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ASSEMBLING MULTIPLE SCANS OF ONE IMAGE...
The Scanning Process.
It is often the case that when scanning images we need to make two or more scans and assemble them into one coherent image for printing or publishing on the web. There are some simple methods to make the assembly process easier...
First, look at the image in relationship to the scanner bed size. How many scans will it take to complete the image? If you're lucky, it's only two, but if you don't have a digital camera that's up to the task it can be intimidating to try scanning something as large as a poster. For example, the Space Jam poster below was assembled with more than twenty scans. It took a long time, but the result was a quality image. (Click the thumbnail below to see the result.)
We wouldn't recommend you start out with assembling something so involved, but you can apply what's here to any number of scans.
Once you've figured how many scans you'll need to make, you need to think about the area of the overall image that's most important to get right. Choose that area for your first pass, and take as much time as you need within your scanning software to make all of your color and tonal adjustments as near as you can to the original. (For some rules of thumb about image prep, see Tutorial 2b.) Don't worry now about making adjustments you may have in mind to improve the image - that is best done after you've finished assembly.
Now note your settings: This is important! You are going to scan every successive pass using the same settings no matter how the preview looks. Use that first scan as an anchor for the remaining passes. Otherwise, you'll have to adjust every scan to every other scan manually - a nightmare! The only setting change you can safely make is rotation. Another thing to mention here is that you may need to remove the cover of your scanner to access the center area of larger images - if you have a lid that must be removed using tools you're on your own. We can't make that judgment for you, but if you decide to go ahead the risks are yours. Clean your scanner glass and dust the art thoroughly.
General tips to make your scans easier to assemble:
Show an edge wherever possible to aid in alignment. Get each scan as "level and plumb" as you can. Overlap each scan as much as possible without adding to the total; if you can't get an overlap of more than a bit, add a pass. Keep the work absolutely FLAT on the scan bed; if you must mount the image on board to do this, it's worth the effort.
Use a logical method to scan and name each pass, as if you're reading a book, from top left to bottom right the names should be in order. Think "rows and columns." This is really important if you're assembling more than four scans. (By the way, as long as your settings are based on the anchor area, you do not need to start with that area.) Save all scans in one folder as they're made.
(We're going to use a simple two scan assembly for this tutorial, but the process will work on any number of scans as long as you and your computer can handle the complications and precision needed to work with that many layers.)
Here are the two scans we'll be working with, courtesy of Terry Jasinski:
Preparing for assembly.
First, open each scan file one at a time and perform a rotation to correct any "out of plumb" scans. It seldom happens, as shown in the shot below, that the scan angle is exactly on zero degrees. Don't let it worry you if it's off a little - it's easy to correct in Photoshop.
Choose one of the long border lines (a long straight edge anywhere will work when it's all you have, but it's a bit more complicated). The edge you choose should be seen across as much of the overall image as possible. First, zoom in to a size where you can see the chosen line clearly on screen.
Choose the measure tool...
...and click on the chosen line (the top or bottom, but precisely on the line); drag all the way to the other end of the line; center your cursor in the same precise position relative to the line as the first click, and release. If you look at the top right of the ruler options palette, you'll see that an angle has been measured.
Now, depending on whether you're working with a border edge or an "interior" line, you'll do one of two things:
If it's a border edge, simple go to Image>Rotate Canvas>Arbitrary and you'll see a dialog box with the necessary correction already entered for you in the window... how easy can it be? Just click OK, and your scan is now "plumb and level."
Here's what a rotated image looks like:
If you're working with an interior line, it gets a little more complicated in that you'll need to either:
Let the auto correction work and assemble your layers at a consistent offset angle
- or -
Do the math to adjust all images to "plumb."
What' our recommendation? Use the auto correct when possible; it's complicated enough when you're working with many layers. Just remember that you are attempting to rotate ALL scans to the SAME angle, so take some time to plan ahead. You can, and probably will, need to rotate a layer individually later on, but get them as accurate as possible now. Perform this operation on every individual scan and save your work. Now we have two or more individual partial image files.
What's next? Some simple addition. Open all of the scans along one side of the image; note their height and add them all together to get a total height for the new file you're about to create. Do the same for the image files along the top or bottom edge. This gives you a total of dimensions including overlaps and borders, so it may seem oversized. For now, use these dimensions anyway - they leave plenty of room for adjustment. Create a new file: Cmd-N. Enter your totaled measurements for width and height; enter the same resolution and mode as that of your scans; click the "Transparent" content button, then "OK."
Beginning with your first scan, open each of the individual scan files, select the area inside of shadows, lines or distortions, copy your selection (Cmd-C). Click anywhere in your new document window to activate it and paste (Cmd-V). Photoshop automatically creates a new layer for your pasted selection.
Choose the "move" tool...
...and move your pasted section loosely into place. Perform this operation on every successive image until all of them are pasted into the new document. Save as a layered Photoshop document - DO NOT FLATTEN.
Next, begin a careful alignment of each layer. Zooming to 200 or even 300 percent can be very helpful here. It also helps to show only the layer you're moving and the layer you're aligning to. To do this click the "eye" icon on the left side of the layer palette to turn visibility on or off. (You can option-click on the icon to hide or show all other layers.) Now you can see why naming the layers in order is important.
As you align the layers, you can make adjustments to rotation, etc., but if you have ample overlap don't worry too much if edges appear to be out of whack a little. Once all the layers are placed as accurately as possible, crop the overall image more closely and save your work.
Here is our assembled image:
If we zoom in on our image, we see that the color doesn't match on both sides of the overlap...
(This is very typical of the problems with multiple scans. Scanners tend to be inconsistent, with a sweet spot and areas of subtle distortion. The better the scanner, the less this occurs. Usually, this is not inconvenient or even noticeable. With assemblies, we have to work with what we get. We can't cover every technique you may need to implement to achieve a great result when the assembly is complex, but in most cases, simple processes will do the trick - especially if the scans were carefully made in the first place.) To correct this, we activate the bottom layer and adjust using the Image>Adjust>Brightness/Contrast dialog...
But that leaves the other side too dark you say? Yes it does, but now we invoke the mighty history brush (a large, soft version) and go over the "too dark" areas to bring them back in line. You may need to set the source of the history brush to get back to the proper place...
Now it's looking pretty good...
The next step is to eliminate areas that just don't want to cooperate.
This time, it's the eraser tool. Choose a large, soft airbrush style for the eraser and work back the overlapping edges. This process can almost magically correct alignment problems! Use a harder brush to remove areas more precisely. If you don't like what you're getting, go to the history palette, select a few steps back and click on the trash icon to throw those steps away. If you accidentally throw away too many, use Cmd-Z to undo and then throw away fewer steps.
Here's what our bottom layer ended up looking like...
After all the hard work, you now have a good single image.
Congratulations are in order, but don't stop there. Flatten your image and save your work. Now create an outer border, make color and tone adjustments, eliminate dust speckles... whatever you can do to bring the image as close to the original as possible.
Here's our final image; a larger version can be seen here.
Finally, it may be the case that no matter what you try, the images just don't line up, or a mistake gets saved (easily done with many layers). The only thing you can do is re-scan the areas that are troublesome, and lay them on top of what you already have. In the case of the poster assembly mentioned early in this tutorial, the area that Taz was in would line up with any two, but not three sides. He was re-scanned using the original settings and laid over the work already completed. By the time it became evident how much time such a large assembly was taking, we were past the point of no return. That experience prompted the use of a digital camera to shoot the other Space Jam posters.
Life's too short.
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