Compositing pictures in GIMP

[Yora]

I want to make a picture of how the sky would look like in a world I created. For this I am using a landscape picture and want to add to it a big sun and a moon. I already made a simple mockup of how it#s supposed to be.

http://leavesofkaendor.com/wp-content/uploads/2018/04/lake-1524386824714-6744.png

The main problem I have now is with the shadow of the moon. It's a picture of Neptune that obviously is on a black background with much of it being in black shadow. To add it to the colored sky here, I need to make all the "blackness" from that image to be translucent instead. Maybe not entirely translucent even, as the dark side would get some reflection of sunlight from the surface of the Earth. I am pretty sure there is a simple tool for that, but I don't know what that would be called and where to find it.

Another issue is that the moon would have to be behind the clouds. My first inntuition would be to make a copy layer of the sky, make it partly translucent, and add it on top of the moon. Same method to make the trees cover over the bottom of the moon.

Anyone knowledgabe in these things having advice for me?

[Lvl 2 Expert]

Good questions. Translucent is easy. Layers->transparency->color to alpha, select black as your color. But I don't think that will give the right effect for either of them.

For the sun you could try it the other way around. Select the sun (remove the square around it first, you wat only the circle itself), then copy that selection-shape out of the background image and paste it as a new layer over the sun, and make that new layer translucent. Try out a few different colors as alpha, but something close to black should work. Maybe something like dark green or blue to protect the red colors better. That will probably look decent for the sun itself, but it might look horrible combined with the rest of the image. You'd almost have to overlay a translucent layer of the background image over everything (that is: the non translucent version of itself) and that probably looks bad too.

For the moon you could try a couple of options under layer->colors. Either brightness-contrast, hue saturation or color balance could give a nice effect. (These things are hard to do right while keeping a natural look, but after a bunch of tries it often works.) The problem again is getting it to look like it's behind the sky. Behind the tree is easy, either copy the tress to go over it or cut out the moon in those places. It might take some time but it will look fine. But just as with the sun it's the translucency that could give you problems.

One way to find out...

(Note: I use an older version, menus might not be the same.)

[Yora]

Color to alpha is exactly what I was looking for. Thanks.