Spitzer IRAC 3.6 μm images illustrating the morphological structures considered in our environmental masks. Section 3 explains the mask construction scheme in detail. Bulges and discs are defined on photometric decompositions of near-infrared images (Salo et al. 2015 for S⁴G galaxies; Laurikainen et al. 2004 or new fits otherwise). The sizes of bars, rings and lenses are defined visually and their ellipticity is measured via ellipse fitting (Herrera-Endoqui et al. 2015 for S⁴G, with additional measurements from the literature). Spiral arms are identified as peaks on unsharp-masked 3.6 μm images followed by log-spiral fits in polar coordinates, the width being assigned based on CO emission (Herrera-Endoqui et al. 2015 and new measurements). The galaxies shown are, from left to right, NGC 2775, NGC 628, NGC 1300, NGC 3627, NGC 3351 and NGC 4457; they are all displayed using an arcsinh stretch.