Although more sophisticated image formats have existed for some time [1], JPEG is still one of the most common. Whether you are working on a smartphone, compact camera, or professional photo device, you'll find an option for generating images in JPEG format. The flood of JPEG images caused by so many millions of users snapping photos actually sucks up lots of storage space on file hosting servers. This problem led cloud provider Dropbox [2] to look for a better way to store photo images. The result of this effort is the new Lepton image format, which reduces the space requirements for JPEG images by up to 25 percent. Lepton works losslessly, meaning the image remains exactly the same after you reconvert it (Figure 1). Dropbox says it has already applied the new format to 16 billion images and has thus saved multiple petabytes of storage space. The Dropbox cloud transparently converts JPEG images to Lepton format and changes them back into the original format as soon as someone accesses them.

Figure 1: Lepton doesn't make any changes to the image when converting and reconverting: The image remains exactly the same.

Unlike JPEG, Lepton doesn't reduce the visible image – image reduction often results in known artifacts with JPEG. Instead, Lepton is designed to reduce the data needed for the image information using the Middle-out algorithm, a type of arithmetic coding that is also used in the VP8 video format. A Dropbox blog post provides a detailed technical explanation about how the Lepton format reduces the size of a JPEG file [3].

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