Chroma folds the spectrum into the twelve pitch classes: all the Cs in every octave pool into one value, all the C sharps into the next. The result is a compact harmonic fingerprint that hears a C major chord as the same object whether voiced low or high.
Music information retrieval leans on chroma for chord recognition, key detection, and finding cover versions, since melody and production change while harmony persists.
For reactive visuals, chroma is the honest route to harmony-aware behavior: palettes that shift with the key, structures that recognize the chorus’s chords returning. It is heavier than band energy but it hears music rather than sound.