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 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 compression, 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, gives out a map and 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'.