how to implement spout?

Sure, I want that too :-). We also had a request to implement Siphon (for MacOS). Somebody must find out how to use Spout, I suggest you do it. Check out a branch and try it!
Adding external libraries: See several examples in Stellarium's code directory.

I currently managed to do this:
https://drive.google.com/file/d/0B3AXCpr5o5fScGtkZUpjb0E3UEE/view?usp=sharing

but I only get a Milkyway background and what my plugin draws;
how can I solve this?

I have no idea how you even did just that, but congratulations, it looks very interesting.

If you share your branch of sourcecode on Launchpad and are willing to describe what you have to do to get spout working, and how to use that "live" texture e.g. in Unity, and are willing to contribute your development to the project (GPL), there would be even more interest here. Apparently "getting Stellarium to render a texture for Syphon, Unity, Spout, 5-port dome projection, ..." and whatnot is the big demand of the month.
I am too busy until at least late March to get also deeply involved in this, but maybe I have a few ideas. Or anybody else of course.

I did it:
https://drive.google.com/file/d/0B3AXCpr5o5fSRGxkdFJOcjl3TEE/view?usp=sharing

Great! Now I am to ask how you did it. Are you willing to provide a plugin?

I couldn't do it in plugins so I implemented it in stellarium main code. In
stelMainView code to be exact! I will publish it soon.

On Wed, Feb 17, 2016, 18:38 gzotti <email address hidden>
wrote:

> Your question #285284 on Stellarium changed:
> https://answers.launchpad.net/stellarium/+question/285284
>
> gzotti posted a new comment:
> Great! Now I am to ask how you did it. Are you willing to provide a
> plugin?
>
> --
> You received this question notification because you asked the question.
>

Any update on your development?

Great work! I wonder if/when this feature will be added to the mainstream Stellarium release?

Is hessi kaveh willing to contribute the code?

Hi, I developed the code on my work computer so I had to check with my
superintendent and he didnt allow me to publish my code!

On Mon, Oct 10, 2016, 18:27 gzotti <email address hidden>
wrote:

> Your question #285284 on Stellarium changed:
> https://answers.launchpad.net/stellarium/+question/285284
>
> gzotti posted a new comment:
> Is hessi kaveh willing to contribute the code?
>
> --
> You received this question notification because you asked the question.
>

Well, I hope your superintendent did not sell your code? Else there may be a conflict with the GPL, apart from moral issues. Or maybe I should hope he did?

No, he did not. But he wants to use the code with another software which
the company wants to sell and I dont know how the licensing is applied to
that software

On Tue, Oct 11, 2016, 01:28 gzotti <email address hidden>
wrote:

> Your question #285284 on Stellarium changed:
> https://answers.launchpad.net/stellarium/+question/285284
>
> gzotti posted a new comment:
> Well, I hope your superintendent did not sell your code? Else there may
> be a conflict with the GPL, apart from moral issues. Or maybe I should
> hope he did?
>
> --
> You received this question notification because you asked the question.
>

FYI: My solution for self-compilers: https://code.launchpad.net/~stellarium/stellarium/gz_spout