Stitching Scans
Our example piece is a Portrait Adoption piece customized for and adopted by the lovely Carrianna! Originally published in EMG-Zine.
2006-11-18
Oh, the joy that is working on a large canvas! So much room for juicy details, so much space for expression. And so much frustration trying to find a scanner to fit it onto when you're done.

Alternatives

There are three alternatives to stitching together a large piece in multiple scans: Buy a larger scanner, have the piece professionally scanned elsewhere, or photograph it.

Photographing a piece of artwork is the subject of an entire article, but it definitely requires more than just a point-and-shoot camera and your living room floor. A piece of artwork has to be thoroughly and evenly lit, and be completely perpendicular to your camera. Best results come from being fairly far off and using a good zoom lens. Most camera-shot images look dirty, are off-color, and are warped by the photography process. We'll take a closer look at this in a future article, but the short story is: You aren't going to get good print files from your camera without a lot of time and setup.

Speaking of setup, a good option is always to have a scanner of the size that you work and just eliminate the whole stitching process. Good luck with that if you like to work really large! An A3 (approximately 11 x 17 inches) scanner will generally run up about $1000 for a good model. Mustek makes one that is vastly more affordable, about $200 new, but I've read mixed reviews on the quality of scan - more good than bad, but not a consistent enough positive to start peddling them myself. Before you buy, try to see if you can't get a salesman at your local OfficeMart to let you try it out and see if the color quality suits you.

And of course, if you just don't want to deal with scanning things yourself, you can take your artwork to a local copyshop and have them burn the file onto a CD. The charge for this varies per shop, but runs between $7.50 per scan and $20. Some places will even stitch them for you if they haven't got a large enough scanner - for a fee.

Scanning Tips

Having, for whatever reasons, discarded any of those alternatives, let's go back to scanning the old fashioned way - in parts.

First, it's got to be large enough to print! A 72 dpi scan will allow you to make only teensy little prints that you'll need a magnifying glass to see. Always scan for printing at 300 dpi or more. There is no need to go to the 2400 enhanced scanning that some machines boast - 300 dpi is perfectly fine and provides lots of information without requiring enormous files.

Second, make sure you scan into a .tif file, or directly into your art program. JPG and BMP and GIF files all have loss associated with them, and many printers and art directors will groan and wail when they receive files in this format. Some of the formats are so nasty that even saving them into a better file format doesn't do any good - the damage has already been done.

Make sure that you press the piece down flat onto the glass. Sometimes scanners have a lip around the glass, and, particularly if your work is too big for the scanner, it ends up sitting a little off the glass. This is bad and causes a shadow - called scanner burn - to develop over your scanned file. If possible, keep the artwork pressed flat against the glass. You might even have to lean on the scanner lid or on the artwork a bit while the actual scanning takes place.


The most important rule of scanning in parts is to overlap. The edges of a scan will nearly always be darker and more prone to scanner burn than any other area. Overlap means we can crop out these undesirable sections and break up any straight lines. Don't try to 'save time' and scan each piece that you will be stitching together right to the place you will be stitching - overlap is our friend. In my example piece here, I have about 6 inches of overlap. This isn't overkill!



Make sure you're scanning straight. Putting a piece of art crooked on the glass means a whole new series of corrections are needed - it's quite simple to use the straight edge of your scanner to make certain that your piece is straight and save yourself fixing time later.

There are a lot of little quirks and fidgety details about scanners that are individual to each machine. You'll need to experiment to find out if your scans should be a touch darker than the default, or if you need to make any other adjustments when you scan. It is useful when scanning in parts to use the advanced options that usually come
with scanner software, and make sure that your settings (darkness, contrast, etc) are identical for each part of the scan.

Stitching

Though commonly called 'stitching,' we won't be using needle and thread to put these parts together. My example will be in Photoshop, but you will be able to do most all of these commands in any high-end image manipulation software.

The first step is to create your canvas. I've scanned this 11 x 14 inch mixed media piece in two parts. Note the generous overlap! My first step will be to rotate both pieces into their final orientation (image > rotate canvas > whichever direction I need to go). Then I pick one of them (in this case, the bottom half) and go to image > canvas size. Don't squeeze yourself too much! If you know your piece is 14 inches, make your canvas 15 inches - we'll crop out any extra later.




Now, copy and paste the other part of your scan into the enlarged canvas. Ew, gross, look at that ugly dark streak across the middle! No worries, we'll get rid of that first thing!



Make sure that the file with the ugliest bit goes on top, and select that band. But don't just delete it! First, let's feather our selection. 'Feathering' a selection is like blurring the edges - as we go further out from our selection, we're picking fewer and fewer pixels. A feather of 25 is probably good, don't go more than 50 pixels for these purposes. Then we can delete.



Woops, now it becomes clear if our two halves weren't perfectly lined up before! Fortunately, that's easy to fix. Because we feathered before we deleted, the edge of the top layer is semi-transparent. We can drag that layer around until we get everything lined up exactly the way we want. If you want, you can even temporarily adjust the transparency of the whole layer. That's the little 'opacity' bar on the layers toolbar. Pulling it down makes the layer more transparent, and you can line things up a little more easily. If you're thinking ahead, you can even make a small mark on your original - in the margins where it will get cropped out! - that you can use to line things up at this stage. I hardly ever remember to think that far ahead, and end up lining up to some of the details in the piece. If you find at this stage that you line up at one side of the scan, and the other side of the scan isn't right, you may have scanned one of the pieces crooked!



Select the entire layer, and hit ctrl-T. This activates the transform command. See the little target in the center of the selection? This is the pivot point for rotations. You can grab that (click and hold), and take it down to where you've got a corner carefully lined up. Now, move your cursor to the other corner, where it isn't lining up, and as you move off the corner of the selected area, you'll get a little curvy arrows symbol. Click and drag to rotate your layer until that corner lines up, too.



Now, chances are good that the color is not exactly, perfectly the same from each half. Gross differences in color can be changed by adjusting color and hue (image > adjust > hue and saturation), but some changes may not be really apparent to your eye, and you're still noticing where that stitch line is! Part of this has to do with the effect of a straight line. In something organic, like art, a straight line will pull our attention and distract us from what we're looking at. The solution? Break up the straight line. Pick up a soft-edged (fuzzy) eraser, and start mimicking the direction of your artwork. Any differences in color will fade right away. This is where overlap is particularly important, since we're actually erasing off the top layer! Don't get too crazy with this, or you'll start running into the scanner burnt areas of your lower layer. Stick close to the area where the join was made.



The final step is to crop out all those messy edges!


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