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The disclosed method provides means of image focus measurement
for JPEG compressed images, recognizing the artifacts induced by JPEG and
how to manage them.
English (United States)
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100% of the total text.
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Focus (video gradient) measurement of JPEG compressed images
Image focus is one of the key attribute of the image quality. Video gradient, a direct measure of image focus, is computed by taking the difference of video of neighboring pixels, followed by determination of the maximum video gradient. The gradient is normalized with respect to the image brightness by known techniques.
The prior method must be modified for images compressed with JPEG. The problem arises due to the blocking effect produced in the JPEG decompressed images. Each 8 X 8 pixel block is treated independently in JPEG compression. As a result, when JPEG image is decompressed, the image has a distinct blocks at 8 pixel intervals in both X and Y direction. Enlarged section of a JPEG image, shown below, shows the blocking effect.
Since the "blocking" JPEG artifacts are present at the 8 pixel borders, horizontally and vertically, ignoring all pixels at the border for gradient computation provides the desired result. This is a simple but necessary modification that must be applied for true focus measurement.
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Disclosed by International Business Machines Corporation
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