Parent Topic: Mosaicking

Colour Balancing

Uncorrected images can be altered to make the mosaic less noticeable. Colour balancing is achieved by calculating the lookup table required to approximately match the uncorrected image to the georeferenced image. The creation of this new lookup table is called ``histogram matching''. An image histogram is a graph showing the number of pixels with a given brightness, for each possible brightness value. In histogram matching, the image histograms of the uncorrected and georeferenced images are compared, and an attempt is made to make the uncorrected image histogram match the georeferenced image histogram by creating a ``histogram matching lookup table''. All values in the uncorrected image will be passed through this lookup table to create the ``histogram equalized'' (colour balanced) image.

The ``Colour Matching'' panel in GCPWorks provides the opportunity to select areas for histogram comparison. It is important that appropriate selections are made. Like areas should be compared. Comparing bright fields to dark mountains will produce a histogram equalized image with dark fields and very dark mountains, or light mountains and very bright fields (depending on which is the georeferenced and which is the uncorrected).