The sky. IT'S FAKE.

9 min read
SimplyMikey's avatar
By SimplyMikey
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I have hundreds, maybe thousands of photos I have not yet found anything creative to do with them, and I'm constantly searching for new ways to improve on my crappy skills.
When I came across one of my photos, So Still Pt. 2, I had thought to myself "oh. why couldn't you get some stars in it??"
And then it occurred to me, "Lets fake it!" Of course this isn't much of photography, but more of photo-manipulation. As I said to a fellow, more strict photographer - "Art is anything." And set about making star trails from all my failed moving-stars-photoshoots.

This will be a how-i-did-it and what-i-learned-on-the-way, since I think i made a first-of kind of thing in GIMP, the open source photo-editing software.
So Still Pt. 2 by SimplyMikey

The Method

1. Open a photo of some stationary stars (Some rotation will be acceptable) or go all the way and fake that as well.
2. Duplicate the original and rotate the duplicate, around a certain point, with very little degree of rotation, 0.01° for instance.
3. Change the blending mode to "Lighten only".
4. Merge down and repeat steps 2 and 3, Rotate 10 time by 0.01°, then a few more time (to taste) with 0.1°
5. Add some perspective.

In practice

In Photoshop, it should be fairly easy to do this. Once you apply the method once, you can create an action for it. So thats not that special to write a how-to for that, there's plenty on the web. Being a Linux user, i use GIMP, which unfortunately doesn't have automated actions, and maybe I haven't searched enough, but I couldn't find any such plug-in. So i learned basic python and basic gimp functions and wrote one, and then another one!

They are both of a class I called Startrails, one being "Startrails - rotate" and the other being "Startrails - generate". They are both in testing, "alpha" if you like, and presented here with a Share alike-noncommercial-mod as you wish just keep the credit license, and with NO WARRANTY. They are both pretty resource hungry so always backup first.

Startrails - rotate

It does steps 2,3, and 4 of the method. All you have to do is open an image with some stars in it and specify a rotation center.
Activating the plug-in pops up a dialog:
Snapshot75 by SimplyMikey
Now, if you just click ok, without specifying any Vector, the layer would rotate around its center. What is a vector? Well it's used in paths to give a point and a direction. We're using it simply as a point. Click the "Path Tool" ("B") and then place a point in the image - make that one of the stars - without dragging or anything. Select the vector in the pop up and the layer would rotate around that point! As I said, you start with 0.01°, run it 10 times and this will give you a layer that you then rotate by 0.1°. The "Layer" drop down menu is quite buggy, at least in linux - make sure you choose the uppermost layer every time you run the code, both in the menu and by selecting the layer in the layers panel.
The process:

Journal trails by SimplyMikey

Finally, there's blending the stars into the image. There are several ways in doing this, you can mask out the sky of the original image and place the stars on the bottom layer for instance. One other way I've grown to love is changing the stars blending method to "Lighten Only", placing the stars on top of all other layers and then masking out the stars locally in case anything should obstruct the stars in the original photo - in So Still Pt. 2, The stars are masked out so that the clouds will be on top.

Startrails - generate

I don't have many photos of sharp, perfectly still stars and when testing the rotate plug-in I had the idea of generating stars with paint. This is what the generate code is for.
The method is fairly simply, and may change in the future for added realism. When examining photos of stars I noticed two kinds of stars - big, bright, blueish stars and smaller slightly less bright and slightly less blue stars. The plug-in generates an equal amount of the two.
For the bigger stars first we paint a white core, then a bright blue circle around it, then a dark blue circle around them.
For the smaller stars we paint the core and then a pale blue circle around it.
Then we blur the whole thing.

Stars by SimplyMikey
This isn't incredibly realistic, but after rotating and making the trails it will get the job done.
The plug-in outputs a user-specified amount of stars in a random pattern scattered all over the image width and height. The number of stars is actually double of what the user inputs, making N amount of big stars and N amount of small stars.

Snapshot84 by SimplyMikey
As you can see, you can control nearly every aspect of the output image, including density (N stars), the big stars radius, small stars radius, the background colors and the amount of Gaussian blur to apply. The black background is essentially the medium between the stars, and is necessary for making star trails. without it, the stars won't progress in a circle.
Fiddling with the setting is almost mandatory.


If you think about it, you have just rotated a photo around one of its point, on a plane.
Something like that:
Globe1 by SimplyMikey
Wildly not to scale

Well, you can immediately see this is not natural. They Sky isn't flat and the stars actually go over your head. You can think about the earth as being a stationary sphere, and the stars being a bigger sphere rotating around it.
Globe2 by SimplyMikey
Red sphere marks the north star

The bigger sphere is the Celestial sphere, on which the star image appears. The black line marks the Celestial equator. Standing on the globe makes the perspective issue clear:
Globe4 by SimplyMikey
The black line going up is the Celestial Equator, and the black line going down is the Earths.
The person in the illustration is standing near the equator, looking in the North-West direction.
Excuse my Blender skills.

Stars which are close to the axis of rotation, marked by the red dot, will appear to rotate with a relatively small radius, while stars near the celestial equator will appear to move in a straight line, right over your head. You can see this in effect:

Entering The Half Pipe by CapturingTheNight Dunbar Trails 1 Hour by gdphotography House Of Po by aFeinPhoto-com
You need to ask yourself, how much of the sky am I capturing? At 18mm focal length without crop, you capture about 90°, so, as in the illustration above, you need to make the layer distort in a way that one end of the trails are straight, and at the other end there's the north star. At 50mm you capture about 40° of the sky, so about half of the perspective affect should apply.
Luckily, there are some filters you can use to automatically simulate this effect. In GIMP, you have a filter called "Lens Distortion" (under filters->distorts->lens distortion), and in   Photoshop I believe that "Lens Correction" will get you there. Basically, you change the focal point of the distortion (X shift, Y shift) to be the north star and test different amounts of central (main) distortion. Remember not to over-do it (I'm still not sure if I over-did it with So Still Pt. 2). If the rotation center is near the center of the image I imagine it would be on a rare occasion that you should see the celestial equator.

My first journal post is done! Corrections, ideas, suggestions, questions, all very much welcome. Remember to be constructive.

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