Tuesday, December 9, 2014

Merging Images in Photoshop, Part One

One of the most common problems in digitization is how to deal with an image that is too big for your camera or scanner. The simplest solution is to photograph or scan the object in separate pieces, then merge those pieces together, however this can present its own set of problems to those unfamiliar with imaging software.

In this post I will be describing my own method for merging together images. There are many other ways to accomplish these tasks, and if you have a way that works for you, I encourage you to keep using it, but also be aware of its potential pitfalls. The main benefits of my own method are the ability to quality-check your work as you go, and make simple non-destructive edits that can be changed or reversed as needed. Also, for simplicity’s sake, I will be referring to my own Mac OS based workflow for menus and keyboard shortcuts.

Here is the whole image that we’re trying to assemble, and for whatever reason, it’s been captured in two side-by-side pieces in the standard .tiff format. It is crucially important, when capturing, to make sure there is overlap between the captures. This is going to help us check how well-aligned our merging is, so the more overlap the better.

Notice how each side is wider than half of the image

Now that we’ve got our two images, open both in Adobe Photoshop and choose whichever one you want to start working on. I usually go from left to right for simplicity’s sake, so here I will be starting on the left side of the image.

In Photoshop, select the Image drop-down file menu, and select “Canvas Size…” (or use the keyboard shortcut: option+command+C). Click on the canvas width field, and double it. In the “Anchor” field, select the leftmost column of the grid so that Photoshop knows where to put the empty space.

You should be left with an image like this:

It will end up a little wider than is necessary, but it’ll be easier to trim it down after the fact than to add more space. This will now become our “master” file. Do a “Save As” at this point and designate it as such.

Next, go to the second image that we are going to merge into the master (in this case, the right side image). The next step should be familiar to most computer users: select all of the image (command+A), and copy it to the clipboard (command+C). Then go back to the master file and use paste (command+V) to add it into the image.

If you’re paying attention, you’ll obviously notice that this new image is not in the correct position. However, by looking at the Layers panel on the right side of Photoshop you’ll see that the new image is on its own layer, resting on top of the background (if you do not see the Layers panel, select the “Window” drop-down menu and enable “Layers” there). Thus we can edit it without disturbing the original “bottom” layer.

Now, with the top layer selected, click on the “Opacity” field in the Layers panel and set it to 40%. This will make the top layer semi-transparent and allow us to line it up with the bottom layer.

Then, with the Move tool selected (V), begin moving the top layer around and trying to find where it lines up. Look for any solid shapes that are shared by both images, or where the borders intersect. Letterforms provide nice clear and easily-spottable shapes, which is why I have used them in this example, but it can be anything so long as it’s shared by both images.

We’re getting there, but it’s obviously still not right. At this point, find an area of overlap and zoom in closely. Then, with both the top layer and the move tool selected, simply “nudge” the top layer into place using the arrow keys. The arrow keys will only move the layer one pixel at a time, so obviously this is for the finest level of adjustments.


Nailed it!

Now for the final steps! In the layers panel, set the top layer’s opacity back to 100%. Then inspect the images along the borders, making sure that it looks seamless. While checking for quality be sure to zoom in and out.

At this point you can crop the image down to its original size, and it will be ready to go. However, one important piece to remember is that layered .tiffs, in addition to simply being larger files, are also not commonly supported by web or other software. What I like to do at this point is to save the “Master” file with both layers, and then create a new version for common use. The common use version will get flattened (Layer -> Flatten Image) then do a Save As in whatever format is required such as .jpeg or .pdf. This way, if any changes need to be made, we can always go back to the Master version.

And there you have it! A nice, seamless image. In the next post in this series, I will go into more detail for dealing with other problems, such as skew and mismatched backgrounds or details.

Written by Ryland Ianelli

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