Evaluating Alignment


Today’s Question: This may be a bit of a stretch, but… The subject of blend modes reminded me of a technique that I think involved a blend mode [in Photoshop] to determine whether the frames of a panorama are aligned well. I know there are automatic options for assembling composite panoramas, but I like to put them together myself. Can you remind me of the blend mode option, if I’m even remembering these few details correctly?

Tim’s Quick Answer: It is indeed a blend mode that provides the feature you’re looking for in Photoshop, where you’re able to see the degree to which the pixels in two overlapping layers actually match. That blend mode is the Difference blend mode.

More Detail: To use the Difference blend mode for evaluating the alignment of two image layers, simply change the blend mode for the upper image layer on the Layers panel. So, you can click the thumbnail for the upper of the two layers you want to compare, to make sure that layer is active. Then click the popup at the top left of the Layers panel (the popup has the default value of “Normal”) and choose Difference.

With the Difference blend mode, any pixels that are an exact match on the upper and lower layer will appear as black. Any pixels that aren’t an exact match will appear as a color that differs from black to the degree there is a “mismatch” between the pixels.

In other words, when using the Difference blend mode for this purpose, your goal is to have as much black in the image as possible. You can move the image layers, resize the image layers, transform the image layers, and more, in order to achieve the best alignment possible.

While I do find that various software tools (including Photoshop) do a great job of automatically assembling and aligning the individual frames for a composite panorama, I also find the Difference blend mode to be tremendously valuable when I choose to take a “manual” approach for any reason.