Development of the 16–17 January 2021 CME as observed by the Solar Orbiter/Metis coronagraph. First and second row from the top: VL pB and UV Ly-α images, respectively, processed via the NRGF algorithm (unit-less numbers as produced by the algorithm). The time labelling each image refers to the centre of the acquisition; in particular, the UV image shown in the second row is the second image in each data acquisition set. Third row: ratio of the UV to pB images, and last row: difference images obtained from the two pairs of UV images in each data acquisition session.