A mezzanine is the high-quality master source a pipeline encodes from, a pristine file with quality headroom to spare rather than an already-compressed delivery copy. It matters to measurement because it is the true full-reference original: a full-reference metric such as VMAF or PSNR scores each encode by comparing it against this master, so the mezzanine defines the ceiling any score can reach and the apples-to-apples baseline a convex hull and a quality target are measured on. In a regression suite the golden reference source is effectively a mezzanine, chosen with headroom so a good encode lands high on the scale and a drop is visible. The catch is that a mediocre master poisons everything downstream: if the source is itself soft or already compressed, every encode scores low, the metric cannot tell a good build from a bad one, and the ladder you build from it is wrong. Pin the mezzanine and treat it as a fixed, version-controlled input, never re-fetched from a changing source.
