UnBlock
Remove 8x8-pixel "blocky" artifacts from a heavily compressed JPEG image.
How to build on Ubuntu 18 through 22
sudo apt install g++ libpng-dev make
make
This also works on Windows 10 or 11, within WSL.
How to run
./unblock in.bmp out.bmp
./unblock in.png out.png
How to test
make test
Software used
This is a wrapper around an algorithm by John Costella. His webpage includes before-and-after pictures showing the algorithm's effectiveness.
Instead of porting John's C# source code to C/C++, I built on Alexander Balakhnin's C port, in the context of a plugin for AviSynth, based on John's own 2007 C port of "the Costella libraries."
Files in .bmp format are read and written by EasyBMP,
files in .png format by libpng.
Why not .jpg for a JPEG utility?
Because my source images are still frames from mjpeg-format video.
Extracting frames in a lossless format avoids the further degradation that happens with
ffmpeg -i in.avi -vcodec jpg ....
The journal article Stitched Panoramas from Low-Cost Airborne Video Cameras demonstrates an application of this software. The article is summarized in the MIT Technology Review.
Bugs
An image whose width is not a multiple of 16 pixels may misbehave.