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sjpeg: 's' stands for 'simple'.
sjpeg is a simple encoding library for encoding baseline JPEG files. It's also a command-line tool that takes PNG or JPEG images as input to produce JPEG files.
Why 'simple'? Well, the encoder used to be one single source file. And not a large one, moreover. Over time it grew and was split, for readability, but the spirit remains the same.

The sjpeg library:

The main header is src/sjpeg.h. This is mainly just one function to call, defined in src/sjpeg.h

size_t SjpegCompress(const uint8_t* rgb, int width, int height,
                     int quality, uint8_t** out_data);

This function will make a lot of automatic decisions based on input samples.

There is a more fine-tuned variant available too:

size_t SjpegEncode(const uint8_t* const data,
                   int W, int H, int stride,
                   uint8_t** const out_data,
                   int quality, int compression_method,
                   int yuv_mode);

as well as functions with a C++ string-based API.

Also included in the library: some helper functions to inspect JPEG bitstream. They are meant to help re-processing a JPEG source: SjpegDimensions(): quickly get the JPEG picture dimensions. SjpegFindQuantizer(): return the quantization matrices SjpegEstimateQuality(): return an estimate of the encoding quality

The library comes primarily with a plain-C API, but a C++ version is available too, that uses std::string as interface.

SjpegEncodeParam interface:

This should be the most fine-tuned use of the library: SjpegEncodeParam gives access to a lot of fine controls (C++ only). See src/sjpeg.h for details.

In particular, using SjpegEncodeParam is the recommended way of re-compressing a JPEG input. Here is the recipe:

use SjpegFindQuantizer to extract the quantization matrices from the source
use SjpegEncodeParam::SetQuantMatrix() to transfer them to the encoding parameters (with a reduction factor if necessary)
call sjpeg::SjpegEncode() with this parameter

Alternately, one can use SjpegEstimateQuality() and SjpegEncodeParam::SetQuality() to pass a quality factor instead of the matrices.

SjpegEncodeParam is also useful for transfering some metadata to the final JPEG bitstream.

'Riskiness' score:

The riskiness score is a decision score to help decide between using YUV 4:2:0 and YUV 4:4:4 downsampling. The function to call is:

 int SjpegRiskiness(const uint8_t* rgb, int width, int height, int step, float* risk);

which returns a recommendation for 'yuv_mode' to use in the above API.

The 'sjpeg' command line tool:

examples/sjpeg is a very simple tool to compress a PNG/JPEG file to JPEG.

sjpeg in.png -o out.jpg -q 70 # compress the PNG source with quality 70

sjpeg in.jpg -o out.jpg -r 90 # recompress JPEG input to ~90%

sjpeg has various options to change the encoding method. Just try sjpeg -h.

The 'vjpeg' command line tool:

examples/vjpeg is a (non-essential) visualization tool, built with OpenGL/GLUT. It allows changing the compression parameters with the keyboard, and visually inspect the result. This is mostly educational, since most of the parameters from SjpegEncodeParam can be triggered from the keyboard.

Building the library and tools:

Please read the instructions in the INSTALL file.

Basically, you can use the supplied Makefile or generate your build with cmake:

make -j, or
mkdir build && cd build && cmake ../ && make -j

Discussion list

The following forum can be used to report bugs or just discuss sjpeg:

https://groups.google.com/forum/#!forum/sjpeg