bit smarter. You don’t set the angle of the image in stone. If it doesn’t look that great from this angle why not moving it a bit to the side. Or we could put a picture on the wall to distract from the little imperfection. I wish we all had better clients.