How to Scan Oversize Images with a Flatbed Scanner

Need to scan something that's larger than your scanner's window? Here's how to do it with any flatbed scanner and panorama stitching software.

Scanning standard Letter or A4 pieces of paper is pretty straightforward. Most consumer-grade flatbed scanners can handle that size. Some are a bit longer for Legal sized paper (8.5″x14″). While there are some larger flatbed scanners available, they tend to be pretty expensive or come with usability compromises and often aren’t really a practical option unless you do a lot of scanning of large images. You can find them occasionally available for rent, but that depends on your local area. And the self-service scanning services at places like FedEx-Kinko’s can add up quickly when large-format scans are priced per square foot.

Another option is to photograph the original, but that entails a setup where you line up squarely above the original as well as control the lighting to keep it even. So it’s not necessarily easy or practical.

But a lot of times you don’t really need to look for a completely different option. With a modification to the usual flatbed scanning workflow you can get excellent results.

You can use your regular letter- or A4-sized flatbed scanner to scan images that are larger than your scanner’s window. The trick is to scan it smaller sections and then use software to stitch those sections back together. It’s the same principle as creating a stitched panorama–you’re just creating a stitched mosaic instead. It takes multiple parts of an image and assembles them into a single whole.

Here’s how to do it with a regular flatbed scanner and PTGUI, among the most popular and powerful apps for creating panoramas. I’m using an Epson V600 scanner in this example, but it will work just as well with any flatbed scanner.

If you’re looking for a quick and easy option and are not concerned about getting high image quality output or are scanning text documents, I recommend taking a look at Scanner Pro for iPhone or iPad.

Step 1: Scan the Images

The first step is to create the scans that you’re going to work with.

In this example I’m going to scan the cover of this old Look magazine. The cover is about 10×13 inches, but my Epson V600 scanner’s scan window is only about 9×12 inches. I’m going to scan it in two parts, but you can use the same process to scan larger images in more parts–say, 4 or 6 parts.

If you’re scanning a large image that’s fragile or valuable, where laying it on top of a flatbed scanner might damage it, here’s an idea for hacking a Canon LiDE scanner that might be of interest.

When you make the scans, you want there to be some overlap between the images. Something around 30 percent overlap is a good target, but you can often get by with less. PTGUI will take the overlapping areas to map the corresponding sections between the images and see how they should align.

Every manufacturer’s scanning software is a bit different. You can also use Vuescan, which is compatible with a huge number of scanners.

In general, you want to turn off any automatic image enhancements like color restoration or auto-contrast. The reason is that you want each of the scanned images to use the same scan settings so that they blend more accurately when you stitch them together. If one scan is brighter or more saturated than another, it’s going to look strange when you stitch them together.

You can try using any dust and scratches corrections like Ice, but I find that for this type of scanning they often do more harm than good and tend to leave them off and fix any major issues in Photoshop afterwards.

These are the two images that will be stitched together as a mosaic.

Step 2: Stitch the Images

Once you’ve scanned each part, fire up PTGUI and load the images as you normally would.

PTGUI is designed to work with photos from a camera, and its algorithms specifically aim to compensate for lens distortion and perspective issues. The main trick in using PTGUI for a flat mosaic is to fool the software into not applying its sophisticated algorithms. A way to do that is to tell PTGUI to treat the images as though they were taken with a very long telephoto lens.

PTGUI will be confused when it tries to read the lens EXIF data from the scans and won’t be able to automatically find the fields for focal length, etc. So you’ll need to enter something here. You won’t be able to proceed until you enter some values.

In Lens type, uncheck Auto and select Rectilinear.

In Focal length, choose something very long. I use 1000. Basically, you want PTGUI to treat it as though it’s a flat image rather than a curved one.

In Crop Factor, choose 1. The imaging sensor size will then fill automatically–you can ignore them.

Hit the OK button to close the popup.

Choose Align Images. It will then begin processing, and you’ll get the Edit Panorama popup.

In most cases, I find that it does a pretty good job all by itself. This is the result I got. As you can see, there’s really not much I need to do in this case. I can straighten a little if I want, but the alignment and the control points have all worked perfectly in this case.

For this one, I would simply do minor straightening and then hit the Create Panorama tab to go to the settings to generate the final image.

There are many things you can tweak here if you’re inclined, but the ones to start with are setting the output size and filetype. If you hit the Set Optimum Size button, it will automatically update to the ideal maximum size for that image based on the images you’re stitching. You can also make it smaller. I generally prefer to output from PTUI at maximum and resize later. You can also set the file format and where the file will be saved.

Under advanced, try the default settings first. I find that they do a pretty good job.

Once you’re happy with the settings, hit the Create Panorama button.

Troubleshooting

If you’re not getting the results you want, here are some steps to work through.

If you get something that looks like this, it’s probably because you set the focal length far too small. This example uses a focal length of 10mm (rather than the 1000mm used above). As you can see, PTGUI is doing all sorts of algorithmic gymnastics to try to make sense of the distortion from what it thinks is a wide-angle lens.

Another thing to try is to go to the Optimizer tab and change the Minimize Lens Distortion option to Heavy. Then hit the Run Optimizer button at the bottom. You’ll then have to go back to the Project Assistant tab and his Align Images again.

If neither of those options give you the results you’re after, it could be because the images are not aligning correctly. It’s time to start messing with the control points. Basically, these tell PTGUI which part of one image corresponds on the other image. It’s where the images overlap.

When you click on one image, you’ll get a cursor on the other so you can choose the corresponding section on the other. It’s best to choose small details, like sharp edges, spots, or other details that are easy to match closely.

If you’re starting with good scans with a reasonable amount of overlap between the images, those steps should give you good results. There are some more advanced options for fixing, using masks, etc, but I don’t normally need to resort to those when stitching mosaic images like this.

Final Tweaks

Once the image is stitched you can open it in your usual image editing software like Photoshop, Gimp, or Paint.net to do any color fixes or retouching.

View Comments

    • I would expect so, yes. I've used it on maps and charts much larger than that. So long as there's some graphic detail that the algorithm can latch onto, it should work well.

  • David, your technique worked flawlessly. I cannot thank you enough.
    Here's a short version of it.
    1. load the images into PTgui
    2. use 1000mm for focal length
    3. lens type : rectilinear
    4. crop factor : 1
    That's it.. if you have problems come back and read the rest of David's techniques.
    best,

    Sanjay

  • Way too complicated for this human. I need to scan record album sleeves and labels. In your tutorial you scanned a flexible material that will mostly sit flush on the glass. Album sleeves are generally hard card stock which means they touch the glass on one side but angle up and past the opposite edge. The lighting and focus changes all across the surface. Placing a vinyl record on the standard sized scanner has the same problem. Either it's sitting on an angle or, if centered on the glass, is millimetres above it. This changes the focus and lighting, too. I need a 13 inch square platform. All this trouble for a lousy hobby. My search continues.

    • Depends on the scanner. Some scanners have one or more sides that allow the item to sit flush, something that's also useful for doing single pages of books. But that's not going to solve the need for stitching. It might also be worth checking with a local copy shop or something like Fedex Kinkos. A lot of the modern photocopiers are actually scanners, and many of them have large plates. They might also have a specialist large-plate scanner on hand.

  • Thanks very much for this! I have, perhaps, an unusual need for this capability. I purchased a scrollsaw pattern of a dog that is about "actual size". I didn't intend to scrollsaw it, I wanted to cut it out using a hobbyist CNC machine. But, first, I had to get the pattern into the CNC software that generates the cutting pattern (gCode).

    How to do this when the pattern is larger than my scanner? Your article described how!! I scanned the pattern in in about 5 parts, creating .jpgs as I went. I didn't pay any attention to orientation, just tried to be sure there was overlap. Then, I ran trial version of PTGUI using your recommended default values in the EXIF window. Amazingly, the splice was perfect. No need to define any control points manually. The panorama was "upside down" and at an "angle", but, that is also easily corrected later.

    A lot of info about painting and other machining info was also scanned in by default. But, this is easy to remove later using the CNC software. There are many scrollsaw patterns available (for sale) so your process will enable some of us to use the patterns without a scrollsaw if they have a CNC.

  • Personally, I prefer to use PhotoStitcher which is really super to stitch oversize images within a few seconds. The operation is so simple. I used it a lot which saved me much time.

  • Glad to see this. I just found out that FedEx/Kinko's will NOT SCAN large documents unless you can hand them WRITTEN PERMISSION from the copyright owner! Doesn't matter if it's an old newspaper that is now out of business, or a map of China distributed to tourists 20 years ago in Beijing. They must have been walloped in a class-action copyright infringement suit.

    • Interesting. It sounds similar to the issue that some pro photographers run into when trying to order prints at some of the big chain stores--sometimes the photo lab staff question that the customer actually owns the image copyright and didn't just download it off the net.

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