Gap in Background Image

[ares2101]

In relation to a recently mentioned space scene, I tried making a hi res image of the milky way as the background but have this weird thing in the render.  An odd triangular section in the upper left is left completely black.  I know it's not the background image being tilted, as this doesn't show up in the opposite corner and the Milky Way as seen runs straight up and should just continue.  A test render and screenshot of background node info follows.

Any clues? 

[Kadri]

Did you try changing your Sun position?

[ares2101]

Did you try changing your Sun position?

I had not.  There are actually two suns in the scene in very different locations (of one which is even in front of the gap) and I had previously had a third in there for lighting the nightside's of the planets not rendered in the example, so I never suspected that as a cause.  Seems it was the cause though, as the issue is now fixed.  Any idea why that is?  It can't be a shadow.  That test render was done with all planets and objects disabled.

[Kadri]

I do not know if your setup is kinda similar but anyway ...

I had this problem ones but didn't much bother why it was happening .
I remembered only that it was about the Sun.
After your post here i looked a little closer , Ares2101.

If you download the TGD in the attachment and play with it you will see it itself.

Open the TGD.

Open the "Sunlight" node and make it "Stay open"

Go in the "Background" node.


[attachimg=1]


When you play with the Elevation slider in the "Sunlight" node
you will see how the Background changes according the Elevation.
The Heading of the Sun does change the Background appearance too of course.

Now put the original "Background shader" node to the "Background" surface shader input
as it was at default (i changed the color to gray for easier look) .
Play with the "Sunlight" node  Elevation and Heading ones again. It does not change now.
It is all the same color everywhere  independent of the Sun position now.

I did not think about this before but after trying it makes absolutely sense.
We change the Sun position all the time in TG2 .
The background should not be affected by this (in general).
But the other kind of surfaces are dependent of the sun position.
If we put a standard surface in the Background node ,
other then the special Background surface node, it does change its behavior of course.

So if we change the Background surface shader this is a little point to remember as it seems.

[j meyer]

I might be totally wrong,but here are my 2cents:
the background is a huge sphere as far as I can remember and thus you can
get a terminator effect from any sun in your scene unless the background color
is pure black like in the default background shader.

[Dune]

I think Jochem is right, a sort of self shadow of the background sphere. Perhaps if you use the luminosity input for your milky way image, you're saved.

[Kadri]

Try the TGD or make your own one.
The Background shader says in parenthesis "Constant shader" for a reason probably!
The position of the Sun does not matter when you use only this one.
It does not account for the Sun at all as it seems and only uses it own luminosity-color.

What you guys say is only  true when you use any other surface then the default one as we talked above .