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. 2021 Sep 29;55(4):2875–2944. doi: 10.1007/s10462-021-10082-4

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© The Author(s), under exclusive licence to Springer Nature B.V. 2021

This article is made available via the PMC Open Access Subset for unrestricted research re-use and secondary analysis in any form or by any means with acknowledgement of the original source. These permissions are granted for the duration of the World Health Organization (WHO) declaration of COVID-19 as a global pandemic.

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Fig. 21 — Edge detection procedure. a The original image. b Image after application of the Marr-Hildreth operator with $3 \times 3$ kernel. c Image after application of a rank 3 filter. d Binary image after thresholding. e Image after erosion (in Blackburn et al. (1998) Fig. 1)