A rate-quality curve - the perceptual cousin of the classic rate-distortion (RD) curve - plots a quality score against the bitrate that produced it. Bitrate runs along the x-axis, almost always on a logarithmic scale because it spans orders of magnitude; the quality score (VMAF on 0 to 100, or SSIM on 0 to 1) runs up the y-axis. Each point is one encode of the same content at a different bitrate, and the points trace a curve. The shape is the lesson: the curve is convex and flattens as it climbs, so a small bitrate increase at the low end buys a large quality jump while the same increase at the high end buys almost nothing - the diminishing-returns property. The knee, where it bends from steep to flat, is a sensible operating point. The catch is that any comparison of two curves must be apples-to-apples: same content, resolution, reference, metric, and model. These curves are the geometry BD-rate and the convex hull both read.
