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Mosaicking

PCI Geomatics -

Some scanned photos have areas of black, which have a brightness or digital number of 0. In this case, there is no data until a much brighter number, such as 80, is encountered. This is usually due to enhancements applied to the scanned image. The cutline generation algorithm compares the data values (80+) to the blank areas (0) and considers the blank areas to be background data. It then clips out these areas.

There are two solutions for this:

  1. (Recommended) Scan your photos carefully, using a good quality scanner - This will help to make sure that your range of values is reasonable. If you find the image too dark or too light and you are applying enhancements, make sure that you do not use tail-trimming. Tail-trimming will shift the histogram of most pixels, but leave the extremely dark or extremely light values set to 0 or 255. If your values are quite dark, start your enhancement by adding a bias to all pixels (i.e. add 25 to your image).
  2. In a pinch: If you have already created your orthos, then they will already have a black background area. Adding a bias is not appropriate in this case. If you do not have any pixels with a value of 255, then use Replace Image/Photo Values to change all 0 pixels to 255. Run the automatic mosaic, then replace all 255 values with 0.
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