JPEG-LS encoder

A simple JPEG-LS baseline (ITU-T T.87) image compressor (encoder) in C language.

　

Background of JPEG-LS

JPEG-LS (JLS) is a lossless/lossy image compression standard which has the best lossless compression ratio compared to PNG, Lossless-JPEG2000, Lossless-WEBP, Lossless-HEIF, etc.

The standard specification of JPEG-LS is ITU-T T.87 [1], see https://www.itu.int/rec/T-REC-T.87/en

JPEG-LS uses the maximum difference between the pixels before and after compression (NEAR value) to control distortion, NEAR=0 is the lossless mode; NEAR>0 is the lossy mode, the larger the NEAR, the greater the distortion and the greater the compression ratio.

The file suffix name for JPEG-LS compressed image is .jls .

　

Development progress

• gray 8 bit image lossless encode (near=0)
• gray 8 bit image lossy encode (near-lossless, near≥1)
• gray 9-16 bit image lossless encode (near=0)
• gray 9-16 bit image lossy encode (near-lossless, near≥1)
• RGB 24 bit image lossless encode (near=0)
• RGB 24 bit image lossy encode (near-lossless, near≥1)
• RGB 48 bit image lossless encode (near=0)
• RGB 24 bit image lossy encode (near-lossless, near≥1)

　

Code list

The code is in pure-C. See src folder:

• JLS.c : Implement a JLS encoder
• JLS.h : Expose the JLS encoder function to users
• JLSMain.c : A main program with main() function, which call the JLS encoder to achieve image file compression.

　

Compile

Compile in Windows (CMD)

If you installed MinGW Compiler for Windows, you can compile using the command line (CMD).

gcc src/*.c -o JLSencode.exe -O3 -Wall

We'll get the executable file JLSencoder.exe . Here I've compiled it for you, see JLSencoder.exe .

Compile in Linux

Run the command in the current directory:

gcc src/*.c -o JLSencode -O3 -Wall

We'll get the binary file JLSencoder .

　

Run image compression

This program can compress the following three types of image file to .jls compressed file.

• PGM grayscale image file, whose specification is in https://netpbm.sourceforge.net/doc/pgm.html
• PPM RGB image file, whose specification is in https://netpbm.sourceforge.net/doc/ppm.html
• PNM grayscale or RGB image file, whose specification is in https://netpbm.sourceforge.net/doc/pnm.html

There are some PGM and PPM files in testimage directory.

Run image compress in Windows (CMD)

Use the following command to compress a image file to a .jls file.

.\JLSencode.exe  <input-image-file>  <output-file(.jls)>  [near]

Where [near] is a optional parameter of range 0~9 :

• 0 : lossless (default)
• 1~9 : lossy. The larger the near value, the higher distortion and the lower compressed size.

For example, the following command to compress rgb1.ppm in testimage folder to rgb1.jls with near=1

.\JLSencode.exe  testimage\rgb1.ppm  rgb1.jls  1

Run image compress in Linux

Use the following command to compress a image file to a .jls file.

./JLSencode  <input-image-file>  <output-file(.jls)>  [near]

　

Appendix: How to decompress .jls file?

This repo do not have a JPEG-LS decompressor. There are several ways to decompress a .jls file:

• The most convenient method: Try this website to view .jls image online (it may not work).
• Use other JPEG-LS code or libraries such as UBC's JPEG-LS code [3] or CharLS [4].
• Use python pillow library and pillow_jpls library, see [5].

　

Reference

[1] ITU-T T.87 : JPEG-LS baseline specification : https://www.itu.int/rec/T-REC-T.87/en

[2] PNM Image File Specification : https://netpbm.sourceforge.net/doc/pnm.html

[3] UBC's JPEG-LS baseline Public Domain Code : http://www.stat.columbia.edu/~jakulin/jpeg-ls/mirror.htm

[4] CharLS, a C++ JPEG-LS library implementation : https://github.com/team-charls/charls

[5] pillow-jpls library for Python - PyPI : https://pypi.org/project/pillow-jpls

[6] FPGA-based Verilog JPEG-LS encoder (basic version which support 8-bit gray lossless and lossy) : https://github.com/WangXuan95/FPGA-JPEG-LS-encoder

[7] FPGA-based Verilog JPEG-LS encoder (ultra high performance version which support 8-bit gray lossless) : https://github.com/WangXuan95/UH-JLS