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The Area of Interest Tool

The Area of Interest (AOI) encoding feature enables image customization through the preservation or obscuring of detail. AOI enables you to define one or more areas within an image or mosaic and encode these areas at a higher or lower compression ratio than the remainder of the image. Using AOI thus creates a visual contrast between the area(s) of interest and the surrounding image, rendering either “sharper” (lower compression) areas or “blurred” (higher compression) areas. In the image below we’ve drawn a red rectangle around an area that has been defined as an area of interest and compressed at a lower ratio than the rest of the image. Note that more detail is preserved within that area.

GeoExpress offers three methods of encoding an area of interest: the weight, shift and mask methods. GeoExpress enables you to encode an area of interest in MG3, MG4, or JPEG 2000 format. When the output format selected is JPEG 2000, any method can be used. MrSID encoding requires the weight or mask method. For more information about using these methods, see MG3 and MG4 Area of Interest Encoding and JPEG 2000 Area of Interest Encoding.

Although area of interest is a simple concept, there are many factors that come into play when generating both the preview and the final encoded image in an area of interest operation. Some of these factors are:

• the relative size of the selected area in relation to the entire image
• the difference between the two compression ratios applied (inside and outside the selected area)
• the presence or addition of a crop operation in conjunction with an area of interest encode
• the desired output format (MG3, MG4, or JPEG 2000)
• the complexity of the imagery being manipulated

Some of the effects that a user may experience as a result of these factors are:

• slight bleeding of the AOI resolution into the surrounding image
• “outside” area seems less compressed than expected given the specified compression ratios
• area of interest specified as lossless is not always numerically lossless for some encode scenarios (this variation is greater in JPEG 2000 encoding than in MrSID encoding).