The Difference Blending Mode Helps You Manually Align Layers
Editor's Note: This is another great tip by our regular contributor, Scott Davenport. You sould know that he'll be running a workshop in San Diego in September — be sure to watch this short video about it!
I'm a landscape photographer, and 95% of the time am shooting on a tripod. I also often take several photos of a scene, with the plan to blend them together in post processing to craft a perfect scene. On a recent visit to Santa Cruz, I photographed the Shark Fin Cove. I noticed a beach goer dressed in red shorts in a great position and grabbed a frame. A few moments later, I took another photo when the ocean waves were more interesting.
I want to blend these photos together so I have the beach goer in red alone on the rocks with that gorgeous sweep of water on the beach. Perfectly aligned layers are key to make the masking and blending job simple.
When I loaded the photos into Lightroom, I noticed slight shifts in the rock positions between the two photos. As sturdy as my tripod is, there are times when I get variation between photos. Often it is due to field conditions like soft sand or high winds – and the winds were stiff atop the cliffs above this cove. Occasionally, I am completely in touch with my inner oaf and bump my tripod. Hey… it happens to the best of us, right?
The Difference Blending Mode
Let's quickly talk about the 'Difference' blending mode. Understanding what it does will make the next section much clearer. The 'Difference' blending mode tells your layering software to display what is different between two layers. Here, I'm using ON1 Photo for the layers. The same principles apply other layering software like Photoshop, Affinity Pro, etc.
Take any layer and duplicate it, so you have two layers that are exactly the same.
Next, select the top layer and change the blending mode to Difference. The result is a completely black photo.
What happened? There is zero difference between the two layers, so the Difference blending mode has no deltas to display.
Now, take my photos from Shark Fin Cove and apply the Difference blend mode to the top layer:
There's loads of differences. I expect differences in the surf, where people are moving through the scene, and also the rock faces to a degree (lighting is always changing). However, the edges of the cliffs should be close to, if not entirely, black when the layers are aligned.
Manually Aligning The Layers
I do most of my stylization work in ON1 Photo. I really enjoy using this software package, however a long-standing gripe I have with ON1 is that it does not offer automatic layer alignment. For targeted alignments, I can do them by hand and save a trip through another tool like Photoshop. Since I know my masking will be around the beach goer in the red shorts, I focus my attention there when aligning the layers.
Using ON1's Transform Tool, I use the arrow keys to nudge the top layer into place. The edge of the cliff is crisper and the spot where the person is red is standing is mostly dark (few differences).
For my situation, it's OK if other areas of the scene aren't perfectly aligned. The process took less than 60 seconds. If you are curious, here's where I applied my mask to display only the person in red.
Can Automatic Alignment Do Better?
I was curious if Photoshop's Edit > Auto-Align Layers would do the alignment as good as or better than me. I ran the two layers through Photoshop's auto alignment and switched the top layer to the Difference blend mode. Here's the results:
It's very good. When I compare the manual alignment I did in ON1 Photo against the auto alignment in Photoshop, I think the auto alignment is better for the entire scene as a whole. However, examining the foreground area in the lower left that I really care about, I think the manual alignment is a touch better.
The Difference blend mode is a good tool to have in your toolbox. In my case, using it saved me a special trip into Photoshop just to align layers (well… other than launching Photoshop after the fact to satisfy my curiosity). In some cases, you may be able to do a better alignment job than software, especially when you need precise alignment in a targeted portion of your photo and not the whole scene. The 'Difference' blend mode gives you the power to align layers by hand with great precision.
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