Lepton is a tool and file format for losslessly compressing JPEGs by an average of 22%.
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⚠️  Deprecation Notice for the Dropbox Lepton project

Dear Lepton users and developers,

Thank you so much for using and contributing to Lepton over the years! We’ve recently realized that we no longer have sufficient resources to adequately support this project. This includes providing timely fixes to bugs and vulnerabilities such as those that were recently reported to us. While we did ensure that the reported vulnerabilities don’t affect our internal use of Lepton, we unfortunately don’t have the capacity to properly fix these and future issues in this public repo. Therefore, after some consideration, we’ve decided to deprecate and archive the project. We apologize for any inconvenience and appreciate your understanding.

If you would like to continue using Lepton, please check out Microsoft’s implementation of Lepton in Rust or alternatively consider switching to other lossless compression methods such as Brotli or to modern image formats such as WebP or JPEG XL.


Lepton is a tool and file format for losslessly compressing JPEGs by an average of 22%.

This can be used to archive large photo collections, or to serve images live and save 22% bandwidth.

Build Status

Build directions

Using a single core

make check

For multiprocessor machines:

make -j8
make check -j8

Using CMAKE:

mkdir -p build
cd build
cmake ..
make -j8

On Windows

mkdir -p build
cd build
"c:\Program Files\CMake\bin\cmake" ..
start .
REM Double click the Visual Studio project


To roundtrip (compress and decompress) an image, original.jpg, do the following:

./lepton original.jpg compressed.lep
./lepton compressed.lep restored_original.jpg

Or all at once:

./lepton original.jpg compressed.lep && ./lepton compressed.lep restored_original.jpg && diff restored_original.jpg original.jpg && echo no differences

Lepton may also be used with pipes -- be sure to check the exit code when using pipes as if compression fails lepton will produce 0 bytes and return a nonzero exit code (failure). In this case do not assume the 0 byte file is representative of the original.

./lepton - < original.jpg > compressed.lep
./lepton - < compressed.lep > restored_original.jpg

You may specify higher memory limits than the default for lepton to handle bigger images:

./lepton -memory=1024M -threadmemory=128M input_file output_file

Additionally you can configure lepton to process progressive jpegs. Warning: these take more memory to decode than normal JPEGs since the entire framebuffer must be kept in memory for the duration of the decompression, instead of just 2 rows of blocks.

./lepton -allowprogressive -memory=1024M -threadmemory=128M progressive.jpg compressedprogressive.lep

Submitting pull requests to lepton

Please begin by filling out the contributor form and asserting that

The code I'm contributing is mine, and I have the right to license it.
I'm granting you a license to distribute said code under the terms of this agreement.

at this page: https://opensource.dropbox.com/cla/

Then create a new pull request through the github interface


Lepton is designed to be easy to debug, but a command line requirement is necessary to prevent the standard forks that let it do a secure verification in a separate process.

To avoid setting follow fork flags, please pass -skipverify to the command line. This will stop verification and let you debug the app as a single process application. If the bug happens in single threaded mode, also you can pass -singlethread which makes it easier to step through the code without other threads hitting breakpoints.

Bindings for other languages


Many thanks to Matthias Stirner and HTW Aalen University for the development of the uncmpJPG library.

Thanks to the VPX authors for their highly tuned bool reader and bool writer implementations.

Related Work

Be sure to read the groundbreaking research done by Matthias Stirner, Gehard Seelman and HTW Aalen University in their 2007 paper and check out their excellent PackJPG repositories for compression of JPEG MP3 BMP and PMN formats.

Also the paq algorithms combine for a highly compressed JPEG result when decompression time is less critical.

When the compressed JPEG needs to also be a JPEG, there is mozjpg to explore as well.


Unless otherwise noted:

Copyright (c) 2016 Dropbox, Inc.

Licensed under the Apache License, Version 2.0 (the "License");
you may not use this file except in compliance with the License.
You may obtain a copy of the License at


Unless required by applicable law or agreed to in writing, software
distributed under the License is distributed on an "AS IS" BASIS,
See the License for the specific language governing permissions and
limitations under the License.
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