What Is an Area of Interest?

The Area of Interest (AOI) encoding feature is a tool that 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” areas (lower compression) or “blurred” areas (higher compression).

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 greater detail is preserved within that area.

GeoExpress enables you to encode an area of interest in in MG3, MG4 or JPEG 2000 format.

Inner and Outer Areas

All regions inside specified areas of interest are called “foreground”. All surrounding regions are called “background”. “Inner” and “outer” correspond to foreground and background respectively.

In JP2 encoding, the -aoishiftinner and -aoiweightinner switches preserve the quality of the foreground at the expense of the surrounding regions. Similarly, the -aoishiftouter and the -aoiweightouter switches preserve the quality of the background at the expense of the foreground.

In MG3 encoding, inner and outer are not specifically referred to except when using the mask method. The foreground and background are controlled using the -aoicr or the -aoilossless switch. If the value specified with the -aoicr switch is less than the value for the image’s overall compression ratio as specified by the -cr switch, then the foreground will be preserved at a greater quality than the background. If the -aoicr value is greater than the -cr value, then the background will be preserved at a greater quality than the foreground.

In the mask method, the -aoimaskinner switch renders the foreground as a solid color and the -aoimaskouter switch similarly affects the background.

Reading the Log File

The compression ratios for the inside and outside of the area of interest are reported in the GeoExpress log file, along with the overall encode ratio for the entire image.

Using Area of Interest with Image Crop

GeoExpress accommodates a workflow that demands a number of image customizations at a single encode. Accordingly, you can define a crop region and an area of interest for the same encoding event. Also, you can specify an area of interest anywhere in the image, regardless of whether or not it is in the crop region. Obviously, any portion of an area of interest that falls outside the crop region will be lost on encode.

Notes

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
• desired output format – MG3, MG4 or JP2
• 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).