Black hole images under spherical-shell and circular-annulus accretion models in Schwarzschild spacetime: a semianalytical approachBofeng Wu, En-Wei Liang and Xiao ZhangEur. Phys. J. C, 85 6 (2025) 632DOI: https://doi.org/10.1140/epjc/s10052-025-14302-x