Reduced Memory JPEG Decompression for Mobile Devices

摘要

Imagery makes up a large percentage of geospatial data in use today. One feature of this imagery is that it tends to be large, often hundreds or thousands of megabytes. As a result JPEG compression is often used to make geospatial imagery manageable by reducing the file size without greatly reducing the quality of the image. However, the benefits of compression are absent when the image must be viewed. Viewing a large JPEG image requires decompressing and holding the uncompressed version in memory. Holding the entirety of a large image in memory is a burden on many systems and sometimes impossible. However, the entire image is rarely needed at full resolution. Usually only a small area of interest is viewed or processed. This paper describes a method of removing a small area of interest from a large JPEG without decompressing the entire image. JPEG compressed images are streams which cannot be randomly accessed. Viewing a particular area requires that all preceding areas be partially decompressed. This process is more efficient than fully decompressing the whole JPEG, but depending on the area requested the entire image may need to be partially decompressed. To circumvent this problem an index file is created on first decompression which records markers for the sections of the JPEG. The index file allows random access to the JPEG file so that areas may be decompressed without reading the preceding portions of the JPEG. This method of decompressing a JPEG requires a limited amount of memory and with an index file is fast enough to be performed in real time.