Wednesday, December 1, 2010

We Want Tech: When You Can’t Stitch

This wall map was reproduced by stitching together 3 photosHow can Microsoft Research’s Image Composite Editor successfully stitch together 15 photographs of a large wall mural but fail on a 9 photograph map? (See last week's article, "We Want Tech: Stitching Folio Size Documents.")

I decided to try a map of my own.

To make the test as realistic as possible, I flew to the opposite side of the country, drove several hours to a small town in New England and took pictures of a wall-mounted map tucked away behind the coffee machine in the town hall (several rooms attached to the school house). I took three overlapping photographs of the map, drove back to the airport, flew home, and fed the photographs into Image Composite Editor.

It successfully stitched them, but distorted the map something fierce. (See the image to the right.) While passable for general purposes, it might be difficult to use for plotting metes and bounds. Click on the image to magnify it a bit. The full size image is 29 megapixels (4950 x 5879).

The Family History Library wall mural may have been much bigger, but it may have been easier to stitch because it is all orthogonal lines and boxes.

Back to Rencher’s Map

If you can’t stitch your map photographs, are there alternatives? You bet. Microsoft Photosynth is a photograph viewer that makes it easy to move from photograph to photograph based on their overlap. It’s a nice alternative to stitching. You zoom into any image as close as you need. You pan around the image. When you reach an edge, a ghost of the next image appears. Click on the ghost to view it—without distortion.

Below is an example Photosynth of Rencher’s map. I zoomed out far enough to see the ghosts surrounding the central photograph. This view reveals a problem that may have caused the failure in Image Composite Editor.

Photosynth of David Rencher's map of Drogheda, Ireland

Notice the photograph that I colored yellow. It was shot closer to the map (or with greater zoom). It doesn’t overlap any other photo besides the one below it. It leaves a hole in the map. Further complicating things, I was working with low resolution copies of Rencher’s photographs. But even with these problems, Photosynth successfully positioned all nine images. Click the above image to see Rencher’s map of Drogheda, Ireland.

Click this image to see a Photosynth of a map of Halifax, VermontI redid my test map in Photosynth. Click on the image to the right to see the result. Compare it with the stitched result from Image Composite Editor at the top of this article. Do you prefer working with a deformed stitched image? Or with a non-stitched, non-deformed Photosynth?

Photography Tips

As we’ve seen, success depends in part on how carefully you take the photographs. Computers are stupid and easily confused. Try to keep things simple:

  • Don’t change camera zoom between images.
  • Keep the camera level.
  • Light the document well and uniformly.
  • Overlap adjacent images by an ample amount, perhaps as much as 25%.
  • Use a tripod.

Stay tuned for more of David Rencher’s technology wish list.

3 comments:

  1. Maybe I am misunderstanding what you see as a problem here, but ...
    Both versions have an inverted keystone shape, suggesting that the lower part of the map was at camera level and you tilted the camera up to get the upper part. Tilting or panning works if you are stitching a landscape, but to get a square document to come out square in the stitching you have to move the camera to keep it pointing directly at the center of the part of the document you are currently photographing.

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  2. Hello,
    Stitching photos has been around for a long time and is hugely represented by many sophisticated software programs and even camera manufacturer's programs like Canon's software. There are some extensive sites online discussing the panoramic photo process. It looks like I will have to write something about this.

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  3. An excellent FREE alternative is Hugin. Obtain from:
    http://hugin.sourceforge.net/

    ReplyDelete