Issue 20    


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Interlude #2: Frankenpainting

Or How to Scan A Painting

The Problem

Most pieces of art are larger than the bed of a typical scanner. Shooting a photo of a piece involves either a nice camera and a lot of setup or a professional to shoot for you. Both of those options can be costly. If you have ever priced scanners big enough to scan a reasonably sized painting, you know how prohibitively expensive they are. What is an artist on a shoestring budget to do?

One solution is to scan the artwork in sections and stitch them together in a photo manipulation package, such as Photoshop, that can work with layers. This is the option that I employ and which I'm going to cover in this issue. It takes a little bit of work, but with practice it can be done fairly quickly and effectively. So roll up your sleeves, grab a painting, and let's get to work.

Step 1: The Scans

You want the initial scan to be high resolution. You won't necessarily work with the files at this resolution, but it is better to scale down than up (or, to put it another way, it is better to have too much information than "just enough"). In fact, you will get a cleaner, smoother final image if you scan in high resolution and scale the image down to the final resolution than you will by scanning directly at the final resolution. Try to scan at a minimum of 600 dpi, and don't be afraid to scan at 1200 if you want.

How many pieces needed for a good scan will depend on how large your piece is. An average scanner will scan areas of around 9 inches by 11.5 inches at a time (with slight variation from brand to brand). It is important... no, wait, it deserves more attention that that. It is IMPORTANT to make sure that you have at LEAST several inches of overlap between your chunks. You will see why in a few steps.

To the right is a diagram showing how I divided "The Blue Flame." It was an easy division - at 11 inches across, I could scan in the full width. I scan in the top, bottom and middle as three pieces of the puzzle. While some art will require more than one scan horizontally as well as vertically, the concept is the same.

Once you have the separate chunks scanned in, save each piece AS IS. Don't scale down, crop or stitch together any of the pieces. Don't adjust the color or contrast. Simply save each piece as its own file. Use names that are sensible and clear, such as (for example) "blueflame_scan_big1.tif." There would be a "...big2.tif", "...big3.tif" and on. The "big" lets me know that it is the high-resolution, raw scan. You could use "raw" or "highres" or "oompaloompa" as long it will make sense to you at a glance months later. The number and the size of the files will help remind you.

As you may notice, the files named above are TIFs. I recommend the TIF file format for this step because they are lossless and yet small. When you see the file sizes, you may not agree that they are small, but relative to what the files would be as PSDs or BMPs, believe me, they are small.

Step 2: Gathering the Pieces

You now need a blank file large enough to hold all of those pieces at once. At 600 or 1200 dpi, that makes for a HUGE file. DON'T PANIC. You can scale down the separate pieces first. In fact, I recommend it. Trying to work with the full sized images all at once would take lengths of time familiar to glaciers and mountains. Reducing the resolution by even half reduces the file sizes by about 75%.

I recommend scaling the resolution down to 300 or 450 dpi. Add an inch to the length and width dimensions of your original piece of artwork, then make a blank file that size. Scale down each of your piece files (DON'T SAVE THEM!). Copy each file and paste it into your blank work file. If you are working with Photoshop, this should automatically put each piece on a separate layer. Other programs may expect you to create a separate layer before pasting each piece.

Arrange each piece into roughly the right pattern (the top left piece in the top left, etc). Save this as a layered file such as a PSD. Close all of the individual raw scan files WITHOUT saving them. Remember, you scaled them down, and if you save them, you lose that extra data that might come in handy at some future point.

Step 3: Lining Up The Pieces

What you have now is one very large file full of pieces that don't match in color and in some cases don't line up. In short, you have an ugly picture.

Leave the color alone for now. The main issue at this point is getting the pieces of the puzzle to line up. The first problem you will find here is that scanners often smush or stretch the edges slightly. Solution: cut off about an inch of edge on each piece. You did scan in several inches of overlap, didn't you?

Pick a corner piece to start with. Move that piece into the corner, leaving only a half-inch or so between the edges of the artwork and the edges of the file space. If you have guidelines you can drag over to the edges of the artwork, do so. If not, eyeball it to the best of your ability. If the edges are not perfectly horizontal and vertical, rotate the entire layer until it is straightened out. On some computers (like the one I used for years) rotating can take a minute in a file this large. Be patient and, if the feature is available, type in a rotation instead of dragging the layer. This will allow you to rotate a layer (for example) 1.2 degrees, see whether it is too much or too little, then undo the rotate and redo it at 1.3 degrees, 1.0 degrees, or whatever is appropriate. Check, undo, and rotate until it is lined up perfectly. Don't simply rotate the layer one way or the other the .1 or so degrees you are off! Each time you rotate a layer, the software has to reinterpret the detail. Doing so several times will slowly fuzz and blur the image. It is much better to find the right rotation amount and then perform the transformation once. Remember this process - you will be doing it for each piece.

Now start working with two pieces at a time, one that is already set (we'll call it Layer A) and one right next to it that is not (Layer B). In other words, work away from the first corner as you go. Move Layer B above Layer A in the Layers menu, then change the opacity of Layer B to 50%. Now you should easily be able to line up the major features of the two layers. Zoom in and line up the layers down to the pixel.

If you are extremely lucky, Layer B will be at the same rotation as Layer A. More likely, they will be slightly off. This makes the alignment frustrating - one side will line up well and the other will be off by several pixels. Remember that rotation process? Start doing that with Layer B until the entire edge lines up with the previous, set Layer A.

Once all of the pieces are aligned as closely as possible, you may still have slight maladjustments between layers. Once again, DON'T PANIC. For one thing, if you have worked carefully enough you will notice the errors more than anyone else. For another thing, you still have one trick left up your digi-sleeve.

Remember that overlap you accounted for when you scanned? It's going to pay off again. Some areas hide misalignment better than others, and a moving, random edge is less noticeable than a straight one. Select the eraser tool, select a brush with a fuzzy edge, and create a wavy edge that targets those areas that hide problems better. The picture to the right shows the final edges in The Emerald Dagger. I have highlighted them a bright red.

Eventually you will have a picture that seamlessly lines up all the way across (and down). Congratulations, your Frankenpicture has now been stitched together. The hardest part is done.

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This document last updated Mon Oct 2 11:00:31 2006.