Butteraugli

butteraugli is a perceptual similarity metric designed to estimate how noticeable differences between two images will be to the human eye. Instead of simple pixel math, it models aspects of human vision—color sensitivity, spatial masking, and contrast perception—to highlight differences that viewers actually see. The core tool outputs a single “distance” score along with per-pixel or per-region maps that show where artifacts are most objectionable. These maps make it practical to tune compressor settings and confirm whether bitrate reductions are visually acceptable. The metric has become a common yardstick for objective image quality when comparing codecs or encoder tweaks that target web or mobile delivery. Because it is deterministic and fast, it can be used in automated pipelines to gate releases on visual quality, not just file size.