A tool for measuring perceived differences between images

Butteraugli is a project that estimates the psychovisual similarity of two images. It gives a score for the images that is reliable in the domain of barely noticeable differences. Butteraugli not only gives a scalar score, but also computes a spatial map of the level of differences.

One of the main motivations for this project is the statistical differences in location and density of different color receptors, particularly the low density of blue cones in the fovea. Another motivation comes from more accurate modeling of ganglion cells, particularly the frequency space inhibition.

Butteraugli can work as a quality metric for lossy image and video compression. On our small test corpus butteraugli performs better than our implementations of the reference methods, psnrhsv-m, ssim, and our yuv-color-space variant of ssim. One possible use is to define the quality level setting used in a jpeg compressor, or to compare two or more compression methods at the same level of psychovisual differences.

Butteraugli is intended to be a research tool more than a practical tool for choosing compression formats. We don't know how well butteraugli performs with major deformations -- we have mostly tuned it within a small range of quality, roughly corresponding to jpeg qualities 90 to 95.

Only a C++ interface is provided. The interface takes two images and outputs a map together with a scalar value defining the difference. The scalar value can be compared to two reference values that divide the value space into three experience classes: 'great', 'acceptable' and 'not acceptable'.

Install Bazel by following the instructions. Run bazel build -c opt //:butteraugli in the directory that contains this README file to build the command-line utility. If you want to use Butteraugli as a library, depend on the //:butteraugli_lib target.

Alternatively, you can use the Makefile provided in the butteraugli directory, after ensuring that libpng and libjpeg are installed. On some systems you might need to also install corresponding -dev packages.

The code is portable and also compiles on Windows after defining _CRT_SECURE_NO_WARNINGS in the project settings.

Butteraugli, apart from the library, comes bundled with a comparison tool. The comparison tool supports PNG and JPG images as inputs. To compare images, run:

butteraugli image1.{png|jpg} image2.{png|jpg}

The tool can also produce a heatmap of differences between images. The heatmap will be output as a PNM image. To produce one, run:

butteraugli image1.{png|jpg} image2.{png|jpg} heatmap.pnm