Easy to use software to merge foreground with tracked milky way?

Started 2 months ago | Discussions thread
Lyle Aldridge Senior Member • Posts: 1,207
Re: Easy to use software to merge foreground with tracked milky way?

This isn't always easy, but it's also usually not outrageously difficult, and GIMP (freeware) can accomplish this task just as Photoshop would, possibly even better. Most nighttime MW images show the sky lighter than the foreground, and that makes it pretty simple to create an operative layer mask using the "Levels" and  "Threshold" adjustments on the "Color" menu. The process, as usual, takes almost infinitely longer to describe than it does to actually perform.

At its simplest, you start with foreground as a layer on top of your sky image. When you create a layer mask in GIMP, one of the options on the menu is to use the image on that layer as the layer mask, and another option is to invert it. (I haven't used newer versions of Photoshop, but the older versions I have used didn't have this nice option.)

So if you check both those options, and create a inverted layer mask of the foreground image, that lighter sky will be darker on the mask. After making the mask visible (right-click on the layer dock and check "Show layer mask"), do a "Levels" adjustment of it, pulling the middle slider right to enhance the sky-foreground contrast. Here, I've taken the fair use liberty of using the Milky Way image from that Luminar demo (linked in Alen K's reply post) as an instructive example:

With the contrast of the mask thus amplified, the next step is to apply a "Threshold" adjustment to it, pulling the black slider to the right. there will almost always be a point where the adjustment will produce a distinct, detailed black-white transition between foreground and sky.

As you can see, there are some areas in the sky that haven't yet turned black. If I pulled the slider a bit farther right those would also be black, but in this instance, it also blackened more of the foreground than I wanted. There are also some small parts of the foreground that are black, but after clicking the "OK" button to make the layer mask look like this, I just used the paintbrush tool to make the sky parts completely black, and the foreground completely white, while preserving the detailed transition between the two.

While using that MW image on top, I had a cobbled-together moon and sky image on the background layer, so the two layers together look like this:

Yes, I know, the moon looks, strangely oriented, but you get the point, I hope. This works better on larger images, in which details are not as pixelated as the trees on the left were. As an impulsive exercise of personal taste, I've also eliminated the messy-looking branches that were extending into the sky on the left. That's just done by painting black on that part of the layer mask.

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