Last Updated: 2018-Jun-04
One of the lovely things about stacking star trails (or stacking in general, for that matter), is you are presented with many opportunities for a choice of foreground. Did someone walk through your shot wearing a flashing nametag? No problem. Did a passing car blow out that wonderful rock in your foreground… no worries. Did you play around with different lighting and find none of them quite met your desires. No cares.
Invariably at workshops and shooting events something will go wrong with hours long shots. But with a plethora of shots to select a foreground from the odds are greatly improved that you can get what you wanted even if it means working around problems created by uncontrollable elements.
Consider my effort atop Mauna Kea. All the shots are here in this video.
But as you may have noticed, the radio dish was moving nearly constantly and is thus blurred in many shots. When I stacked all the images together in the normal fashion, this is the result:
Unless you’re into that Dali-esque melting radio telescope vibe you may not want that result. Or perhaps some dunderhead walked through the scene with a flashlight making a wicked blow out – or any number of possible complications. What to do?
Answer: Find the image or images with a more desirable foreground and fix it! Fortunately I have LOTS of frames from my 7 hour-long star trail. When I created the image originally I chose this foreground:
Resulting in this image which was selected for the shortlist of the Astronomy Photographer of the Year, 2011.
But, in putting together this article I noticed a foreground I think will work even better. This one:
Here are the steps we’re going to take – you can download the images using the links below and try for yourself if you wish – of course you may not post your derivative work or claim it as your own.
- Start Photoshop
- Load the foreground and background (stacked) image.
- Plop the foreground image onto the stacked image.
- Set the foreground opacity to 40% to see what you are working with
- Use the quick select tool to capture the foreground elements you wish to remain.
- Use “Layer -> Layer Mask -> Reveal Selection”
- Adjust the foreground opacity to 100%
- Flatten and save!
Step By Step
Easiest for me is to drag and drop my images into Photoshop. Choose your own method if you’d rather but beware that depending on your approach, Photoshop may decide to do your layers as “smart objects” which creates more constraints and steps.
Note that you might decide to load more than one foreground so you can mix and match to choose what result you like best!
Once you’ve got your two (or more) images loaded select the foreground image, then Ctl-A (Select All), Ctl-C (Copy) then select the background image and do Ctl-V (Paste). If you have more than one foreground, repeat the process of overlaying the foregrounds onto the background stack.
When done this way with equally sized images, the layers will be exactly over one another. You can accomplish the same layering goal by using the “move tool” and the process described by Harold Davis as “Plopping”. Or if you were really on the ball, you could load the images into Photoshop as Layers. For me that method “Files -> Scripts -> Load Files into Stack -> File / Folder Selection tool” is unnecessarily complicated and forces me to use the more impoverished Photoshop file selection tool. Plus I can almost never remember to look under “File -> Scripts” (I always expect it under “File -> Open” or File -> Automate. Even File -> Import would make more sense!)
We will also adjust the opacity of the foreground to around 45% by selecting it in the Layer tool (Window -> Layer)
After adjusting the opacity, we can see clearly how the layers align and make a determination about which parts of which photos we want in the final image.
A quick look indicates that we can replace everything below the mountains with our foreground making the task trivially simple. We will use the quick selection tool (look under “Magic Wand” if you don’t see it). Select the foreground, then click the selection tool somewhere at the edge and below the sky. Drag across the frame to the other edge and if you’re lucky, the selection will be like this:
After completing the Reveal Selection we adjust the foreground to 100%
For fun and excitement we can clean up the final concoction using various adjustments. Here I’ve used Curves and Hue/Saturation adjustments to reduce the green cast and crisp the image up just a bit.
Now we flatten and save and post and brag!
What if Quick Select Doesn’t Work?
If your selection area is more complicated there are other alternatives.
One alternative is to use a gradient mask. But that may not work either such as when a tree reaches up into the sky with lots of tiny branches. Another approach is to do hand masking/layering. The following provides a rough idea of both how to approach the hand masking problem and how much more difficult the process can become.
Here I used a “Hide All” mask on the foreground. Next I painted in white on the foreground mask (notice that the mask is selected in the layer window). Wherever I paint white in the mask allows the foreground to “replace” the background. In this case it’s messy because the foreground is clearly darker than the background. However this technique can work very well for small improvements. For example did you notice that there is a red streak of car tail lights in the middle mountain above? It would be simple to replace that small area with one (or more) frames from the rest of the stack using this hand masking technique.
If you want to get the full scale, nitty gritty detailed write up of this hand blending technique, I heartily suggest you purchase Harold Davis‘s “The Photoshop Darkroom” book. It will be money well spent!