Wednesday, April 29, 2009

The Role of Seams in High Resolution Image Mosaics

When discussing true orthophoto generation I made reference to the image mosaicking process. I thought I would touch on that more today, with an emphasis on high resolution imagery. One of the main challenges in mosaic production is ensuring the mosaic is seamless. That is, one cannot easily discern where the edges of the input images are. This can be challenging for a number of reasons. One of the most difficult aspects involves the input image geometry. Because the input images have different perspective centers, the geometry of surface objects will vary between images. For example, a tall building in the center of a frame image may not exhibit any building lean, but the same building in the next image will show noticeable lean. So the big challenge in the mosaic process is ensuring the seams between input images conceal any mismatches. While seams between the images can be automatically generated, a quality control check must be performed to ensure there are no issues. Seams usually need to be manually edited if there are any problems.

Here's an example: the image below shows a multi-story building with a seam cutting right through it. Because the angle of building lean is different in each images, it looks like a mess.

In this example, the seam has been edited so that it shifts north of the building. This is ok, but it also reveals that there is a lot of building lean in the image portions chosen for inclusion in the mosaic.

This example shows the same building with the seam edited the other way. Instead of diverting north of the building, I've moved it to the other side. You can see that this provides a better top-down perspective on the building, which will generally make for a better output mosaic.

Another typical problem area: bridge decks. The image below shows a seam cutting through a freeway overpass. It is easy to see that the edges are misaligned.

Below, the seam is edited to run parallel with the bridge instead of cutting through it.

Note that accuracy issues in the terrain model used to create the input orthophotos, such as not having accurate enough terrain or the existence of errors in the terrain, can also introduce "mismatches" between images. In the bridge example above I used a DEM where I modelled the terrain with reasonable accuracy (performing terrain editing in stereo), but I stopped short of modeling the bridge decks. This means that the image area of the ground is accurate, but there is some offset for the bridge. While the seam editing technique hides the error and creates a visually-appealing result, the fact remains that the ortho may be considered flawed because the seams in the mosaic simply mask the error - they do not eliminate it.

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