a) Mass-to-light-ratio of groups as a function of group mass in different environments defined by the luminosity density field. b) Mass-to-light-ratio of groups inside and outside filaments with luminosity density values Den8 < 1.5. c) Mass-to-light-ratio of groups inside and outside filaments with luminosity density values 1.5 < Den8 < 5. d) Mass-to-light-ratio of groups inside and outside filaments with luminosity density values Den8 > 5.