footage-rift.webm: 97 frames, rows 424-844, 1.67 MB, VP9 CRF 4
  rings (before minus after, blurred 4 px): RMS 0.871 L*
  kept: correlation 0.990, RMS ratio 0.999
  contour pixels a frame, lossless -> decoded: before half 39 -> 1362, after half 0 -> 431
  the after half after the blur, decoded against lossless: RMS 0.573 L*