Thanks, Oscar.
I really appreciated your advice and help.
I've been trying to figure it out still now, but I got to know more of the indigo renderings today.

Thanks a lot again.

Ok, I figured it out by reversing the face.

This would be my last question for today, what's the difference between BiDir with MLT and BiDir with Path Tracing?

How do you make it pointing inwards, then?

So I added the exit portal block in the fore ground of the rendering.

Now it blocks the sun light coming from window outside.
How come does that happen?

So, I don't see any exit portal check box...
How do I make that show up?

Thanks, Oscar.

So Oscar, what is exit portals? How do I use it?

I attached my computer condition below.
Thanks.

When you mentioned simple BiDir without MLT, does it mean BiDir Path Tracing?

Also, I checked the light layers, turning off one by one, but I don't think it matters.
How long does it usually take for 4000 resolution by your experience?

Thank you.

Hi,

Thanks for replying on my question.
So the render's been 105 hours with network rendering, but still fuzzy.
Please help me out.

Thank you.

Hi,

I'm rendering an interior unit but still too fuzzy even though it's been rendered for more than 40 hrs with 4000 resolution.

Please help me out with this.

Thanks.