This tool computes (dis)similarity between two or more PNG images using an algorithm approximating human vision.
Comparison is done using the SSIM algorithm (based on Rabah Mehdi's implementation) at multiple weighed resolutions.
The value returned is 1/SSIM-1, where 0 means identical image, and >0 (unbounded) is amount of difference. Values are not directly comparable with other tools. See below on interpreting the values.
- Comparison is done in in L*a*b* color space (D65 white point, sRGB gamma) with chroma subsampling. Other implementations use "RGB" or grayscale without gamma correction.
- Supports alpha channel.
- Only needs C (C99) and
libpngor Cocoa on OS X. No OpenCV or MATLAB needed.
dssim file-original.png file-modified.png
Will output something like "0.02341" (smaller is better) followed by a filename.
You can supply multiple filenames to compare them all with the first file:
dssim file.png modified1.png modified2.png modified3.png
You can save an image visualising the difference between the files:
dssim -o difference.png file.png file-modified.png
The dssim.c file is also usable as a C library.
The amount of difference goes from 0 to infinity. It's not a percentage.
If you're comparing two different image compression codecs, then ensure you either:
- compress images to the same file size, and then use DSSIM to compare which one is closests to the original, or
- compress images to the same DSSIM value, and compare file sizes to see how much file size gain each option gives.
More about benchmarking image compression.
You need libpng, zlib, pkg-config and make
make
Will give you dssim. On OS X make USE_COCOA=1 will compile without libpng.
You'll find downloads on GitHub releases page.
Debian packages for i386/amd64 can be installed for ubuntu (14.04 LTS) from ppa:
$ sudo apt-add-repository ppa:lkwg82/dssim
$ sudo apt-get update
$ sudo apt-get install dssim(other versions on request)
Plain download from here: https://launchpad.net/~lkwg82/+archive/ubuntu/dssim/+packages
Scores for version 1.3 measured against TID2008 database:
| TID2008 Category | Spearman correlation |
|---|---|
| Noise | -0.872 |
| Noise2 | -0.888 |
| Safe | -0.889 |
| Hard | -0.903 |
| Simple | -0.922 |
| Exotic | -0.484 |
| Exotic2 | -0.651 |
| Full | -0.818 |