This program tries to recreate lost precision of DCT coefficients based on quantization table from jpeg image. Output saved as jpeg image with quantization set to 1 (like jpeg saved with 100% quality). You can save smoothed image with original quantization tables resulting in same DCT coefficients as in original image.

You may not notice jpeg artifacts on the screen without zooming in, but you may notice them after printing. Also, when editing compressed images, artifacts can accumulate, but if you use this program before editing - the result will be better.

The original project page is here.

Web version available here. Images are processed locally on your computer. Without multithreading and SIMD optimizations it runs slower than native code.

• Click the "Load" button or drag-n-drop JPEG image into the browser window. After processing is complete, you can save the result by clicking the "Save" button. You can edit the filename field before saving. The options field is passed to the wasm code when you initiate a processing by loading a file.

jpegqs [options] input.jpg output.jpg

-q, --quality n Quality setting (0-6, default is 3)
-n, --niter n Number of iterations (default is 3)
-t, --threads n Set the number of CPU threads to use
-o, --optimize Option for libjpeg to produce smaller output file
-v, --verbose n Print libjpeg debug output
-i, --info n Print quantsmooth debug output (default is 15)
Use the sum of flags: 0 - silent, 1/2/4 - various information, 8 - processing time, 16 - SIMD type.
-p, --cpu n Use to lower the SIMD type if CPU detection fails:
0 - auto, 1 - scalar, 2 - SSE2, 3 - AVX2, 4 - AVX512. (x86 build selects between modes 1-3, x86_64 from 2-4)

• The processing time includes only the smoothing algorithm, jpeg reading and writing time is not included.
• More iterations can make the result look like CG art, can make the photos look unnatural.

The quality setting sets a combination of flags for processing:

3. default
4. adds DIAGONALS flag
   smoother diagonal edges, ~1.5 times slower
5. adds JOINT_YUV flag
   chroma channels will depend from luminance, better color consistency
6. adds UPSAMPLE_UV flag
   non-blurring chroma upsampling, unlike fancy upsampling from libjpeg

• levels 0-2 is the same as 4-6, but with LOW_QUALITY flag
  ~10 times faster, but the quality is lower
  LOW_QUALITY implies DIAGONALS (always set)

• Images 3x zoomed.

Original images:

JPEG with quality increasing from 8% to 98%:

After processing:

If your system have libjpeg development package installed, just type make. Tested with libjpeg-turbo8-dev package from Ubuntu-18.04.

1. Download and extract libjpeg sources:
   1. libjpeg, for example version 6b
      wget https://www.ijg.org/files/jpegsrc.v6b.tar.gz
tar -xzf jpegsrc.v6b.tar.gz
   2. libjpeg-turbo, for example version 2.0.4
      wget -O libjpeg-turbo-2.0.4.tar.gz https://sourceforge.net/projects/libjpeg-turbo/files/2.0.4/libjpeg-turbo-2.0.4.tar.gz
tar -xzf libjpeg-turbo-2.0.4.tar.gz

• For a libjpeg (not turbo) you can build jpegqs in a simpler way:
  make JPEGSRC=jpeg-6b
  This uses static configuration from jconfig.h, which should work for common systems.
  The following items are not needed if you do so.

2. Configure and build libjpeg:

   1. For libjpeg and libjpeg-turbo-1.x.x:
      (cd jpeg-6b && ./configure && make all)
   2. For libjpeg-turbo-2.x.x ./configure script is replaced with cmake:
      (cd libjpeg-turbo-2.0.4 && mkdir -p .libs && (cd .libs && cmake -G"Unix Makefiles" .. && make all))

3. Tell make where to find libjpeg includes and libjpeg.a
make JPEGLIB="-Ijpeg-6b jpeg-6b/libjpeg.a
   For a newer versions libjpeg.a is located in a .libs/ dir.

The jpegqs makefile can download sources, extract and compile libjpeg for you. Replace %VER% with a version.

• libjpeg: make jpeg-%VER%/libjpeg.a Tested versions: 6b, 7, 8d, 9c
• libjpeg-turbo:make libjpeg-turbo-%VER%/libjpeg.a Tested versions: 1.0.0, 1.4.2, 1.5.3, 2.0.4

It will print you link to archive which you need to download, or you can allow the downloads by adding WGET_CMD=wget to the make command line.

Get MSYS2, install needed packages with pacman and build with release.sh. If you are not familiar with building unix applications on windows, then you can download program from releases.

Similar projects, and how I see them after some testing.

jpeg2png:
 ✔️ good documentation and math model
 ✔️ has tuning options
 ✔️ better at deblocking low quality JPEG images
 ❓ little blurry in default mode (compared to quantsmooth), but can be tuned
 ➖ 10 to 20 times slower
 ➖ less permissive license (GPL-3.0)

jpeg2png can provide roughly same quality (better in not common cases), but significantly slower.

knusperli:
 ✔️ more permissive license (Apache-2.0)
 ➖ you can hardly see any improvements on the image
 ➖ no performance optimizations (but roughly same speed as for quantsmooth with optimizations)
 ➖ no any command line options
 ➖ uncommon build system

knusperli is good for nothing, in my opinion.