The purpose of the scale estimation phase is to compute an estimate for the scale at which the pattern may occur within the image. In the example above, the pattern is about 10% of the image. The third column of the first row shows the pattern scaled to reflect the computed estimate. One can see that the estimate in this example is very accurate. The scale estimation phase uses the amounts of colors within the image and query pattern, but not their locations.
The purpose of the initial placement phase is to identify regions in the image that have a similar color histogram to the color histogram of the query. These are promising regions for the pattern occurrence, and will be explored further in the next phase. The size of each candidate region is determined by the scale estimate from the previous phase. There is preprocessing performed so that at query time the system never examines image locations which obviously do not contain the pattern.
During the final verification and refinement phase, the system checks/verifies that the positions of the colors within a promising image region make sense. At this time it also adjusts/refines its guess as to the scale, orientation, and location of the pattern in order to improve a match distance measure that uses the positions of uniform color regions within the query pattern and image. An iterative match improvement process is directed by the region colors since these colors are unchanged by the allowed similarity transformation of pattern region positions. Eventually, the search rectangle settles (SEDLs) on the image region where the system believes that the pattern occurs.
The point here is that a system might falsely conclude that the pattern does not occur within an image because its scale estimate is incorrect. This can happen even when the system has the correct location for the pattern (as shown above).
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The ideas and results contained in this document are part of my thesis, which will be published as a Stanford computer science technical report in June 1999.
S. Cohen. Finding Color and Shape Patterns in Images. Thesis Technical Report STAN-CS-TR-99-?. To be published June 1999.
Email comments to firstname.lastname@example.org.